HOW CHRONOLOGY IS DETERMINED

Cross section, east slope of Funeral Mound, Ocmulgee National Monument. Arrangement of construction elements confused by erosion and wash from top and side of successive mound stages.

INDIANS COME TO THE NEW WORLD

Most living American Indians share with the east Asians a group of features which are considered to be distinctive of the great Mongoloid division of mankind. These include: straight dark hair, dark eyes, light yellow-brown to red-brown skin, sparse beard and body hair, prominent cheekbones, moderately protruding jaws, rather subdued chin, and large face. Since the question of race determination, however, is one of extreme complexity, it should also be pointed out that while the majority of modern Indians as well as prehistoric skeletal remains in America share enough of these features in common to be regarded as predominantly Mongoloid, they as well as the east Asians themselves, possess other physical traits like stature and head form which vary widely from group to group. Some of these other traits may be explained by the influence of different environments acting over long periods of time, but others point to an admixture of non-Mongoloid features in some of the earliest migrants to these areas. It is just the meaning of this mixture of apparently diverse elements which makes the problem of ultimate origins so difficult; and we shall have to be content for now with the general relationship which seems to have been established. If the earliest wanderers to the Americas were primarily a blend of other racial elements, their influence on the physical type of later American Indians has been largely submerged by the Mongoloid features of the vast majority of later arrivals.

Asia, too, is the closest great land mass to this continent, and from it there are more practicable means of access than from any other area. Even today the Bering Strait could be crossed by rafts, for islands at the middle cut the open water journey into two 25-mile stretches. Eskimos make the trip in their skin boats, or in winter by dog sled over the frozen surface of the strait. In the past, the journey must have been even simpler. During the several worldwide glaciations of the Pleistocene Epoch, a geological period which began more than 600,000 and ended about 10,000 years ago, great masses of ice spread across the surface of the continents in the higher latitudes. Since the growth of these ice sheets was nourished by falling snow, the seas, which supplied the necessary moisture, were reduced in volume as the ice expanded. The maximum drop in sea level has been calculated as between 200 and 400 feet, but the floor of Bering Strait is so shallow that a drop of as little as 120 feet would have been sufficient to create a dry land bridge between the continents. Further lowering must have increased the area and elevation of this passage, but the main effect of this was simply to extend the length of the interval during which the bridge remained open. This may have continued well into the period of milder climate after the time of maximum ice advance.

Another peculiar condition in this region at this time was the presence of considerable areas untouched by glacial ice. These included the foothills and coastal plain along Alaska’s northern coast as well as the great central Yukon Valley. This surprising situation was probably due to the small amount of moisture left in the winds which had passed over the high and cold mountain chains bordering the southern coast and the second great mass of the Brooks Range to the north. Furthermore, the broad Mackenzie Valley, leading south along the eastern slopes of the Rockies, was the area latest to be covered by glacial ice and first to open up with the return of warmer conditions. It may even be that the ice failed to cover this region during the last one or more of the minor advances which together make up the latest, or Wisconsin, glacial period.

Taken all together, therefore, the conditions described provided man with a chilly but relatively dry and passable route from the Asiatic mainland to Alaska and thence to the warmer interior sections of North America. For a considerable period this route must have been flanked with glacial ice lying only a few miles away on one side or both through a total distance of some 2,000 miles. It is one of man’s distinctive qualities, however, that he is able to adapt himself to extremes; and it is probable that the game he lived on was itself acclimated to living close to the edges of the ice sheets. We are less certain about the conditions under which this journey was begun at its Asiatic end; but it seems likely that there, too, ice would have formed in the high mountain masses, but that the valleys and lowland would have remained open as they did farther east.