The Cave of San Josecito, Mexico

Some of the more noteworthy fossil assemblages of birds and mammals known from the North American continent come from the Ice Age. It is perhaps not surprising that this should be the case, since in geologic history the Pleistocene occurs immediately before the Recent and is, relatively speaking, not far removed from the present in terms of years. One would normally expect to find the paleontological records for this next to last stage better preserved than those of earlier periods, since time and a changing earth are some of the factors responsible for an unsatisfactory preservation of fossil material. As a matter of fact, varied and well preserved representations of the vertebrate life of the Glacial Period, exclusive of those of man, are not uncommon in brea deposits, cavern accumulations, and in certain alluvial, lacustrine and fluviatile deposits of the United .States, as well as in the frozen muck and gravels of Alaska.