On land, it walked upright on two legs. But in the warm, swampy water, it swam with a kind of galloping rhythm, the right foot stepping out longer than the left. The creature occasionally kicked the shallow, muddy bottom with the claws of its three-toed back feet. Then it came to the point where the water deepened, and the ten-foot-long dinosaur lifted clear of the bottom to swim away into watery weightlessness.
This happened 200 million years ago, near Hartford, Connecticut. There were no people, no other mammals; even grass had not yet evolved. But there was mud—mud that is now rock. And from rock, we can read, sometimes with surprising intimacy, the great and small events of the Earth.
Rock turns bone to stone, preserves footprints, petrifies fish; rock captures the air bubbles of liquid lava, records the pressure of colliding continents, tells of the whoosh and swirl of ancient streams.
“There’s a story in every rock,” says Richard Little, a professor of geology at Greenfield (Massachusetts) Community College. By looking carefully at rocks and rock formations, you can begin to read those stories; and if you split open a rock with a hammer to look inside, you will be the first person in the world to read the story that particular rock has to tell. “Once you know just a little bit of geology,” says Little, “the whole world opens up to you.”
The best places for collecting rocks are along streambeds and at the beach. These rocks were brought from near and far by water and deposited by the glaciers that covered the continent during the Ice Age, which began 2 million years ago and ended about 10,000 years ago. A good field guide, such as the Peterson or Golden guide, will help you identify them.
There are hundreds of kinds of rocks, but only three large categories:
Igneous rocks usually massive and hard, are formed when material melts and then cools. Some, like granite, originated in molten pools underground; granite’s sparkly minerals formed as the rock slowly cooled and solidified. Another type of igneous rock, basalt lava, often appears as black bands running through other rocks. Many of the tide pools along the North American coasts are old basalt flows.
Metamorphic rocks, which often show bands of light and dark minerals, are rocks changed from one form to another by high pressure. Under the heat and pressure of mountain building, sandstone becomes quartzite, limestone becomes marble; often, the pressure creates beautiful swirling bands, such as those seen in gray-and-white gneiss. Sometimes minerals gleam from these metamorphic rocks: schist shines with mica (the same mineral that makes beach sand sparkle) and may also glow with red garnets.
Sedimentary rocks, as Little puts it, are “eroded pieces of mountains”—gravel, sand, and mud that were washed by dinosaurera streams into valleys. The layers of sediment compacted, and their grains hardened into rock. Marine muds, rich in seashell calcium, become limestone; petrified sands turn to sandstone; gravel becomes conglomerate.
A rock’s type bespeaks how the rock was born. Other features may chronicle its history. In the Northeast, for instance, one story line to look for is written by the glaciers, sheets that covered the land with ice more than two miles deep during the ages when mammoths, shaggy rhinos, and giant camels roamed North America.
If you look carefully along the large slabs of granite in northeast mountains, you will often see the scrape marks left by boulderpocked glaciers. These long impressions look as if the rock were clawed by some monster slipping downslope, desperate for a handhold. The scrape marks are generally parallel, unlike the rougher, more irregular marks left by a bulldozer.
Sometimes, the shape of a rock tells its history. If you find a perfectly round rock, particularly an oddball among many jagged rocks, it may well have come from the bottom of an ancient waterfall or stream channel. Small rocks (basketball-sized or smaller) that become trapped in these spots often act as drills, scouring out stream potholes and waterfall-lunge pools. They are known as “tool rocks,” though they were never used as tools by humans.
Sedimentary rocks, on the other hand, often have history thrust upon them. These are the rocks that record where dinosaurs stepped, where bones petrified, where fishes died—these are the fossils.
The Southwest desert is blessed with vast dinosaur boneyards, among the richest in the world. But bones, prints, and fossil-fish impressions may turn up virtually anywhere in North America. And the experience of finding a fossil often proves unexpectedly thrilling.
“Who would believe that such a register lay buried in the strata?” fossil hunter and geologist Edward Hitchcock, a professor at Amherst College in Massachusetts, wrote in amazement in 1858. “To open the leaves, then unroll the papyrus, has been an intensely interesting though difficult work, having all the excitement and marvelous developments of a romance.”
He was writing about his studies of footprints discovered in shale in Greenfield, Massachusetts. At the time, he was convinced the ancient footprints showed that the area had once been populated by giant, flightless birds and “marsupialold” animals. Instead, the prints belong to dinosaurs. They can turn up in the most unlikely places too. In 1973, for instance, when Bill Gringas was removing shale to make room for a new deck for his home in Grandy, Massachusetts, he discovered three-foot-long prints of a fifty-footlong dinosaur the size of Tyrannosaurus.
Along the beds of certain rivers, “fossil fishing” may prove the most fruitful way to discover these records. Geologist Little advises, “Look for dark shales of the oxygen-poor former lake bottom. With a hammer, split carefully along the layers, and you may be rewarded with coal-black impressions of fish scale, fins, or the supreme catch—a whole specimen over two feet long!”
Another plus of geologizing: all that hammering is probably good for you. A few years back, a survey of the obituaries in the journal Science revealed that geologists had, with the exception of one killed in a rock avalanche, lived longer than other scientists.
Little offers one theory why: “Geologists are getting the physical release of hammering all these rocks. It feels good.”