The First Americans Read online

  The phrase “land bridge” suggests some sort of a fragile isthmus across which the Asian pioneers teetered fearfully on their way to a new land, but it was nothing like that at all. The landmass we call Beringia, which periodically emerged between Siberia and Alaska throughout the Pleistocene down to the end of the last glacial period, was about a thousand miles across from north to south, mostly a relatively barren tundraand steppelike place crossed by occasional rivers with their associated streams and swales, and probably sparsely populated by the same big-game animals the pioneers were familiar with from Siberia—notably woolly mammoths, bison, and horses. No people pioneering a path across this landmass, or along its southern coast, would have suspected they were crossing anything like a bridge or, indeed, that they were changing continents. They were just moving on, following the game, headed generally east toward the morning sun.

  Beringia came about as a direct result of the glaciers, those vast storehouses of frozen moisture mined from the oceans. And the creation of the glaciers lowered the sea level, exposing a great deal of real estate that is now under water. At their most extensive, about 20,000 years ago, the Laurentide and Cordilleran glaciers (along with those covering northern Europe, parts of northern Asia, and South America) dropped the sea level as much as 440 feet below the present level, perhaps more. That was a hell of a lot of moisture locked up in the ice—an estimated 12 million cubic miles more ice than exists today. The Laurentide Glacier was two miles thick in places and almost a mile thick at its southerly edge. The Cordilleran, which reached a thickness of one and a half miles in places, was essentially the growing together of separate montane glaciers into onecontinuous if narrow blanket of ice that extended south well into Puget Sound. Here and there on its western edge, the Cordilleran spread across the exposed continental shelf, calving icebergs into the Pacific but leaving some ice-free coastal areas as well.

  Glaciers form only under relatively special circumstances. Basically, they begin in elevated mountain basins where the rate of annual snowfall exceeds the rate of yearly melting for a considerable period of time. The presence of uplands is critical, and where there are no mountains, as in the so-called Bering Refugium, glaciers cannot and do not form.

  Within the highland basins, snowfields form, and under the weight of subsequent snowfalls the pressure of gravity converts previously porous flakes to rounded particles called “firn” or “névé.” Gradually, the porosity decreases further as the pressure from still more new snow accumulates, converting the firn deep in the snowfall, or recrystallizing it, into ice. This initial glacier is restricted to an amphitheater or half-bowl-like depression (called a “cirque”). Then eventually it grows larger, flows out of the bowl and down into adjacent valleys, and becomes a valley glacier. If the process continues, valley glaciers can conjoin in the foothills to form piedmont glaciers, which ultimately coalesce into great continental ice sheets spreading across the lowlands. The Cordilleran glacier was the consequence of the confluence of a great many mountain and valley glaciers, while the Laurentide was the result of the merging of three ice domes into one mighty ice sheet whose margins crept across the landscapes as long as the glacier was being fed with additional snow.

  The edges of continental ice sheets move in spasmodic thrusts called stadials and often form distinct lobes whose margins may rest or actually retreat a little (interstadials) even during an overall advance cycle. If you could see the speeded-up process, the movement of a continental glacier would seem like that of a huge, undulating amoeba, its edges pulsing as it grows, devouring most of the northern half of the continent. Then, finally, the amoeba would stop growing, its lobes would retreat, its edges would thin, and it would begin to die. The last “death,” the start of an interglacial period, occurred about 10,000 years ago and marks the beginning of the Holocene.

  While we understand rather clearly the factors that influence the short-term health of a glacier, what causes ice ages themselves is a much moremurky matter. Almost as soon as it was recognized that ice ages had existed in the past, various explanations were offered for their genesis. Of the early explanations, by far the most important and in some ways the most enduring was James Croll's Climate and Time, published in 1875.

  Croll believed that the origin of the ice ages lay in changes in the orbital geometry of the earth. Using calculations derived by yet another scholar to describe the orbits of the planets, Croll hypothesized that there had been eight glacial-advance cycles, each followed by an interglacial, between 240,000 and 80,000 years ago. Unfortunately, Croll had the dates of the ice ages all wrong, and when it was shown that glacial interludes had existed up until much more recently, Croll's entire orbital change theory was thrown out.

  Ultimately, Croll's cyclical change hypothesis was resurrected by Milutin Milankovitch, a Serbian mathematician. Between 1924 and 1941, Milankovitch reanalyzed Croll's data, supplemented it with new information, then asserted that there were not one but three cycles that affected the earth's climate. The longest cycle, of 100,000 years' duration, derives from changes in the shape of the earth's orbit. A shorter cycle, of 41,000 years, is due to rhythmic changes in the tilt (back-and-forth movement) of the earth's axis and determines the amount of sunlight that reaches the northern latitudes. The third and shortest cycle, of 19,000 to 23,000 years, reflects changes in the precession (wobble or side-to-side movement) of the earth on its axis. This last cycle influences equatorial latitudes and conditions, which in turn affect the lengths of the seasons. Like Croll, Milankovitch posited eight major and complete glacial cycles, but his were each about 100,000 years in duration.

  Confirmation of Milankovitch's calculation would not come until the 1970s, when deep-sea cores revealed that there had indeed been systematic changes in the earth's climate that corresponded to Milankovitch's predictions. Changes in the ratios of two oxygen isotopes (O18, which is common in the skeletons of tiny one-celled creatures called “foraminifera” when the oceans are cold, and O16, which is more abundant when the waters are warm) confirmed that all three of Milankovitch's cycles had indeed occurred in the past. These cores also revealed that a cold climate with limited mountain glaciation extended back 2.5 million years ago into the Pliocene (the period before the Pleistocene) but that extensive climatic deteriorationaccompanied by large continental ice sheets did not occur until just after 1 million years ago.

  The cores further showed that though the “signature” of the 100,000-, 41,000-, and 19,000-to-23,000-year cycles was readily detectable, only the shorter cycles were directly attributable to orbital changes. In order to have an effect on climate, the long cycle—which has also been documented well into preglacial times—would require an additional stimulus or trigger. In other words, some additional factor or factors would have to be at work to set major, long-term glacial cycles into motion. Some scholars suggest that when the drifting minicontinent of India slammed into Asia, creating the Himalayas and the Tibetan plateau in the collision, that great uplift of land, with its concomitant effect on the jet stream, air circulation, and rainfall patterns, could have provided the additional “push.” Others have suggested changes in the temperatures of oceans and/or shifts in the amount of carbon dioxide in the earth's atmosphere as additional causes. Whatever triggered the recurrence of long-term glacial fluctuations, we do know that the severity and duration of these cycles have differed in the past. Between about 1.6 million and 900,000 years ago, a complete and rather modest glacial cycle occurred about every 40,000 years. Between 900,000 and 450,000 years ago, the cycles grew in length and severity to 70,000 years. Since then the cycles have been a full 100,000 years long, with extreme continental glaciation.

  Whatever their duration, an often overlooked feature of these recurring glacial periods is that the Northern Hemisphere always experienced summers and winters. As my colleague Olga Soffer of the University of Illinois constantly reminds me, if the Pleistocene began 1.6 million years ago, there have been 1.6 million Januarys and 1.6 million Julys�
�an annual cycle of cold and warm (or at least less cold). The summer temperatures as far north as Alaska today get to be 70 to 80° F. Even at the greatest extent of the glaciers during all these periods, it has not been all bad, all cold, all unbearable, everywhere on earth. During the time of the last glacial maximum, about 20,000 years ago, hippos were frolicking in the Sahara, which was not a desert but a place of shallow lakes and often dense lakeside vegetation. Ice Age Florida, though a bit cooler and moister, would still have been a choice destination for your Pleistocene Christmas vacation.

  Between 115,000 and 110,000 years ago, during the last interglacial,the planet enjoyed a period with climates that were actually warmer and drier than today's. Then the climate began to deteriorate, global cooling started again, and by 110,000 years ago what has been termed in North America the Wisconsinan glacial stage was under way. By 70,000 years ago, global temperatures had dropped by some six degrees Fahrenheit, and sea levels had fallen by about two hundred feet. The ramifications were, of course, worldwide—the snow line on Mount Kenya, for example, dropped about two thousand feet lower than it is today—but the cold and its effects were more pronounced in the north and in highland South America.

  Let's stop for a moment and consider the matter of these lowering snow lines. They are among the most distinctive effects of a glacial advance. Their immediate impact is to force high-elevation plants to move down the mountains or go extinct. Typically, plants from the upper regions then find themselves among lower-elevation neighbors, creating what we think of as ecologically anomalous environments. As a result, animals are forced into what might previously have been unlikely—and for the animals probably uncomfortable—associations. This is widely common in glacial times; indeed, one finds the remains of woolly mammoths next door to those of whitetail deer or mastodons sharing territory with elk. One of the chief environmental effects of glaciers is to create what would strike us today as a highly unsettling patchwork of habitats for local plant and animal populations.

  In any event, in the north, the ice pulsed and grew until about 50,000 years ago, when the climate warmed somewhat and the ice began to recede a few hundred miles. Then, 25,000 years ago, the glaciers began their last great advance. By 19,000 years ago, the Laurentide glacier had reached its maximum extent in the region from Illinois to Wisconsin and Iowa but continued to grow in the East, reaching its maximum, in most areas, between 18,000 and 20,000 years ago. Meadowcroft, by the way, lay but a scant fifty miles from the ice front at its greatest extent. Far to the northwest, the glacier continued creeping south until about 13,000 years ago. The effects on the land of this enormous blanket of ice were profound and still lie around us today.

  As the glaciers snatched up moisture that normally would have returned to the sea, thus lowering the sea levels, the shape and size of North America were altered considerably. The west coast extended about anotherfifty miles farther seaward in some areas. On the east coast, where the continental shelf today is far wider, dry land extended some two hundred miles out into the Atlantic. To those who are aware that the notion of terra firma is a highly relative term, what with continents moving about over time, it should come as no surprise that the enormous weight of the ice caused the land below it to sink. This led to a postmelting phenomenon called “glacial rebound,” wherein the earth rises back up. In certain areas around the Great Lakes, the land is still rising at the rate of approximately two-tenths of an inch a year. And as the glacier's lobes pulsed this way and that depending on local and global conditions, the land below was gouged out, planed, polished, bent, and torn. One result was the Great Lakes, excavated by the glacier as it advanced and retreated. One lobe of the Lauren-tide ice sheet came to rest just off the present coast of Massachusetts, having shoved great quantities of rocky detritus ahead of it into a great curved embankment called a glacial moraine; this is what we know today as the southern arm of Cape Cod. Such moraines, large and small, are one of the most noticeable features of glacial advance and retreat, and they were one of the main features besides erratic boulders that tipped off Agassiz and his predecessors to the idea of glaciers.

  ICE AGE ECOSYSTEMS

  During the maximum extent of the Wisconsinan glacial period, the overall global temperature was about 12°F lower than today's, and near the glacier's edge it was—on average—18° cooler. Anticyclonic winds spun clockwise above the glacier, and cold winds tended to sweep downslope from ice to the exposed land near the glacier, which would have tended generally to be tundra underlain by permafrost for perhaps some 125 miles south. All in all, it was a seemingly inhospitable place, but no more so than much of the Arctic today. On the other hand, local conditions at the glacier's edge varied considerably, depending on a host of factors: weather, topography, and so forth, but especially elevation.

  The very presence of the huge blanket of ice had a modulating effect, in fact, on the seasonal weather south of the glaciers (and to their northwest in ice-free parts of Alaska), making summers generally less hot thantoday and winters less cold. One effect of the cyclonic winds was to render the eastern lands of the United States wetter and some western lands drier. At the same time, cooler, moist air was common in today's American Southwest and Great Basin area, with far more rainfall then than now. One result was the occurrence of huge so-called pluvial lakes, some reaching the proportions of inland seas, such as the Lake Michigan–sized Lake Bonneville in Utah, once more than 1,200 feet deep, the remains of which can now be seen in Great Salt Lake and the Bonneville Salt Flats, where maniacs set land-speed records on the dead-flat bottom of the old lake. The closest thing we have today to those pluvial lakes is Kenya's Lake Turkana, which, being so shallow, changes its size annually based on rainfall. North America's pluvial lakes occurred in the Southwest's many closed basins and achieved their maximum extent and depth at the very height of the glacial maximum. They seethed with life, serving among other things as seasonal homes for what were probably millions of migratory birds, their marshy shores attracting all kinds of life-forms, including, at some point, humans.

  Even at the time of the glaciers' maximum extent, meltwater burst from beneath them, depositing sediments in valleys and on outwash plains—vast quantities of silt and sand sometimes a hundred feet deep. Strong winds blew the silt and sand elsewhere. Huge areas of active sand dunes formed downwind, and minute particles of silt, called loess, blew across much of the Midwest, mantling vast areas far beyond the narrow, tundralike region near the glacier's edge. Today we owe a great deal of the agricultural productivity of the American Midwest to this blanket of loess created between 29,000 and 12,000 years ago, when the glaciers were still advancing and the lands beyond were cold, dry, and windy.

  At the end of the glacial period, virtually all paleoecological maps show a thin line of tundra vegetation next to the glaciers, with pockets of tundra leading southward along the tops of the Appalachian Mountains. In the east, in a wide belt south of the tundra, such coniferous trees as jack pine, fir, and spruce marked the landscape, and south of that, temperate forests of southern pine, oak, and hickory dominated. Florida, a much wider peninsula then, supported cypress, gum, and sand dune scrub. The southern plains were mostly grassland, while the western mountain region was dominated by mosaics of plant communities ranging from desert scrub to juniper and pine woodlands. All of these life zones were in relativelyconstant flux as the local conditions changed. As we will see later, this snapshot, with its strip of tundra between the glacier's edge and the lands to the south, is extremely soft-focus—that is, coarse-grained. I would find out soon enough that the world in those days at the end of the Pleistocene was not so simple. And it would only get more complicated.

  THE HOLOCENE SCENE

  In eastern Washington State one finds an astounding landscape called the Channeled Scablands. It is a place of huge labyrinthine channels cut into bedrock, enormous basins carved from basalt headlands, some up to eight miles wide and two hundred feet deep. Tremendous boulders and chunks of basalt ha
ve been moved around through the channels. Extinct waterfalls abound. Not far off, in the bottom of an extinct glacial lake in Montana called Lake Missoula, geologists have found ripple marks like those you see in beach sand and on river bottoms, but these Bunyanesque ripples are twenty feet high and spaced as much as three hundred feet apart. Here, obviously, there be monsters, or at least Paul Bunyan.

  The first scientific exploration of the Channeled Scablands was by a geologist, J. Harlen Betz, who wrote about them in 1923, explaining that no normal erosional process would have had the energy to produce such gigantic features. Instead it had to have been …yes …a catastrophic flood. He reasoned that an ice dam holding back the waters of an enormous glacial lake filled with meltwater in western Montana had failed, and untold quantities of water had exploded onto the land to the west. His account was met with extensive derision on the part of his colleagues. After all, catastrophic floods were still a sensitive matter among geologists, who had only recently suffered being reviled by religionists for knocking Noah's Flood as the Big One. Just when all that dust had settled and catastrophism was out, here comes Betz challenging uniformitarianism with this talk of a fantastic flood of mythic proportion. But the later discovery of the ripples of Lake Missoula proved Betz right. Only the passage of water over some kind of ground, like sand on a flat beach, causes ripples.