thousand years to radiocarbon dates for the period of 15,000 to 12,000 years ago in order to discuss the time of the Great Adventure: Thus the radiocarbon date, 11,000 C-14 yr BP+/- 75 becomes a rough 13,000 years ago, in this case less than a hundred years off the true recalibrated date. For the purposes of telling my story, that’s generally close enough. On occasion, radiocarbon dates are necessary to pin down the precise timing of the opening of the ice-free corridor or the brief panoramic window of Clovis. When you see an exception to rounded-off dates in this book, such as 13,300 or 12,900 years ago, that means the data is translated from the radiocarbon calendar and has been cited several times in peer-reviewed literature. I’d like to attempt to keep it simple.
| Here is a sample of some rounded off dates that appear several times in the text:30,000 years ago: Marks about the earliest date humans could have appeared in North America. The evidence? Not much: A single site in the Siberian Arctic and inferences from genetic and linguistic studies on extant populations.15,000 to 27,000 years ago: A very cold time of advancing glaciers. By about 20,000 years ago the ice sheets were at their maximum, closing off all routes from Alaska to lower North America. The great megafauna still roamed the ice-free far north but there is no record of humans in the Arctic during this period.13,000 to 15,000 years ago: Very rough dates that denote the time of the so-called pre-Clovis people. It was a time of global warming and rising seas. The archaeological data indicates a couple sites around 14,000 years ago in northeastern Siberia and, about 700 years later, several more along the tributaries of the upper Yukon River in Alaska. South of the ice, a number of credible pre-Clovis dates come from the United States and South America.12,800 to 13,100 years ago: This is the time of Clovis, which probably begins several hundred years earlier than 13,000 years ago, by which date the culture, marked by its iconic projectile point, was full blown and spread across the southern half of the continent. There’s lots of archaeology to pin these dates down. About 12,800 years ago, the warming period is interrupted by a cold snap. The American megafauna, which might have been in decline for a few hundred years, suddenly goes extinct, and Clovis disappears from the archaeological record. |
Chapter 2
The Lair of the Short-faced Bear
Forbidding Glaciers, Man-eating Predators
and Poisonous Plants in Ice-age America
The great ice fields of the last glacial advance loomed across the entire North American continent and at their maximum around 20,000 years ago constituted an impenetrable barrier to human migration. The glaciers themselves served as barometers to ice-age peoples and when they began to melt about 15,000 years ago they signaled the onset of approaching climate change.
Even today, I note that the question of where the ice is located still resonates in my personal life. In my small world of mountains and wilderness, recognition of the hard face of modern global warming comes in large measure not from reading the pile of scientific research which I receive most every day, but from old friends and some new ones who climb mountains. I am blessed to call among my friends a handful of the world’s great mountain climbers. They climb in British Columbia, Alaska, the Andes, Africa and, most of all, in the Himalayas. My small forays and treks along the Continental Divide of the Northern Rockies and coastal British Columbia are meek in comparison to their expeditions to the high white wilderness of glaciers that mark the roof of the world. But the topic is the same: the disappearance of the world’s glaciers, especially the finite ones we know well and explore like the body of a secret lover.
My own fragile and diminutive mistresses lie along a great traverse of grizzly bear country along the Continental Divide in Montana’s Glacier National Park. The trek starts from the paved road the locals call “Going to the Six-Pack Highway” and departs from all trails as it leads you north towards Canada. It’s not an easy bushwhack and the weather can shut you down. I’ve made the traverse over a half-dozen times beginning forty years ago, often alone or with a trusted friend and, the last, just a few years ago, an abbreviated trip with my own daughter.
As you climb up to the divide and wind around the peaks, Glacier’s famous ice fields come into view. Most are tiny hanging glaciers clinging to the northeast-facing cliffs. A few of the larger ones have names: Vulture, Gyrfalcon or Two Ocean. You walk on snowfields and the tundra of the high country, where you can scarcely put a boot down without stepping on a grizzly dig. You finally scramble your way out of a perfect glacial cirque punctuated with a circular turquoise tarn, using your ice axe here and there for balance and backsliding, and step onto the Continental Divide. It opens up into a narrow expanse of subalpine splendor with stunted fir trees framing chains of shallow mountain lakes. Grizzly sign is everywhere; some of the high meadows look plowed by the long-clawed bears who dig corms and tubers from the thin soil.
Once I followed a well-used animal trail around the shore of small lake up there and almost stumbled into a garbage can-sized dish-shaped depression next to a tree; the grass at the edge of the grizzly daybed sprang upright from the weight of the bear who had just arose from his nap. Considerably more cautious, I crept down the trail. That’s way too close.
In the 1970s, the small glaciers at the head of Valentine Creek still contained blue ice. By the late 1980s, these glaciers had become mere snowfields. Twenty years later, the snow was gone. Now, the ribbon of ice above lonely Gyrfalcon Lake is but a shadow of its hefty parent only thirty-five years ago. On my last traverse of the basin, I climbed up to the hanging glacier and broke off an icicle, sucking the water out of it like a last good kiss.
All this is but a brief moment in a single life watching small, beloved ice-fields shrink and die. A similar tragedy is playing out in the Wind River Range of Wyoming, where large glaciers, though diminished, still flow. My friends tell similar stories of the giant ice fields of the Himalayas. It is a tale of loss, of the end of something beautiful in a melting century.
One can imagine the minds of ice-age explorers dancing with images of glaciers. The first Americans of the Late Pleistocene would not have walked or boated down the icy defiles without a distinct foreboding of approaching change and an appreciation of the coming need for humans to pioneer new habitats, to confront these never-before-seen animals and endure shifting climates.
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The Pleistocene or Great Ice Age lasted about 2 million years (published estimates run from 1.6 to 2.6 million years), during which the northern polar caps surged and ebbed on a cycle of every 100,000 years or so. The last advance, the Wisconsin, started almost a hundred thousand years back in North America and, within that cycle, oscillations in the ice produced smaller advances and retreats. The Late Wisconsin, 12,000 to 38,000 years ago, was generally a time of expanding glaciers that reached their maximums about 20,000 years ago. This period is when humans first show up in North America.
Non-polar glaciers often have their origins in the high country when a cooling climate dumps more snow in the mountains in winter than melts during summer—accumulation exceeds ablation. As the snow builds up over decades and centuries, the pressure causes snow to granulate. The weight of ice compacts the glacier until, like a plastic, it begins to flow downhill. The rivers of mountain glaciers may coalesce and become ice sheets, like the Cordilleran of western North America.
The history of ice in North America is important because it outlines the great mysteries of continental colonization: Who were the first Americans, when and how did they get here and what routes did they take to get south of the ice? Since the archaeological record from the American Late Pleistocene is not robust, reconstruction of ancient environmental habitats contributes substantially to understanding when people could live there. And these habitats depended on where the glaciers were. The ice altered environments and climate. Constantly changing vegetative communities, chewed into a patchwork by the huge Pleistocene grazers and browsers, were the norm. The presence of
