7
Witness
Testaments of Sand
The sediments are a sort of epic poem of the Earth. When we are wise enough, perhaps we can read in them all of past history.
BURIAL
The girl sat on the sand by the river, sifting the grains over and over through her fingers, as three-year-olds tend to do. Her attention was divided. First, she needed to keep track of her mother, who was moving down the riverbank through the sparse trees, picking what fruit she could find. But upstream, above the hills, something fascinating was happening. A great, dark, turbulent mass of clouds had been rolling over the landscape, shot through with lightning and shaking with thunder. The sky had turned a strange earthy color in the late afternoon sun, and the ash cloud from the volcano on the horizon added its brushstrokes to a scene that transfixed the little girl. A distant rumbling sound began and rapidly grew louder. The girl turned and saw her mother running toward her. But it was too late: out of the river valley roared the first wave of a flash flood, tumbling the girl like a ball, forcing her into the bank, where the torrent of sand and pebbles quickly covered her.
Over three million years later, Zeresenay Alemseged picks away the sand grains from the girl’s skeleton, one by one; he has been at the task for years. Dr. Zeresenay is a paleoanthropologist, working at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, far away from his home in Ethiopia. But it was Ethiopia that yielded to him his treasure, the skeleton of the toddler in her tomb of sand. She had lived her short life in what is today the Dikika region of the Afar, the searingly hot depression at the northern end of the East African rift valleys, close to the Red Sea. The rift valleys have long marked, as their name implies, the rent in the Earth’s crust along which Africa threatened to split apart; in the Afar Depression and the adjacent Red Sea, this rip is active today, as evidenced by volcanoes and faults that create new land every year. But in occasionally more benign times, this was the home of our ancestors. It’s where the celebrity fossil Lucy was found, but the “Dikika baby,” remarkably preserved, is perhaps on her way to taking over the star role. Lucy and the child are of the same species, Australopithecus afarensis, but the child lived much earlier than Lucy. They are representatives of a crucial stage in our evolution, and the Dikika child, tiny though she is and still not fully revealed, has already shed light on critical questions. The sand, in burying her instantly but not violently, has preserved incredible details of her small body. Her face, braincase, lower jaw, all but two of the teeth (including adult teeth still waiting), collarbones, vertebrae, ribs, fingers, and kneecaps have all been revealed so far. Zeresenay has found, in her throat, the minute hyoid bone that is crucial for human speech. Though many of her features demonstrate that she had advanced only in some ways from her ape ancestors, she walked on two feet, and it is clear that the Dikika girl was on our evolutionary journey.
Sand preserves. Not only did it preserve the skeletons of Lucy and the Dikika girl, but it preserved itself. The layers of sediment in which our ancestors are found tell us about the rivers and the lakes, the volcanoes and the floods. Zeresenay works with colleagues from around the globe—geologists and experts on fossil animals and plants—to build up a picture of the world in which Australopithecus afarensis lived, a picture that is recorded in the character and contents of the sands and interpreted through the eyes of the geologists.
MAKING ROCKS
In the last few chapters, we have followed societies of sand grains on the move in today’s world, journeys that take place over great distances and long periods of time. But however epic those journeys, in the end all sediment is deposited somewhere, and there it may remain. The sand that engulfed the Dikika baby was buried by the cargo of further flash floods. Sand accumulates, building up layer by layer, each layer the product of a separate depositional event—the waning of a river in flood, an avalanche of sediment down a submarine canyon, a winter storm. If the circumstances are right, these layers, or beds, can build up over time into enormously thick piles of sediment that record long periods of the Earth’s history. Look at the Grand Canyon, for example, or the Roraima of the Venezuelan tepuis, the foundation for the highest waterfall in the world. The accumulation of sediment layers is one of the Earth’s great construction projects, ultimately building entire landscapes.
FIGURE 31. Turbidite landscapes of the South African Karoo. (Photo by author)
Figure 31 shows one such landscape, that of the Karoo in South Africa, where vast thicknesses of seemingly endless layers of sand-dominated sediment have built up from repeated turbidity currents hurtling down submarine slopes more than 250 million years ago. But if these were once sands and muds on the sea floor, how have they become solid rock, and how have they come to form an arid African landscape?
In the Karoo, loose, unconsolidated sand has been turned into sandstone, or lithified. Just as cooking turns cake batter into a cake (or something resembling a rock, if overdone), so lithification—a complex cooking process of temperature, pressure, and chemistry—turns sand into sandstone. The recipe for turning loose sand into solid rock reads roughly as follows: Pile up layers of sand successively so that the weight of the overlying sediment squeezes out the water from between the grains of the lower layers; allow the water to escape. Utilize the increasing temperatures below the Earth’s surface to dissolve some of the grains in the remaining water, at the same time allowing minerals to precipitate and cement the grains together. The greater the mineral and chemical variety of the grains used, the more complex will be the result. Approximate cooking time: millions of years. As we saw in chapter 2, adding bacteria can speed up the job; as the head of the research project looking at Bacillus pasteurii at the University of California at Davis, has commented, “Starting from a sand pile, you turn it back into a sandstone.”
But sediments themselves are only part of the story—after all, they can’t simply keep on piling up unless the base of the pile is sinking. And wherever great thicknesses of sediment have accumulated, that’s exactly what has happened: areas of the Earth’s crust have subsided over long periods of time, constantly creating the space for further accumulation of sediment. Fill up a bathtub with sand and there’s a limit to how much it can contain—unless somehow the bottom of the tub is sinking, in which case, as long as it continues to sink, more and more sand can be added. This is exactly what has been happening in the Afar Depression, preserving Lucy, the Dikika baby, and countless others of our ancestors. The Afar is called a depression for good reason—it is the lowest region of Africa, parts of it 150 meters (500 ft) below sea level, the result of the rifting of the land on either side. The Afar continues to sink, and sediments continue to pour in: it is a sedimentary basin, the geological term for our bathtub. Basins have been the great repositories, the storehouses of sediments throughout the Earth’s history. They come in many shapes and sizes, depending on the causes of crustal subsidence. And there’s feedback that amplifies the subsidence—the more sediments accumulate, the greater their weight, which further depresses the foundation of the basin. As long as subsidence continues and as long as there is a supply of sediment, then a basin will continue to grow. And the very fact that a basin is a depression almost guarantees a supply of sediment—the basin is a low point, surrounded by higher points, and in between, by definition, are slopes, down which sediment is inevitably transported.
Basins can form as plates collide and override each other, the crust sinking in front of a growing mountain belt; they can form where the crust is pulling apart, as in the Afar, and in California, where lateral movements cause subsidence (and uplift); and they can form in the middle of continents or at their edges. Two hundred million years ago, the Atlantic Ocean began to form, the old continent (as we shall see later in this chapter) breaking apart, as the Afar is today, eventually to the point where molten material from the Earth’s mantle surged up through the rift, creating new, oceanic, crust. That process continues today at the Mid-Atlantic Ridge; the upwelling of the molten material is a source of heat and uplift and, as the continents drift apart away from the ridge, their broken edges subside continuously and over long periods of time. The subsidence creates space to accommodate the volumes of sediment pouring off the continent (not to mention the constant rain of organic debris), and a basin is formed. Our Susquehanna sand grain, carried out onto the continental shelf of the eastern United States, was following the journeys of countless of its predecessors that had accumulated since the ocean began to open. There are areas of this continental shelf where the sediments have built up to a thickness of 10 kilometers (more than 6 mi). Basins are, indeed, truly massive sediment warehouses.
And when sand is buried to a depth of 10 kilometers, it experiences a huge weight of overburden and temperatures of up to 200°C (400°F), a recipe for change. Put the ingredients for your favorite cake together and they are stable at room temperature; put them in the oven and that stability is gone—the ingredients change their chemistry and the batter is turned into a cake. It is the same for sediments heated and squeezed in the depths of a basin. Minerals that had been dissolved in the water of the pores, the spaces in between the grains, precipitate and solidify. Different waters from deep in the sedimentary pile will flush through the pores, depositing new minerals. Vulnerable and unstable grains—for example, the feldspars that we saw weathered into clay in the first chapter—will decay and, again, change into clay and similar minerals. Even quartz grains, the survivors, will begin to dissolve, particularly along their edges, where they are crushed against one another. The net result? Grains are soldered together and the pore spaces between clogged up—the whole thing is glued together, cemented into solid rock, the finished cake of the Earth’s kitchen (Figure 32). The chemical and physical—and sometimes biological—processes that turn sediment into rock are technically termed diagenesis. They are much studied but still far from well understood—rather like the mysterious behaviors of proteins and carbohydrates investigated by chefs who are experts in “molecular gastronomy.”
FIGURE 32. Very thin slices of sandstones viewed through the microscope. The darker material between the grains is the cement. (Left) A 280-million-year-old dune sand, almost entirely quartz grains, cemented by silica. (Right) Billion-year-old river sand, angular and poorly sorted; the large mottled grains are feldspar. (Photos courtesy of Tony Dickson, Cambridge University)
So, from sand a sandstone is formed in the bowels of the basin. And there it will stay unless the processes that have been causing the subsidence of the basin cease and, indeed, reverse themselves. The crust may rebound or the contents of the basin may be caught up in battles between the plates, wrenched and bulldozed upward, eroded, sculpted, and exposed for the benefit of inquiring geologists—and others. The ancient sands of Roraima and the Grand Canyon, our sand grain’s relatives along the banks of the Susquehanna, and the endless landscapes of the Karoo have all experienced this history.
One key dimension of diagenesis is that the original sediment’s character is preserved. It may have been altered and lithified, but the rock has not melted; many of the grains are intact, and the sedimentary patterns, texture, and architecture that were imprinted and constructed when the sand was deposited are, like the Dikika baby’s bones, still there for all to see.
These great piles of sedimentary rock layers are ledgers: they record conditions of sediment accumulation and changes over long periods of time. As the last few chapters have described, on every scale from the individual grain to the basin, written in these ledgers is the planet’s biography as told by individuals, tribes, and societies of sand grains. The stories are written in an arcane script, and it is the geologist’s task to decipher it.
IDEAS
Stand in front of a pile of sandstone layers in the Karoo, or by a road cut, or next to a beach cliff: how do you read their stories, put together a mental image of their origins? The history of the deciphering of the language of sediments is a detective story and a soap opera of controversy, dogmatism, rivalry, and politics—a typical tale of the development of scientific ideas, with a colorful cast of characters.
Revolutionary ideas have often come from seemingly unlikely places. Nicolaus Steno, born in Copenhagen in 1638 as Niels Stensen, was a geologist for all of three years—in spite of the fact that the name Stensen means “son of rock.” Yet his ideas were far ahead of their time, ideas that would contribute to the birth of modern geology 150 years later. Steno was from a wealthy family and trained in medicine, becoming physician to the grand duke Ferdinand from the Medici family of Tuscany. After dissecting a shark’s head, at the duke’s request, he realized that the teeth were identical to objects found entombed within rocks, at that time referred to as glossopetrae, “tongue stones,” which were thought to have fallen from the sky or grown organically within the rock. Steno took the simple view that the objects looked like sharks’ teeth because they were sharks’ teeth, buried in the sand after death, and that the sand had now become dry land. Other contemporaries felt the same way (notably Robert Hooke in England, who had correctly declared earthquakes a cause of uplift of the land), but Steno took the whole matter several steps further, interpreting the layers of rock within which such fossils were found. In 1669, he published the ponderously titled work De solido intra solidum naturaliter contento dissertationis prodromus (roughly, Preliminary Discourse to a Dissertation on a Solid Body Naturally Contained within a Solid). In it, he described how the layers are formed by the settling of grains from water, that the older layers are over-lain by younger, and that all layers are originally formed horizontally—any deviation from this must reflect later events.
Herein lay the foundations of stratigraphy, the study of sequential sedimentary layers to interpret the story of the time period over which those layers were deposited. The simple notion of superposition, that a sequence of layers represents the passage of time, was radical—but it is fundamental to our reading of the ledgers today. Unfortunately, even before this extraordinary work was published, Steno had converted to Catholicism and found that this conflicted with his science, which he abandoned, becoming a bishop later in life.
The same conflict that turned Steno away from science would inhibit development of his ideas until the end of the eighteenth century. Religious doctrine required a young Earth (born at 9:00 A.M. on October 26, 4004 B.C., according to Archbishop Ussher) and held that all its features could be explained by the multifarious activities of the Great Flood.
Uniformitarianism: the challenge of saying this word belies its meaning, a simple but powerful idea about how to look at the Earth’s processes. Uniformitarianism asserts that the things we can see going on today can account for everything we see of the Earth’s past: how continents and sand grains came to be, the landscapes and deserts of eons ago. It is often summed up simply as “the present is the key to the past.” Without this, any attempt to describe our planet would be nonsense: causes and processes could be explained by any harebrained, unrealistic, unnecessarily complex, mystical, unobserved, and unrepeated mechanisms that might, for whatever reason, appeal. Uniformitarianism works extremely well. The idea was proposed in the eighteenth century by the British geologist James Hutton but was dismissed by those who favored catastrophism, the shaping of the Earth by sudden, cataclysmic events, such as the Great Flood; catastrophism was consistent with traditional ecclesiastical thinking. Hutton, like Steno, was fortunate to be born (in 1726) into a wealthy family, in his case landowners and merchants in Scotland. And, like Steno, he studied medicine; the focus of his interest was, however, chemistry. In 1747, he fathered an illegitimate child and, probably because of this, moved to Paris to continue his studies. He eventually returned to Britain, took up farming and chemistry, and developed an interest in geology—or “natural philosophy,” as it was then called. He read Steno and Hooke and participated in the scientific society of the time, James Watt and Adam Smith being among his friends. His farming led him to observe soils and the underlying rock, and to put together ideas on erosion and deposition, none of which sat well with the doctrine of the time. He saw geological renewal and long cycles of events, all of which required the Earth to have a long history: as he remarked in a 1788 paper presented to the Royal Society of Edinburgh, he saw “no vestige of a beginning—no prospect of an end.” The most fundamental of Hutton’s observations was made during a boat trip with friends, including fellow geologist and mathematician John Playfair, along the coast of Scotland. At Siccar Point, a location that has become the destination of geological pilgrimages ever since, Hutton displayed the astonishing insight that was to change our understanding of our planet forever. Figure 33 shows these remarkable rocks, essentially the same today as when Hutton first saw them.
FIGURE 33. Hutton’s unconformity at Siccar Point. (Photo © Dr. Clifford E. Ford)
At their base are layers that are almost vertical, but these are truncated along a rough surface, above which are essentially horizontal layers of red sandstone. In a groundbreaking leap of the imagination, Hutton read the story of these rocks. The vertical layers are older, had once been horizontal, but had been tilted, uplifted, and eroded. The layers above are younger sediments deposited on the old eroded land surface. The boundary between the two groups of rocks must reflect a long period of time for this to have been accomplished—not thousands, but millions, of years. (This type of boundary is termed an unconformity, a break in the sequence of events recorded in the rocks.) We now know that the horizontal sandstones (of which we shall see more later) are tens of millions of years younger than the vertical layers below.
Hutton concluded that the ledgers could be interpreted by understanding the Earth’s dynamic processes today and applying that understanding to the evidence of its past. The term uniformitarianism would not come into use until later, but the ground was prepared. In his Royal Society of Edinburgh paper, Hutton wrote:
In examining things present, we have data with which to reason with regard to what has been; and, from what has actually been, we have data for concluding with regard to that which is to happen here after. Therefore, upon the supposition that the operations of nature are equable and steady, we find, in natural appearances, a means of concluding a certain portion of time to have necessarily elapsed, in the production of those events of which we see the effects.
Hutton’s reasoning was revolutionary—and highly controversial, even heretical. He was attacked from all sides. His cause was not helped by the fact that his writing style was somewhat odd and opaque, often described as virtually unreadable (the quotation above displays unusual clarity). He was far better at defending his ideas in conversation; in spite of his manner being described as “peculiar,” he was articulate, lively, and enthusiastic. Although his ideas would have to wait until after his death to be fully explained (by his friends, notably John Playfair), his intellect was remarkable, and his contribution to our ability to read the Earth’s ledgers was radical and essential. Among those influenced by reading the unreadable Hutton’s works was Charles Darwin.
During the nineteenth century, profound developments in knowledge and ideas about the Earth came thick and fast, always interwoven with soul searching over, and conflict with, religious doctrine. Those who found their first love to be geology included farmers, doctors, lawyers, and independently wealthy gentlemen; the stages on which they strode were three of the world’s great mountain ranges: the Alps, the hills of Scotland and Wales, and the Appalachians. Among these geologists was Louis Agassiz, born in Switzerland and later the founder of the Museum of Comparative Zoology at Harvard University. One of the founding fathers of American science, Agassiz brought his knowledge of glaciers to the understanding of the ice ages and glacial landscapes and, in spite of being a firm opponent of Darwin’s ideas, he became a major contributor to biology and ancient life studies. Charles Lyell, whom we met in chapter 5 observing the crumbling coasts of England, took Hutton’s and Playfair’s ideas on uniformitarianism many steps forward; only a few days before his death in 1875, he was working on revisions to the twelfth edition of his Principles of Geology. The list of the geologists of this period is long, their bewhiskered, serious faces staring out at us from the geological hall of fame.
Reading the script of sandstones, interpreting the ledgers, is intimately entwined with the entire science of geology. The nineteenth century saw the rules developed—rules that today seem blindingly obvious, but the intellectual insights that led to those rules were, in fact, extraordinary. Building on the work of Lyell and others, two men stand out during those years as founders of sedimentary geology and stratigraphy. Amanz Gressly, born in Switzerland in 1814, spent much of his life working in the Jura Mountains—indeed, today he is celebrated in his native land to a far greater extent than elsewhere. Trained, again, in medicine, he turned to geology at the University of Strasbourg. In the course of his work, he met Agassiz, who encouraged and promoted his ideas. Apart from having a dinosaur named after him (the obscure Gresslyosaurus, based on a few bits and pieces of bone), Gressly’s legacy is one single publication—but it is an insightful and widely influential one. Fundamental to his thinking was the simple idea that if ancient sediments can be read correctly, then we should be able to make paleogeographical maps of the geography of the Earth’s past, showing lagoons, tidal flats, beaches, dunes, and barrier islands, in the same way we make maps of the geography of today. To do so, Gressly argued, we must recognize the characteristics of a sediment (and therefore a sedimentary rock) that indicate the environment in which it was deposited—river versus beach, deep water versus shallow, and so on. The principle of uniformitarianism is vital here—if we observe the shape and structure of a point bar today and see exactly the same character preserved in an ancient sandstone, then the simplest explanation is that the sand was originally deposited in a point bar; if we see ripples on the surface of a sandstone, then their geometry can be compared to ripples today to reveal the environment in which the sandstone was deposited. Gressly approached this in a way similar to that used for the recognition of facial character, defining associations of features with a particular facies, a specific depositional environment. Each facies is distinctive: the characteristics of sediments deposited by a meandering river are different from those of a braided one, and all of them are completely distinct from those of a barrier island. Aiding diagnosis, marine fossils will not be found in a river’s floodplain.
He then reasoned that if you walk today from the landward side of, say, a barrier island, toward the ocean, you will cross a number of different but contemporary environments—lagoon, dune, beach, foreshore, and shallow marine sediments. These same kinds of lateral changes can be recognized by tracking changes in ancient sediments. And, critically, imagine that sea level rises, encroaching across the barrier island. If the sediments are preserved, then shallow marine sediments will be deposited on top of the earlier beach, beach sediments on the earlier dunes, and so on. The facies are stacked vertically in a sequence that is equivalent to the lateral-sequence and record changes in depositional environment over time. These are among the most fundamental rules for interpreting the Earth’s history that continue to be used today. There is, however, something of a mystery as to how Gressly derived these revolutionary ideas; after his single 1838 publication on these principles, he seems to have had nothing more to say about them, devoting the rest of his life, between bouts of mental and physical illness, to collecting and describing fossils.
Henry Clifton Sorby was an entirely different character, renowned as the greatest scientist that the steel town of Sheffield, England, has produced, and acknowledged as the father of sedimentology. Sorby was twelve when Gressly published his famous paper, and Ralph Bagnold was twelve when Sorby died. Sorby thus bridged nineteenth- and twentieth-century geology. His father owned a tool-manufacturing business that provided Sorby with financial independence. He was determined from the start to be a scientist, but since there were no appropriate courses, he never attended university. Instead, he simply set up a scientific workshop and laboratory in his home. Sorby, in the spirit of Antony van Leeuwenhoek, was fascinated by the potential of the microscope and pioneered a method of grinding slices of rock so thinly that light could be shone through them and they could be examined microscopically (as in Figure 32). He understood, despite the derision of some of his fellows, that much could be learned about the large scale from careful study of the extremely small. In 1858, writing on the microscopic structure of crystals for the Geological Society of London, he remarked: “In those early days people laughed at me. They quoted Saussure [a giant of Alpine geology] who had said that it was not a proper thing to examine mountains with microscopes, and ridiculed my action in every way. Most luckily, I took no notice of them.”
Sorby’s work fascinated John Ruskin, the widely influential art and social critic, painter, and philosopher on the natural world. However, their friendship did nothing to alleviate Ruskin’s agonizing over the aesthetic relationship between science and art (not to mention the century’s ongoing conflict between science and religion). He was fascinated with the details of nature that Sorby revealed and at the same time dismissive. In a letter to his friend, the Pre-Raphaelite artist William Holman Hunt, he declared: “There’s nothing makes me more furious than people’s looking through microscopes instead of the eyes God gave them.”
Sorby applied his microscopic innovations to metallurgy and biology, but he is best known for his interpretation of the features and relationships of mineral grains and what they tell us about sedimentary processes and ancient rocks and landscapes. He also initiated the quantitative analysis of sedimentary structures, recording the relationships between sand ripples, cross-bedding, current velocity, and grain sizes. In 1908, in the introduction to the summary of his life’s work, “On the Application of Quantitative Methods to the Study of the Structure and History of Rocks,” he wrote: “My object is to apply experimental physics to the study of rocks.” The young Ralph Bagnold was ready to take Sorby’s objective further. And the early years of the twentieth century saw rapidly developing and revolutionary ideas: Johan August Udden’s obsession with sand (chapter 1) was bearing fruit; Alfred Wegener, a young Austrian meteorologist, was making his first expedition to Greenland and formulating his ideas on continental drift; the newly established U.S. Geological Survey was interpreting the libraries of the Earth’s history revealed by the landscapes of the American West; and the age of the Earth was finally being established.
VESTIGES
James Hutton’s inability to perceive any vestige of the Earth’s beginning set in motion a century of anguished and often vitriolic debate. In 1851, in the thirty-sixth volume of his Works, Ruskin summed it up: “If only the Geologists would let me alone, I could do very well, but those dreadful Hammers! I hear the clink of them at the end of every cadence of the Bible verses.” Charles Lyell did not place any limits on the age of the Earth, but by the end of the nineteenth century, that view would be refined. Charles Darwin, somewhat rashly but in the interests of making the point, reasoned that 300 million years would have been required to create the amount of erosion he observed in southern England. Darwin’s estimate set off a new round of debate, with scientists queuing up to disagree with him on this (and other issues). The most influential and formidable of these was the great Scottish physicist William Thomson, perhaps better known as Lord Kelvin. Kelvin’s reasoning was based on thermal conductivity: his view was that the Earth has continuously lost its initial heat and that to cool from its primordial molten state to its condition in modern times would have taken between 20 and 400 million years. He settled on an estimate of 100 million. Even though his assumptions—and many of his statements—could be best described as sweeping, Kelvin’s reputation as a brilliant physicist carried a great deal of weight. Darwin was shaken by the criticism: he withdrew his estimate and, as recounted in Tony Hallam’s review of the controversy, wrote to Lyell to warn him, “For heaven’s sake take care of your fingers: to burn them, severely, as I have done, is very unpleasant.”
In spite of well-founded geological reasoning that threw doubt on his figures, by 1897 Kelvin had cantankerously reduced his estimate to twenty-four million years. But Henri Becquerel’s discovery in 1896 of radioactivity in uranium, and subsequent demonstrations by Pierre and Marie Curie and Ernest Rutherford that radiation produces heat (thus counteracting cooling of the Earth), put an end to the debate. In 1904, Rutherford was to give a lecture at the Royal Institution in London, part of which would deal with the implications of his work for estimating the age of the Earth. When he walked into the lecture theater, he saw to his dismay that Kelvin was there. As Hallam relates, he later wrote:
To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me! Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the earth, provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium! Behold, the old boy beamed upon me.
It was now clear that the age of Earth should be measured in at least hundreds of millions of years. But exactly how old is our planet, and how can its age be measured? Rutherford set the direction by noting that the steady decay of radioactive elements could provide a clock. Early attempts to use that clock suggested that billions rather than millions of years would be the appropriate scale, but the methodology needed refinement. Committees for the British Association for the Advancement of Science and for the U.S. National Academy of Sciences in the 1920s and 1930s devoted a great deal of effort to this, and today, as we saw in chapter 1, the oldest direct evidence from Earth (from sand grains) points at an age of at least 4.4 billion years, while evidence from meteorites suggests an age of around 4.6 billion.
So, between the covers of our planet’s ledger we have over four billion years’ worth of records to interpret. How detailed and consistent are these records? Is the ledger complete? The answer to the second question is a resounding “no, nowhere near.” Look again at Figure 33: Hutton’s famous unconformity marks the position of tens of millions of years of missing records. But, of course, these missing records may have been preserved elsewhere; if we follow that break in the rocks far enough, perhaps some new layer of sand—a missing entry—will appear. Follow it further and what had been the break may now be represented by a huge thickness of layers—an entire missing account. But this reconstruction has to be painstakingly put together following the rules of our geological forebears. The process is like assembling a jigsaw puzzle, but given that a stack of sedimentary layers is the story of a period of geologic time, it is a four-dimensional puzzle.
FLOODS
Although the expanse of geologic time had not yet been fully defined before his death at the age of fifty in 1919, Joseph Barrell, a geologist at Yale University, published some remarkable ideas about reading the layers of the Earth’s history. Barrell recognized the importance of knowing how much time is not represented in an apparently continuous pile of sedimentary layers, how many entries and records are missing from the ledger. Barrell, again oddly not considered among the conventional pantheon of geological heroes, spent time observing Appalachian rivers ancient and modern (he developed the explanation for the Susquehanna water gaps described in chapter 4) and recognized the key role of base level. Major periods of sediment deposition occur when sea level is rising, not falling, and local variations in base level will have the same effect. He described rhythmic and cyclical changes in climate and other environmental factors that determine whether sediment will be deposited or not in any particular setting—and proposed that by far the majority of the ledger is missing. Even in an apparently continuous stack of sedimentary layers, all perfectly parallel to one another, there are multitudes of what archaeologists refer to as lacunae; Barrell referred to them as diastems. Despite these gaps, Barrell used his own work and the results of early attempts at radioactive dating to quantify geologic time. He proposed that the great explosion of life on Earth, carefully documented in the rocks of the Cambrian period in Europe and the United States, occurred between 550 and 700 million years ago. Today’s consensus is 590 million years.
Barrell’s insights and research also laid to rest (more or less) some of the problems with uniformitarianism. Hutton’s original ideas had been variously translated, interpreted, and corrupted into two broad schools of thought: first, a literal interpretation that natural processes have always proceeded in the same way and at the same rates as we see them operating today, and, second, that while the laws of nature have always remained the same and worked in a consistent manner, the intensity with which they have operated has varied considerably over time. The latter view accepts that, on the grand scale, there have been periods in our planet’s history when, for example, volcanic activity has been more widespread than today, and the temperature of the atmosphere has been hotter or cooler; it also accepts that rates of erosion would have been different before plants colonized the land. As we have seen, human activities change the rates of sediment delivery to the oceans (the period of human influence on the planet has variously been referred to as the psychozoic, the anthropocene, and, more facetiously, the mental). On a small scale, every sedimentary layer represents a unique event, separated by Barrell’s diastems, or periods of inactivity. Some sedimentary events record the periodic occurrence of great floods—but not the great flood.
The residents of New Orleans will vouch for the import of sudden events. Just as rivers burst their banks during times of flood, carrying vast volumes of sand out onto the floodplain, so did the London Avenue Canal during the storm surge of Hurricane Katrina on August 29, 2005. Flushing sediment out of Lake Pontchartrain, the surge burst the levee, and as it subsided, it left deposits of sand up to 1.8 meters (6 ft) deep in the city’s streets and backyards; cars were buried in sand. Following in the footsteps of Sorby and Bagnold, geologists used the structure of the sand deposits—cross-bedding and so on—to reconstruct the direction of the current flow, its magnitude, and how porches, cars, and kitchen designs influenced deposition and erosion. This was not a depositional event consistent with a passive uniformitarianism; it was a catastrophic event. After all, deposition of sand occurs because the energy in a current, be it water or wind, is high enough to transport the sand, and as the energy level drops, so does the sand—but it’s the high energy that starts the whole process.
The flood deposits of Hurricane Katrina are, understandably, not preserved, but others are. Today, there is gathering interest in what is lightheartedly referred to as paleotempestology—reconstructing the records of old storms and hurricanes by taking cores of the sedimentary layers in key environments around the coasts, particularly around the Gulf of Mexico and the U.S. eastern seaboard, and recording and dating the telltale layers of sand. While debates over hurricane frequency and climate change continue, these studies contribute real data over a period of thousands of years. Following Barrell’s rhythms, there seem to be different cycles of hurricane activity, some over a scale of tens of years, some over much longer periods—some evidence suggests that catastrophic hurricanes pounded the Gulf Coast more frequently between 1,000 and 3,500 years ago than they do today.
Understanding the rhythms of destructive natural events is key to planning how to deal with them, whether floods, storms, hurricanes—or tsunamis. If you look at the “before” and “after” satellite images of the coasts devastated by the Indian Ocean tsunamis of December 2004, you will notice that beaches have often disappeared. Where has the sand gone? Inland. In the aftermath of the tsunami, vast areas of low-lying land along the shore were swathed in layers of sand, each layer scoured out from the seabed and the beaches and carried far inland by successive tsunami waves. The sand layers often show graded bedding, the coarsest grains settling out first as the pulse of water subsided. Tsunamis (literally “harbor waves” in Japanese, reflecting their destructive effects on ports) are very different from the normal ocean waves discussed in chapter 5. For a tsunami, the wave is not just the surface form of the water, with the water being less and less disturbed with depth; a tsunami moves the entire volume of water. Whether it’s caused by the sudden faulting and shifting of the ocean floor, a giant submarine landslide, or a volcanic eruption, the ocean is displaced, like a bathtub full of water that has been tilted. And, in shallow coastal waters, there’s nowhere for the water mass to go but upward, building into a towering wall that surges landward rather than breaking like a conventional wave. A tsunami has the power to carry and dump huge volumes of sediment far inland, covering tidal flats, fields, marshes, floodplains—a continuous sheet of sand where such a thing should not be, but sand that through its composition, thickness, sorting, grading, and other characteristics gives us vital information about how tsunamis work—and how, in the future, people can avoid the areas that they threaten.
Go to the flat lands around the mouth of the Salmon River on the central Oregon coast and dig a trench. Not far below the surface, after digging through the soils and muds typical of the place, you will come across a layer of sand. It may be only a few finger widths thick, but it clearly differs from the dark soils above and below it. If you look at it with a magnifying glass, it looks like beach sand and contains the shells of microscopic marine organisms. It’s clearly out of place, not part of the normal sequence of events—it’s a tsunami sand. In places, tsunami sand has been found directly on top of the remains of fire pits dug by the native inhabitants—perhaps abandoned in panic as the water surged toward them. Local stories and legends, like that of the raven with which this book began, talk intimately of the sea; but some are violent, recounting walls of water that rise up, flooding the land, destroying villages. The Pacific Northwest has a history of earthquakes and tsunamis because of its address—directly above the Cascadia subduction zone, where the Pacific crust is being forced down beneath North America (hence the volcanoes). After the earthquake and tsunami recorded around the Salmon River, the ground sank and the sand was quickly buried and preserved. This particular event has been recognized from British Columbia to Northern California and has been dated (in part thanks to evidence like the fire pits) to a period of twenty years around A.D. 1710. No historical records are available from North America, of course, but there are detailed accounts from Japan of a major tsunami on January 27,1700. All the evidence, starting with the layer of sand, points to a Cascadian earthquake during the night of January 26, potentially as big as the Sumatran one of 2004. The resulting tsunami spread out across the Pacific, hitting Japan the following day. This was not the only tsunami to strike the Pacific Northwest in historical times—numerous other out-of-place layers of sand are building up the picture of frequent events over the past few thousand years.
While we associate tsunamis primarily with earthquakes beneath the sea floor, anything that displaces ocean water can cause one—sometimes in unlikely places. Along the east coast of Scotland is found a layer of sand that doesn’t belong where it is, sandwiched within coastal peat. It contains the remains of marine organisms and has been dated to around 6000 B.C. It looks like a tsunami sand—but how could a tsunami have originated in the North Sea, which is far from any significant earthquake territory? The story came together when Norwegian geologists identified and dated a series of gigantic submarine rockslides on the floor of the Norwegian Sea. Around 6000 B.C., sections of the Norwegian continental shelf and slope failed catastrophically, sending debris hurtling into the deep water; when it settled, the mass of sediment covered an area the size of Scotland. Known as the Storegga slides, they were very likely the cause of the tsunami. Thus a thin layer of sand is detailed testament to a major natural event that occurred thousands of years ago, terrifying, no doubt, our Scottish ancestors, who were still dealing with the rising sea level following the retreat of the glaciers. The presence of wild cherries and the growth pattern of fish bones buried in the sand suggest that the tsunami struck in the autumn.
As we peer further back into the Earth’s past, archaeology turns into geology. But there is a long period of time over which the two disciplines overlap, and today the light that can be shed on our own history and mythology by detailed geological investigation is being increasingly demonstrated—often by sand and its associates. Helen of Troy, whose face launched a thousand ships, would have gazed out at a very different scene—today, the ancient city of Troy lies far from the sea. But geological analysis of the sands and silts that clogged up the harbor and the estuary allows reconstruction of a geography consistent with Homer’s accounts. Elsewhere in antiquity, the underwater sandbar that later grew to connect Tyre with the mainland probably enabled Alexander the Great to conquer the city. The development and decline of ports and settlements around the Mediterranean and elsewhere tell similar stories: the ill-fated harbor at Aigues-Mortes (chapter 5), Roman dams in North Africa that attempted to stop sand from destroying agricultural areas, and even, possibly, Atlantis.
Plato’s accounts (hardly firsthand) put Atlantis in front of the straits called the Pillars of Heracles—an ancient name for the Straits of Gibraltar—and describe the island’s destruction as coming from the Atlantic in the form of violent earthquakes and floods around twelve thousand years ago. Today, the Straits of Gibraltar mark one of the places where the tectonic struggle between Africa and Europe that formed the Alps continues. In 1755, Lisbon was all but destroyed by one of the largest earthquakes ever (it was felt as far away as Finland) and its aftermath. Tsunami sands around the Iberian coast record these events, and deep in the Gulf of Cádiz is a succession of turbidite deposits, each one bearing witness, as off the Grand Banks, to a major earthquake. These events have happened, on average, every couple of thousand years, and the largest of these deposits, potentially triggered by a very large earthquake, has been dated to around twelve thousand years ago. West of the Straits of Gibraltar is a large submarine bank, today 50 meters (160 ft) below the surface. Twelve thousand years ago, as sea level was rising after the ice age, the bank would have comprised a number of small islands. They wouldn’t have been far enough above the surface to have been habitable; however, if the movements of regular great earthquakes caused subsidence, as they often do, could there once have been a larger area of land, an island of the size Plato describes—Atlantis?
EPISODES
The volume of sand and sandstone in and on the Earth’s crust is enough to build the Great Wall of China around the equator fifty million times. And if every sand grain has a story to tell, that’s a lot of stories—libraries of ledgers. For the remainder of this chapter we shall read, successively further into “deep time,” a sampling of those stories, vignettes, of dramatic and workaday episodes of our planet’s past to which sand bears witness.
The Mother of All Tsunamis
The dramas of modern earthquakes, tsunamis, and volcanic eruptions cannot compare to what happened on a uniquely bad day sixty-five million years ago. An extraterrestrial projectile around 10 kilometers (6 mi) across (although there are alarmingly smaller estimates), traveling at perhaps twenty times the speed of sound, slammed into what is now the Gulf of Mexico. As catastrophes go, this one is virtually indescribable. In the blast of the impact, molten rock, dust, and noxious chemicals were flung into the atmosphere, where they remained for hundreds of thousands of years. The hole excavated instantaneously by the impact is more than 180 kilometers (110 mi) wide; this crater is now buried beneath younger sediments of the Gulf and the Yucatán Peninsula, where the town of Chicxulub has, unpronounceably, given its name to the feature. The catastrophe is notorious for causing the extinction of the dinosaurs—together with upward of three out of every four species on the planet. There are, as usual, debates about this. The extinction was not instantaneous, but rather a slow death over several hundred thousand years, and odd creatures—crocodiles, for example—survived. There is even some evidence that one or two small dinosaurs made it. But given that the atmosphere was poisoned, the light of the sun shut out, the land covered in fires, the rain acid, and the air choked with dust, it is hardly surprising that these were difficult and complex times. The details are still being worked out, particularly with the aid of crater data gathered through drill holes, and one of the challenges is that the immediate area of the impact retains very little evidence of the event itself, since the impact effectively removed its own evidence, creating a gap, a diastem, lasting hundreds of thousands of years.
Away from the crater, however, there is ample and dramatic evidence—as well as numerous unconformities. The impact created the mother of all tsunamis, as did the water rushing back into the hole, and some extraordinary deposits of sand resulted—extraordinary simply because of the material and the velocity at which the currents were moving. Researchers from Japan, well versed in the art of modeling tsunamis, have made some rough calculations of the waves, suggesting heights of 200 meters (650 ft). These horrifically colossal waves hurtled great distances, leaving death, destruction, and deposition in their wake. Evidence of both the depositional and erosional effects of these waves is found all around the Gulf of Mexico and the Caribbean, across the Mississippi Valley, along the Atlantic coast of the United States, and possibly at locations in South America and Europe. The composition and structure of sand layers around the Gulf Coast record reversals of current direction as the waves came and went; the grain size variations tell the story of velocities and frequencies, and unusual cross-bedding tells of extremely high and rapid flow. Gravity slides and turbidites record the collapse of submarine slopes. And most of the sands contain strange glassy grains known as spherules or microtektites. The impact not only melted rocks, it vaporized them, ejecting a fireball of material into the atmosphere. There, the vapor (and the molten material) condensed and fell back to Earth as a rain of minute glass spherules. These are now found as grains of sand, but grains with a very unusual chemical and mineral composition; analysis of their chemistry indicates that they were formed at temperatures of more than 1,500°C (2,600°F).
The sand deposits and grains tell extraordinary stories of a (fortunately) extraordinary event. However, as in all good detective stories, the evidence is sometimes conflicting: layers of sand contain spherules in the wrong part of the sequence, and there are other possible origins for the sand deposits. If all the evidence were straightforward, the story would be less exciting.
Dunes, Dilatancy, and Dinosaurs
Even allowing for what awaited them sixty-five million years ago, the dinosaurs had a good run, and they live on dramatically in the popular imagination. But for a long time, they were known only from fragments of skeletons, bits and pieces (like Gresslyosaurus), often imaginatively put back together. Then, in the 1920s, expeditions from the American Museum of Natural History discovered a treasure trove in the Gobi Desert, a beautifully preserved abundance of what the ancient Chinese called “dragon bones” even nests with eggs were buried, essentially intact, in the sand. Today, the museum and the Mongolian Academy of Sciences, together with other research organizations, cooperate to reveal this treasure. In particular, the site at Ukhaa Tolgod, the Brown Hills, is a gold mine of paleontological riches. Ten million years before the catastrophe on the other side of the world, dozens of species of mammals and reptiles enjoyed a good place to live, and, from a paleontologist’s point of view, it was also a good place for them to die, for their remains are exquisitely preserved. Tiny mammals and dinosaurs sitting on their eggs have been painstakingly removed from the sand. But what kind of sand, and how could it achieve this extraordinary preservation? The creatures, like the Dikika baby in Africa, must have been rapidly buried, allowing no time for predators to dismember the bodies. On the face of it, the rich red sandstones in which the fossils are entombed bear all the hallmarks of desert dunes, ancestors of the Gobi sand seas of today. But today (with the possible exception of Cambyses and his army—see chapter 6), creatures are not suddenly buried by sand dunes—they move too slowly and predictably, and even dinosaurs couldn’t have been stupid enough to simply stand around, waiting to be buried by a dune. Besides, how could dinosaurs have thrived if the place was like the depths of the Sahara today?
A clue came from the type of sand in which all the fossils were found. Associated with the fossil-rich layers were great thicknesses of sand that showed the characteristic cross-bedding of dunes (and, occasionally, dinosaur footprints), but the sand that entombed the dinosaurs had no such features—it was, in fact, featureless, a simple, structureless mass. Layers of mud between the dunes attested to the very different climate of the time, warmer and wetter, not unlike that of the Sand Hills of Nebraska today, where periodic torrential rains saturate the dunes and cause massive flows of waterlogged sand, which have been known to fill up buildings located in the shelter of a dune. In Mongolia, sandstones of the same age as at Ukhaa Tolgod show the remains of burrows made by creatures, like those today, escaping the heat of the day, but there are signs that they had to excavate new burrows as the old ones were plugged up with mud from the rains. At Ukhaa Tolgod, layers of sand were found that were cemented by a kind of calcium carbonate found in arid environments and known as caliche. This would have effectively blocked water from draining away, out of the dunes. Combine these pieces of evidence and perhaps the mystery is solved. Cloudbursts were probably more frequent at Ukhaa Tolgod then than they are in Nebraska today, and the dunes were significantly bigger. Saturate a towering dune face with water, prevent the water from draining away, destabilize the sand through the effects of dilatancy, and the slightest tremor, perhaps the wind, perhaps an irritated dinosaur, would cause an instantaneous slide of huge volumes of sand slurry—burying the irritated dinosaur. Here are desert processes, diagenesis, the physics of granular materials seventy-five million years ago, and forensics at work.
Breaking Up
The dinosaurs of the Gobi had cousins all over the world. In Philadelphia in 1787, Benjamin Franklin was presiding over a meeting of the American Philosophical Society. George Washington was also at the meeting, and together they examined a large, heavy bone presented by Caspar Wistar, a local physician, who had dug it up in New Jersey. It was identified as the thigh bone of an unusually large human. In 1802, the young and gloriously named Pliny Moody, working on his father’s farm in Massachusetts, plowed up a slab of red sandstone on which was imprinted a series of large, three-toed footprints. They were decreed by religious authorities to have been made by “Noah’s raven.” Had either of these finds been correctly identified, they would have qualified as the earliest recognized dinosaur fossils, but that honor went to a discovery by the Reverend William Buckland in England some decades afterward. Although the term dinosaur would not be coined until 1842, Buckland, who consorted with Lyell and Agassiz and trod thoughtfully the difficult path between the Bible and the new geology, described to the London Geological Society in 1824 his finding near Oxford of the “Megalosaurus or Great Fossil Lizard of Stonesfield.”
Dinosaurs were clearly as at home in New England as in Mongolia, and the eastern United States continues to yield a trove of bones and footprints, not of the very first dinosaurs (that distinction belongs to South America), but of some of the earliest. The red sandstones in which Moody discovered the footprints are found up and down the northeastern United States, and they record a major episode in the Earth’s ledger: the fragmentation of a continent. The dinosaurs first appeared during the chapter of geological history called the Triassic, a chapter that lasted from around 250 million to 200 million years ago and whose beginning followed, as we shall see, a dramatic ending. By the beginning of the Triassic, most of the planet’s continents had been welded together into one vast landmass, Pangea—or Pangaea, depending on which part of the now fractured supercontinent you have ended up on (Figure 34).
FIGURE 34. Pangea assembled, 260 million years ago (left), and fragmenting, 160 million years ago (right). The darkest areas show ancient oceans, and the light areas between the shapes of today’s continents show their true structural extent, the continental shelves. (Images generated by Cambridge Paleomap Services Limited, modified by author)
Two hundred and sixty million years ago, the northeastern United States lay just south of the equator, the United Kingdom and much of Europe just north. As Pangea drifted slowly northward, the western United States dwelt for a while in the same latitude as today’s Sahara: go to Zion National Park and look at the Navajo Sandstone from this time, with its gigantic sweeping cross-bedding, and you will see evidence of ancient sand dunes. The architecture of the cross-bedding helps us reconstruct continental movement through different wind systems at different latitudes: although researchers at the University of Nebraska (who also work on the Gobi dinosaurs and the Sand Hills) have demonstrated that the story, as is so often the case, is not simple, these ancient dunes have tales to tell us about the wandering supercontinent.
This arrangement of the Earth’s landmass was not to last, however. Convection and churning of the Earth’s molten interior would begin to break up the super-continent; as if the brittle crust on the top of a crème brulée had been shattered, fragmentation began. It was not an overnight process—it took close to 100 million years for what we now know as parts of the Atlantic Ocean to form, new oceanic crust solidifying from the molten upwelling between the fragments. The process continues today.
Pangea broke up slowly and far from simply, not along a single crack, but through a mosaic of intersecting fracture systems, some of which would continue to open, others of which would stall. The network propagated over time, like a system of gigantic zippers, Africa pulling away from North America first, Europe from Greenland, and southern Africa from South America much later. The process began with faults, fractures in the Earth’s crust allowing the crust to pull slowly apart. The result was a series of rift valleys, bordered by faults whose movement successively dropped the valley floors—think of the topography of the Great Basin or the Rio Grande today, or East Africa, the Red Sea, and the Gulf of Suez. Rift valleys were where our human history began—Lucy and the Dikika baby were both found in the great East Africa Rift system. These are among the most dynamic of tectonic and sedimentary environments, with towering valley sides along the faults and broad valley floors; movement on the faults constantly shifts the local base levels, and erosion of the bordering highlands constantly pours sediment into the valleys. It was across this kind of landscape that Moody’s dinosaur strode.
These great Triassic rift valleys spread not only along the northeastern edge of North America, but across the United Kingdom and into Europe, and northward past Greenland (Figure 35). Although the continents were drifting northward over this period, for much of the time the rift valleys were forming under equatorial and desert climates. Tropical weathering, with iron minerals coating the grains and being concentrated during later diagenesis, resulted in almost all the sediments deposited in the rift valleys being red. For a long time, this group of rocks was known as the New Red Sandstone (and yes, there is an Old Red Sandstone, but more of that later). I grew up in Nottingham, in the English Midlands, to which visitors flock in search of the vestiges of Robin Hood. What they find is a (relatively modern) castle built on a craggy outcrop of red sandstone of the Triassic period. The sandstone is easily excavated, and Nottingham’s foundations are riddled with an entire world of tunnels and rooms: a tannery, a brewery, homes, and storage spaces. The oldest pub in England was partly excavated into the red rocks, and countless wells produced freshwater from the sandstone. Look at the walls of these excavations and you will see pebble beds, cross-bedding, muddy layers, and other diagnostic features that tell a story of lake beds, flash floods, and river sandbars.
FIGURE 35. The rifting pattern that led to the breakup of Pangea. (Image generated by Cambridge Paleomap Services Limited, modified by author)
In the countryside around Newark, New Jersey, and Amherst, Massachusetts, you will find the same features. Travel around Nevada or the Gulf of Suez today and you will see the modern setting for these deposits: steep valley walls, along which cascades of erosional debris build up great fans of sediment; rivers, dried up for much of the year but rushing torrents after rain; lakes that dry up to leave pale surfaces of salt and cracked mud; sand dunes; and, occasionally, the sea encroaching as the land level shifts. The terrain is shaken by earthquakes, which change the elevation of the valley floors, divert drainage, and empty or fill lakes—constantly shifting base level. Rift valleys are complex environments and the sediment record extends deep below the surface; Triassic rifts can contain 2,500 meters (8,000 ft) of sediments successively deposited as the valley floor foundered. Detailed work on these sediments on every scale—the reconstruction of sedimentary environments and input over space and time, and the directions of sediment transport—sheds light on the movement of faults and the development of the rifts. It tells the story of the breakup of a megacontinent.
These rocks have also had their say in modern history. Triassic sandstones built New York’s brownstones. And the rifting of the crust allowed molten rock from below to find its way to, or close to, the surface—volcanic rocks are often associated with the red sediments. One such rift valley ran through Gettysburg, Pennsylvania, and the resulting topography played a critical role in the Civil War battle. The volcanic rocks, hard and durable, formed the higher ground occupied by the Union forces: Cemetery Ridge, Culp’s Hill, Little and Big Round Tops. The Confederate troops moved through the lower ground, underlain by the more easily eroded sediments. The volcanic outcrops and boulders provided command of the battlefield and some cover but prevented digging in—the Union casualties were high. Nonetheless, the Triassic rocks, which mark the breakup of a continent, helped prevent the breakup of a nation.
The Mother of All Extinctions
As noted earlier, the ledger’s entries predating the Triassic ended with a cataclysm. The final entry was the Permian, a climatically difficult period for life—but life was nevertheless thriving, both in sea and on land. Then, in a series of pulses of mortality or in one geologically sudden event (the evidence and schools of thought vary), 90 percent of marine species and 70 percent of land life were obliterated. Whole families and genera of creatures disappeared forever.
As an undergraduate student over forty years ago, I stood on an outcrop in the Arctic, one foot on the Permian, one on the Triassic. I admit that I did not fully appreciate the drama represented between my feet—perhaps because much of the science of mass extinctions had not yet been done. Beneath my Permian foot was ample evidence of healthy marine life—large corals and other fossils—beneath my Triassic foot, nothing but lifeless sandstone and shale. Corals were one of the marine groups hit hard at the end of the Permian, and those beneath my feet were never seen again.
The turbidite sandstones of the Karoo (Figure 31) are Permian in age. Toward the top of this sequence of sediments, the character of the marine environment disappears, reflecting a drop in sea level, and the sediments become typical of those deposited in rivers. Careful study of these river systems sheds important light on what was going on at the end of the Permian. Immediately beneath the Permian-Triassic boundary are sandstones and shales with all the characteristics of channels and floodplains of a gently meandering river system (as in Figure 21). But above, the sandstones are thicker, coarser-grained, and characterized by very different internal structures: the rivers are now braided (as in Figure 22). Braided rivers today form when the sediment supply is abundant, clogging up channels, building up channel sandbars, forcing the river to change course and break up into multiple channels. The extinction had demolished much of the plant life on the land—trees, bushes, and grasses—leaving the forces of erosion free to sweep across the exposed surface, generating a huge increase in the supply of sediment to the river systems. A similar change was seen in local river patterns after the eruption of Mount St. Helens.
The “smoking gun” for this devastating extinction is nowhere near as clear as for the event that wiped out the dinosaurs. There is chemical evidence of major climatic change, dramatic lowering of ocean oxygen levels, acid rain, and increase in atmospheric carbon dioxide. In Siberia, one of the world-record series of volcanic eruptions was going on around this time, and an extraterrestrial impact has been identified in Australia that may or may not prove to be the culprit. But here again, some fascinating details are being provided by microscopic forensics, this time from oil. Oil is formed by the degradation of organic material—microbes, plants, and animals—and these donate bits and pieces of their molecular identities to the oil. Even if it is only found as minute droplets preserved in microscopic cracks in sand grains, the oil reveals those molecular identities (known as bio-markers), which often tell us far more about the living world of the past than do conventional fossils. This kind of biochemical forensic work has suggested a cause for the great extinction at the end of the Permian that has nothing to do with an impact, but everything to do with a catastrophic change to the biological balance of the planet. The Permian oceans seem to have teemed with life, and the bio-markers indicate waters with an ample supply of oxygen. This benign environment was then abruptly terminated: oxygen levels plummeted, and the oceans were taken over by bacteria that can’t tolerate oxygen but thrive on hydrogen sulfide, the poisonous gas with the smell of rotten eggs. The oceans had become stagnant and toxic, breathing their poisons into the atmosphere with lethal consequences. Did the Siberian eruptions create a runaway greenhouse world with temperatures rising so rapidly that the atmosphere could no longer oxygenate the oceans? And were many of the complex creatures that had evolved defeated, wiped out by primitive microbes? The forensics are still inconclusive, but the identities of the victims are well established.
Mountains Appearing and Disappearing
Where did the monumental volumes of sand that swept across the devastated post-Permian Earth and filled the rift basins of the Triassic come from? They came from the erosion of mountains—where most sand originates. The supercontinent of Pangea had been welded together by the closing of an earlier great ocean that for hundreds of millions of years had been the predecessor of the modern Atlantic. Like the western Pacific today, this ocean’s story involved a complicated sequence of moving plates, large and small, volcanic island chains migrating as small ocean basins behind them were consumed by subduction. Fragments of continents and volcanic islands collided with one another and with the major continents over time, each of these collisions forming a mountain belt along the edges of the ocean. The Appalachians were the result of some of these crustal fender benders and pileups. And the Appalachians are part of a continuous chain of mountains from the Ouachitas in the American South through Newfoundland, Ireland, Wales, Scotland, and Norway, and reaching north into Greenland and Spitsbergen (where I stood astride the Permian extinction). This mountain range, collectively known as the Caledonides, after the Roman name for Scotland, was a bustling laboratory for the early days of geology, the source of great discoveries and great controversies, and continues to be today.
The Appalachians were constructed by three major pileups, 500, 400, and 300 million years ago, the last event completing the welding of Pangea. Each one crumpled the edge of the continent and bulldozed indiscriminately a miscellany of geological material up onto that edge—oceanic crust, volcanic islands, bits of other continents, older sediments. All of this, as newly upthrust mountains, began to crumble and erode. Indeed, some of the erosion was happening while the bulldozing continued, and piles of sedimentary detritus were cannibalized into the growing heap. The stories of mountain building, orogeny, whether it be in the Caledonides, the Alps, the Urals, or the Himalayas, are about the most complicated of any of the planet’s history. How to unravel them, when most of the evidence has been removed? The Appalachians today are, after all, mere stumps, worn-down remnants of their former glory. Erosion may proceed slowly, but the Appalachians today are being worn away overall at an average rate of 3 millimeters per century—that’s 30 meters (100 ft) every million years. On a geological scale, that’s a lot of rock removed. (Interestingly, the valleys may be eroding faster than the summits, making the Appalachians increasingly rugged.) Young mountain ranges, where the variation in elevation is greater, show rates of erosion perhaps ten times that. It’s in all that sediment, much of which is sand, that the opportunity lies for us to reconstruct the story of the mountain range, to put back together the sequential stripping away of the pile of bulldozed rock, rather like visualizing a tree from the sawdust around its stump.
This is where provenance really enters the game. Like a family genealogy or DNA forensics, the makeup of a family of sand grains preserved in a sandstone tells the story of its provenance, or where the family came from. Just as sand grains can be made of many things, so too can sandstones, but by far the majority of sandstones are made up of quartz plus a few other key ingredients. For a long time, sands and sandstones have been given names, or classified, on the basis of three key components: quartz, feldspar, and lithic (rock fragment) grains. Some sandstones that have endured multiple cycles through the mill of weathering and erosion have lost all components except durable quartz. Other, less mature, sandstones still retain grains of feldspar and rock fragments, which, in the longer term, are less stable and will disintegrate with time. Take a large sample of sandstones from the same sequence of beds; make, as Sorby did, very thin slices of them; peer down a microscope and count the grains, thousands of them, of each type; and finally plot the proportions of these types on a triangular diagram, with each of the three major components occupying an apex. (This sort of diagram is known as a QFL—quartz, feldspar, lithic—plot.) Different sandstones will cluster in different areas of the triangle, and different areas of the triangle are given different names—thus you will have classified your sandstone. You might have identified it as a sublitharenite or a lithic subarkose, arcane names that will not detain us here but are loved by sedimentologists. What is important is the principle of separating out different kinds of sandstone according to their families of grains, which in turn reflect their family tree.
As the theory of plate tectonics took hold, its power to link a particular tectonic setting (such as a volcanic island chain above a subduction zone, a rift basin, or the passive margin of a continent moving away from a spreading ocean ridge) with the associated sediments became clear. In the early 1970s, a geologist at Stanford University, Bill Dickinson, took this idea, did the work, and set out the rules. He took the traditional QFL diagram and defined areas within it that circumscribed sandstones derived from different plate-tectonic settings. The broad groupings were sand originating in old stable continents; sand recycled out of mountain belts; and sand deriving from volcanic islands. Over the years, the scheme has been refined and new approaches of mineralogy and chemistry brought into the equation—the science of provenance has been honed into a fine art. Dickinson went on to become one of the tectonic and sedimentological heroes of his generation. He recently published a lengthy and exquisitely detailed study of the provenance of sand grains included in prehistoric pottery (temper sands) of the Pacific Islands. Dickinson’s intimate knowledge of the geology of the Pacific and his meticulous methodology documented patterns of migration, settlement, and commerce that had hitherto been impossible to define—all through the study of sand grains.
But back to the Appalachians. Provenance studies, combined with measurement and mapping of sandstone sequences, facies, and structures, make it possible to interpret the story of the sequential erosional stripping of a newly formed mountain range and the sedimentary environments in which the sediments were deposited. The three major episodes of the formation of the Appalachians each resulted in the accumulation of vast thicknesses of sediment on the flanks of the uplift—the very weight of the bulldozed pile depresses the crust and a deep basin forms in front of the advancing pile, a ready receptacle for the cascades of sediment. One of these sediment piles in particular qualifies as a famous sandstone, a well-studied section of the ledger: the Old Red.
The Old Red Sandstone—named to distinguish it from the New Red Sandstone—records over sixty million years of history (from the end of the chapter known as the Silurian period through much of the subsequent Devonian), over a huge stretch of territory from today’s Arctic to the Gulf of Mexico. Such a scale of time and distance necessarily covers a considerable variety of geologically newsworthy events—there was simply so much going on at different times in different places. The opening and closing of small ocean basins and the consequent tectonic impacts in what is now Europe differed in timing and effect from what was happening on the Appalachian side of the story. A collision was not always head-on, but sometimes more of a glancing blow, tearing parts of the continent sideways (rather like California today), along laterally moving faults that ripped open deep depressions, into which sand poured. Seas invaded and retreated at different times in different places. Pieces of crustal flotsam drifted around the ancient ocean, beaching, colliding, piling up. It was a period of ponderous tectonic chaos that completely changed the geography of the world.
The Old Red represents a wide variety of sediment genealogies, and for this reason the term is used fondly but only informally today—these gigantic piles of sediment have been subdivided into more manageable and locally meaningful chunks. But the Old Red continues to have a place in geologists’ hearts because of its prominent and dramatic role in the growth of geology as a science. The sandstone above Hutton’s unconformity (Figure 33) is the Old Red. Agassiz discovered the remains of early fishes in the Old Red elsewhere in Scotland. And two members of the pantheon of nineteenth-century geologists, Roderick Impey Murchison and Adam Sedgwick, in 1839, used the spectacular sequences of the Old Red in Devon to define a new chapter in the Earth’s history: the Devonian. Nevertheless, a great deal of effort was needed to establish the Old Red’s importance. A colleague of Murchison’s describes how a visiting foreign geologist told him, “You must inevitably give up the Old Red Sandstone: it is a mere local deposit, a doubtful accumulation huddled up in a corner, and has no type or representative abroad.”
Murchison, thankfully, declined to follow this advice. The breadth of its stupidity can perhaps be portrayed by Plate 13. The spectacularly exposed sections of the Old Red Sandstone in Greenland record a thickness of over 6 kilometers (4 mi) of sediment accumulation. Even though the photograph was taken from an aircraft, cross-bedding can be distinguished in some of the layers.
The Devonian is an extraordinary record of an extraordinary period in the history of our planet. It is not surprising that it is, in itself, a textbook in sedimentology, displaying examples of almost every kind of sediment, every permutation of facies (not always red), from deep-water turbidites to sand dunes. As Richard Fortey wrote in Life: An Unauthorised Biography, “The two different faces of the Devonian—marine versus the deposits of lakes and mountain basins—are a kind of temporal schizophrenia. The non-marine rocks were lumped together as Old Red Sandstone, and it was some time before it was proved to everybody’s satisfaction that these richly coloured red rocks recording life’s greatest adventure were the exact contemporaries of unremarkable pale limestones and dark shales.”
Here, in this passage from Fortey, is the other reason for the Old Red Sandstone’s fame: “life’s greatest adventure.” For contained within the Old Red are the stories of the flowering of a sophisticated plant world and of some remarkable fish, together with tales of the rise of the vertebrates and their first expedition onto the land, recorded by their footprints in the sand. According to Mark Twain, in his 1903 essay “Was the World Made for Man?”:
So the Old Silurian seas were opened up to breed the fish in, and at the same time the great work of building Old Red Sandstone mountains eighty thousand feet high to cold-storage their fossils in was begun. This latter was quite indispensable, for there would be no end of failures again, no end of extinctions—millions of them—and it would be cheaper and less trouble to can them in the rocks than keep tally of them in a book.
For a sampling of these great stacks of sediment that poured off each newly formed but crumbling mountain range, we shall return to the work of Barrell. In 1913, Barrell drew a map that showed a range of Devonian mountains extending from New York into Pennsylvania and, on their western side, what he called the Catskill Delta, in cross section a wedge-shaped apron of sediment flanking the mountains, extending and thinning (hence the “wedge”) to the west and south. He proposed that the red color of many sediments told of an arid or semiarid climate and that the droughts resulting in such a climate were the impetus for animals to develop lungs. The sandstones along the Susquehanna that our sand grain hurtled past in chapter 4 belonged to the Catskill Delta. In places, the Catskill sediments are more than 3,000 meters (10,000 ft) thick. We now know that this accumulation of geological waste is not the result of a simple delta; rather, it represents a compendium of all the varied sedimentary environments that we might expect in front of a mighty range of mountains. The initial subsidence of the basin under the stress of the growing mountains was rapid. Early sediments in the sequence were turbidites, cascading into the basin, filling it up, flooding westward, until the sediments built up above sea level. The early river systems were braided, clogged with sediment, the later ones longer and meandering as the land built out and the sea retreated. Imagine taking the Appalachian Trail 380 million years ago. You are traveling considerably south of the equator, in a climate like that of Namibia’s today. It’s thirsty work, with little shade, no trees. Stop and look westward: in the foreground, steep valleys, rivers flowing out in wide braided channels onto broad floodplains; in the distance, meanders, sand banks, lakes glinting in the Devonian sun, in some places fringed by dunes. The scene shimmers in the heat. In the hazy far distance, the shoreline, the coast of the wide sea stretching far to the west, tidal flats, deltas, barrier islands, beaches—on which our ancient ancestors leave their footprints. The sea is slowly retreating westward, driven back, except for an occasional countersurge eastward, by the ever-growing land along the flanks of the mountains.
To take an Old Red Sandstone field trip today, stroll around the streets of New York, Washington, and other East Coast cities—your tour will reveal a diversity of facies (as will a tour of Scotland). The original curb and paving stones of New York are Old Red, and much of Yale University was built from it—as was the spectacular but infamous Dakota Building, home of John Lennon and the scene of his death. Mark Twain, who seems to have had a great fondness for Old Red, wrote of “that poor, decrepit, bald-headed, played-out, antediluvian Old Red Sandstone formation which they call the Smithsonian Institute.” Twain was muddled: the Smithsonian is made of New Red, but never mind, they’re building stones, brown-stones and curbstones, New Red and Old Red.
Devonian sandstones have been the subject of intensive geological scrutiny for more than two hundred years, and increasingly sophisticated methods continue to be brought to bear, for there remains much to learn. But through reading the stories told by their facies, grain sizes, provenance, chemistry, and all the other aspects of their character, the biography of a mountain range, its birth and death, has been told.
Time and Tides
If our ancient vertebrate ancestors on the Devonian shores had playful moods and, like Edward Lear’s Owl and Pussycat, danced “on the edge of the sand . . . by the light of the moon,” the Moon would have appeared to them to be bigger than it appears today. And the days they spent going about their ancestral business would have been shorter than they are today. The evidence is in the sands.
The most likely origin of the Moon is an almighty collision between the Earth and another planetary body shortly after the Earth was formed, the Moon spinning off into orbit, captured by the Earth’s gravity. As water formed on Earth and the oceans gathered, they came under the tidal influence of the Moon—and the Sun. But we know that the Moon is slowly pulling away from us; direct measurements show this happening at a rate of 3.8 centimeters (1.5 in) every year. Celestial mechanics demonstrates how the never-ending cycle of tides puts a brake on the rotating Earth, energy is lost, and the Moon slips away. As the Earth slows down, the days become longer and there are fewer of them in the year; but regardless, the tide still comes and goes.
Tides, as we know, move sediment and thereby leave a record. The layers of sediment deposited by tides can be seen today (they are, not surprisingly, called tidalites), but they can also be seen preserved in very ancient rocks. In parts of South Africa, there are tidalites formed 3.2 billion years ago, when the planet was young and life did not add up to much yet. These rocks consist of multitudes of stacked repetitions of twinned layers, a fine sand deposited as the tide came in, and silt and mud as it went, more calmly, out. The layers vary systematically in thickness and in grain size, reflecting variations in the strength of the tidal currents, and these variations result from the tidal cycles that we see continuing today. Twice a month, the Sun and the Moon are lined up (the condition known, memorably, as syzygy) and conspire to produce a larger tidal pulse—the spring tides. Spring tides have nothing to do with the season, but refer to the leaping up of the tide. When the opposite configuration occurs and the Sun’s pull detracts from that of the Moon, then the tides are subdued, the neap tides. A further cycle results from the fact that the Moon’s orbit is not perfectly circular, and it moves closer and further away over the course of a month—the origin of the idea that lunacy is influenced by the proximity of the Moon, as in Othello’s gloomy observation, “It is the very error of the moon, / She comes more near the earth than she was wont, / And makes men mad.”
The cycles of the Moon influence the strength of tidal currents, which in turn determine the character of the tidalite layers. Measure the cycles of thickness and grain size in ancient tidalites, apply some mathematical statistical analysis, and the length of the day and of the lunar month can be estimated—along with how far away the Moon was. Three billion years ago, a month was around twenty days long, there were approximately 550 days in a year, and the Moon was probably 25 percent closer to the Earth than it is today.
Now, given how far geologists are peering back in time, with associated uncertainties over measurement, statistics, and interpretation, it is not surprising that this kind of conclusion is the subject of lively debate and dispute. But the approach continues to be refined, using tidal sands from different ages around the world—and it remains the only insight we have into the early relations between ourselves and our nearest celestial neighbor.
I suspect that James Hutton would have been delighted by tidalites and their stories. Interpreting the incomplete ledgers of our planet’s history takes, as I hope this chapter has shown, hard work and imagination. In the words of John Playfair, describing in 1803 his friend Hutton’s earlier reading of Siccar Point: “The mind seemed to grow giddy by looking so far into the abyss of time; and while we listened with earnestness and admiration to the philosopher who was unfolding to us the order and series of these wonderful events, we became sensible of how much further reason may sometimes go than imagination may venture to follow.”
To see a world in a grain of sand.