The Pleistocene is a term applied by geologists to the latest division of geologic time. It includes the present; and it reaches back beyond the present to the beginning of a special series of events, which, marking a decisive epoch in geologic history, have brought about the present relations of land and sea, the conditions of climate, the peculiarities of soil, the specialization and distribution of living forms, and in general all the phenomena that distinguish the modern from the geologically ancient world. No part of the geologic record is at the present time receiving more attention from students of world history than that which belongs to the Pleistocene; and in no part of the world are certain chapters of the Pleistocene record clearer, or fraught with greater interest, than in our own fair Iowa. Before the beginning of the Pleistocene, Iowa had been subject to numerous vicissitudes of climate; for long eras it had lain beneath sea level and so received its load of limestone, sandstone and shaly sediments; at other times it was part of the nascent continent; in common with other parts of the globe it had undergone numerous gradual, but complete changes in its animal and plant life. The pre-Pleistocene history of Iowa was varied; but it was on the whole progressive; and eventually the region became fairly established as a part of the dry land. That portion of geologic time which immediately preceded the Pleistocene is generally known as the Tertiary; but with many geologists the term Tertiary is now discarded, and the time which has usually been assigned to it is divided into two periods known respectively as the Eocene and Neocene. During the Neocene Iowa was a fair and sunny land, clad in forests of tropical species, and revelling in all tropical luxuriance. Birds of gay plumage flitted back and forth in the open glades; savage beasts related to the lion and the tiger sought the shadowy recesses; herbivorous animals not very different from the elk, the camel, the rhinoceros and the horse, found pasturage in the grassy savannas, while troops of monkeys swinging from branch to branch, and from tree top to tree top stirred the woodland echoes with noisy exclamations. Now it is against this background of genial climate and abounding tropical life that we are to project the picture of the early Pleistocene in Iowa. Some cause or causes, at present not well understood, brought all the happy conditions of the Neocene to an end and introduced a series of changes whereby the cliamte and conditions that now obtain in central Greenland were established over some of the most favored areas of Europe and North America. Iowa was involved in the general change, and, together with all adjacent regions, was buried under persistent accumulations of snow and ice. Two things stand out distinctly in this part of the history. The change from Neocene to Pleistocene was attended by a very great depression of temperature; it was followed by centuries of unparalleled precipitation of snow. Many attempts have been made to assign a cause for the stupendous climatic changes recorded in the Pleistocene deposits of Europe and America, but no explanation of the facts, so far offered, has met with anything like general acceptance. Dana, Croll, Geike, Wallace, Upham, and a host of others, have each sought to find a cause, either in upward movements of the earth's crust, in changes of oceanic currents, in increased eccentricity of the earth's orbit, or in some other event, or combination of events, assumed to have been concurrent with the oncoming of glacial conditions. But it must be said that unanimity of opinion has not yet been reached |