Date of this Version
There is an abundance of soils and paleosols as well as terraces and terrace fills and erosional forms of pediments to aid in working out a precise stratigraphic sequence for the Quaternary animals. The dating and correlation of Quaternary deposits are complex, and we must realize that there were numerous glacial advances and retreats. A continuing reexamination of the relationship of the mammalian faunas to the paleosols, terrace fills, volcanic ashes, and other geologic features must be done in order to provide a proper framework for the interpretation of the mammalian stratigraphic sequences.
The horses and camels, which had so successfully lived in the Great Plains Region for some 40,000,000 years, had some very aggressive competitors during the latter part of the Quaternary when the giant bison, mammoths, and other large mammals arrived from Asia prior to the Illinoian Glaciation. These migrant forms consumed great quantities of grass and herbs, which hitherto had supported the native horses and camels. Fossil evidence indicates that the vast herds of camels and horses began to diminish as the intruders increased in numbers.
The inhospitable climates of the Illinoian and Wisconsinan glaciations contributed greatly to the reduction in numbers of the grazing animals. As much as 56 meters of loess was deposited on the Yarmouth Interglacial sediments and elsewhere in some localities in the Central Great Plains. The diminution in size and their ability to adjust to environmental changes undoubtedly contributed to the success of the bison to survive the rigorous climates.