The Last Giant of Beringia

The Pleistocene Epoch, commonly called the Ice Age, began about 1.75 million years ago. It ended just ten thousand years ago, though some scientists think we are still in it, merely enjoying an “interglacial” reprieve—a day at the beach in geologic time. During this period, the climate was not uniformly cold, but fluctuated in great cycles. Warming trends followed cooling ones, sometimes oscillating in periods of tens to hundreds of thousands of years. In the most recent of these cold cycles, temperatures fell starting about twenty-eight thousand ¹ years ago and continued falling until perhaps eighteen thousand years ago. Brutally cold conditions, unlike anything known on the planet today, locked down on the polar regions and the adjacent latitudes. The effect was greater in the Northern Hemisphere owing to peculiarities of weather patterns and ocean currents, which owed in turn to the direction of the earth’s rotation, differences in the distribution of land and sea, and irregularities in the earth’s orbit. In colder regions, more snow fell in the winter than melted in the summer. So, it accumulated. It built up in layers that melted slightly, condensed and recrystallized. Each new season’s snowfall compressed the layers beneath it. The result: masses of glacial ice.

Year after year, glaciers thickened until the leading edges pushed out over the land, merging with one another. The Laurentide Ice Sheet, the largest in North America, piled up to a height of nearly two miles. Centered on Hudson Bay, it grew to cover practically all of what is today Canada. It joined with the Greenland Ice Sheet to the east, and plowed its way south nearly as far as what is now the state of Kentucky. At some points, the Laurentide Ice Sheet likely joined North America’s other great mass of ice, the three-thousand-mile-long Cordilleran Ice Sheet, which draped itself along the coastal mountains of western North America from Puget Sound to the Aleutian archipelago. Advancing ice also blanketed Northern Europe, scattered parts of Asia, the world’s principal mountain ranges, and, of course, Antarctica. Amazingly, glaciers did not penetrate Interior Alaska, where relatively arid conditions inhibited their development.

Where the ice floated on the sea, as pack ice and icebergs, it did not affect sea level. A floating iceberg displaces the same quantity of water that the berg would produce if liquefied. But most of the world’s ice was not floating. It sat ensconced on the land and so did affect sea level. Evaporation continued to remove water from the sea, the winds transporting clouds of it over the land, where it fell as precipitation, often snow. Some of this returned to the oceans via the rivers during the cool summer, but much of it remained on the land as ice. Ice sheets held roughly one-twentieth of all the world’s water, half of that in the Laurentide Ice Sheet alone. Consequently, the sea fell during the Ice Ages, eventually dropping an estimated one hundred twenty-five meters, or about four hundred feet below its present level.

As the sea receded, the shapes of the continents changed. The eastern seaboard of North America gained real estate as the shoreline migrated hundreds of miles east to the edge of the outer continental shelf. On the Pacific side, the coastline did not change as noticeably because the continental shelf does not extend very far offshore. But north of the North Pacific, in the region of Bering Sea, a shallow continental shelf stretched continuously from Alaska to Siberia. And as the sea pulled away from the shore, Asia and North America began to reach for each other like the outstretched arms of God and Adam on the ceiling of the Sistine Chapel. When the finger tips touched, a charge of new life streamed into the Americas.

Not all at once, of course. During the most recent glaciation, the first linkup probably occurred with a continuous strip of land running from Siberia’s Cape Chaplin, through St. Lawrence Island to the southern part of Norton Sound. But temperatures—and therefore the sea level—continued to fall. The shelf broadened until perhaps eighteen thousand years ago, the height of glacial activity. Then the continental shelf between Alaska and Siberia stood proud of the water for over nine hundred miles, north to south, encompassing the Alaska Peninsula to the south and extending beyond Wrangell Island to the north. North America and Asia were joined at the head like great, sprawling Siamese twins.

For Alaska, the effect was extreme, and it involved much more than a strip of land that permitted overland passage between two continents. With each appearance and disappearance of the Bering Land Bridge, Alaska essentially switched continental allegiances. When it was joined with Asia during the cold cycles, it was perforce cut off from the rest of North America by the very glaciers that had lowered the sea. Then, as a matter of biogeographical reality, Alaska became part of Northeast Asia. During the warm cycles, however, the glaciers melted, causing the seas to rise and the land bridge to flood. With a water barrier in place to the west, and no glacial barrier to the east and south, Alaska again became a province of North America. She was like a fickle lover: when times were warm, she accepted North America’s embrace; but when the air grew cold, she reached for Asia.

Traditionally, scientists thought that there were four Ice Ages in the Cenozoic, the current era, extending back about seventy million years. American scientists have named them after states: the Wisconsin, Illinoian, Kansan, and Nebraskan. And between each of these came a warmer, interglacial period they have called, respectively, the Sangamon, Yarmouth, and Aftonian. In Northern Europe, scientists working in the Alps named the four corresponding glaciations there after rivers that flow out of the Alps into the Danube: the Würm, Riss, Mindel, and Gunz. To help keep things crystal clear, researchers from Britain, Northern Germany/Holland, Poland, European Russia, and Siberia have all offered terminology of different partisan provenience. Hence, the last glacial stage is variously known as the Wisconsin, Würm, Monastirian, Devensian, Weichselian, Vistulian, Valdaian, Sarta, and Ermakavo.

By the 1960s, however, drilling records were showing many more glacial tills than just those four. In the 1970s, deep-sea drilling yielded unambiguous evidence for as many as sixteen glacial stages, separated by interglacials, during the last million years or so. The Bering Land Bridge would have connected the continents during each of the glacials, as soon as the sea level fell to about one hundred sixty feet below the present level, while the Bering Strait seaway would have existed during all or most of the interglacials, whenever sea levels rose higher than minus one hundred sixty feet. The exact number of appearances of the land bridge might not be known for a long time.

Of particular interest to Beringian scholars is the last land bridge, which existed at least during the extremely cold period between roughly twenty-five thousand and fourteen thousand years ago (but which may have remained in place more or less continuously through the last interglacial, when sea levels oscillated around the breech point). Great circumpolar plains extended from southern Europe into central Asia, east to Siberia, and across the land bridge into the middle of Alaska. Much of these interior regions, like Interior Alaska, escaped glaciation throughout the Ice Ages because mountains or distances impeded the migration of coastal moisture, inhibiting the snowfall necessary for glaciers to develop. But even if largely free of glaciers, Beringia was still a harsh place, dry and windy. Loess (windblown glacial silt) filled the air and piled up in shifting dunes. The vegetative cover may have been sparse, the land a polar desert, a drier version of today’s tundra intermittently established on frozen silt.

Notwithstanding these extremes, Beringia seemed to have supported a great bestiary, more impressive even than its modern African counterparts. Woolly mammoths ranged throughout Beringia, protected from the icy winds by six-inch-thick hairy coats hanging in long skirts. From their domed heads fifteen feet in the air, massive tusks up to thirteen feet long curved and recurved like an art nouveau motif. Ground sloths, torpid behemoths that could attain six thousand pounds, nonetheless displayed wicked claws and teeth. The long-horned and probably fierce steppe bison might have stood as tall at the shoulder as a smallish mammoth. Alongside the forlorn looking faces of muskoxen, broad horns dropped down and terminated in little flips, like the starched caps of Dutchwomen. From a distance, with their humps, rumps, and swaybacks, they looked like double haystacks. On the run, with their guard hair reaching nearly to the ground and all but obscuring their short legs, they seemed to fly over the tundra as if they were riding magic carpets. Horses evolved in North America and crossed the land bridge into Asia (and a good thing they did, because their New World populations were totally extinguished, until European explorers brought their descendants back aboard sailing ships). The saiga antelope, a goatlike antelope with a greatly enlarged muzzle, seemed to be borrowing an evolutionary page from the trunked beasts. Giant stag moose with enormous, long-beamed and many-tined antlers alongside caribou and wild sheep—all these inhabited the land bridge during the Ice Age. Meanwhile, stalking the hoofed creatures on padded feet were the fiercest predators this side of the Jurassic: heavily muscled saber-toothed cats with serrated, six-inch-long canines capable of slicing through the thick neck skin of a large ungulate; lions larger than those in Africa today; packs of timber wolves; and the giant short-faced bear, bigger and more menacing than an Alaskan grizzly.