The Peorian interglacial stage and the preceding Iowan glacial stage have received much attention within recent years. In 1917 (Geol. Surv. Iowa, vol. XXVI, pp. 49–212), Alden and Leighton presented the results of their studies on the Iowan drift and the loess associated with it. In 1918 (U. S. Geol. Surv., Prof. Pap. 106, pp. 1–356), Alden dealt with the Quaternary geology of southeastern Wisconsin. The results of these investigations have been to establish the fact that a sheet of till intermediate between the Illinoian and the Wisconsin had been laid down, that which had already been designated as the Iowan; furthermore, that immediately following this there was deposited a covering of loess. It was further concluded that this is the main loess deposit, much of what has been regarded as Sangamon loess being really loess of a later stage, the Peorian.
As no Iowan drift is known to be present in Illinois to separate the loess of the Sangamon from the Peorian, it must be difficult, often impossible in our present state of knowledge, to distinguish the one from the other. The Sangamon loess was laid down probably long after the Illinoian ice disappeared, so that there was time for the Illinoian drift to become leached and otherwise modified and for the accumulation of old soils and peat-beds.
On the other hand, the old soils of the Peorian stage are likely to overlie the loess. Unfortunately, the desired indications of geological age are not always present where bones and teeth are found; or, if present, are not always observed. We must, therefore, make our assignments of fossils to one stage or the other with great circumspection or leave the decision in abeyance.
Reference has already been made to the presence of Kansan drift in western Illinois and of black soils intervening between it and the Illinoian. Such soils must be referred to the Yarmouth interglacial stage. Whether or not still older glacial or interglacial deposits occur in Illinois is problematic.
In Illinois any considerable number of species of fossil vertebrates are rarely found together. The localities are widely scattered and a single species or two in each is the rule (map [38]). In later glacial deposits around the south end of Lake Michigan have been discovered the dogfish Amiatus calvus and a sun-fish belonging to the genus Lepomis. Baker (Univ. Ill. Bull. XVIII, p. 85) reported the humerus of the merganser, Mergus serrator, from the same region. The ground-sloth Megalonyx jeffersonii (pp. 33–34) has been found at Urbana, Galena, and Alton.
The few horses are described on page [187]. Peccaries have been found at three localities (p. [218]). For the specimens of deer that have come to light, see page [229]. A species of Cervalces and the moose Alces americanus have been met with in Will County (p. [107]). The reindeer has been recognized from poor materials found at Alton. The prong-horn Antilocapra appears to have lived in the region of Galena, as shown by Wisconsin specimens. The remarkable antelope Taurotragus americanus has been found at Alton (p. [339]). As to the musk-oxen and the bisons, the reader may refer to pages 251, 259, 268; for the mastodons and elephants, to pages 100, 140, 152, and 176.
Of the rodents, the muskrat has been found about Chicago; the pocket gopher at Alton and Galena; the ground hog at the same places (p. [343]). The beaver (p. [339]) likewise occurs at Alton. The giant beaver, Castoroides ohioensis, has been collected at four widely removed places (p. [279]). The rabbit, Sylvilagus floridanus, was included among the animals found in the lead crevices of the region about Galena, where also have been found an extinct species of raccoon, Procyon priscus, what appears to be a large dog Canis (or Ænocyon) mississippiensis, the coyote, Canis latrans, and the fox Urocyon cinereoargenteus. The bear, Ursus americanus, and the common gray wolf, Canis nubilus, appear to have existed in the middle Pleistocene at Alton.
A skull of Felis couguar, the yet existing panther or mountain lion, has been found in Randolph County, in the bed of Kaskaskia River. It probably belongs to the late Pleistocene.
A considerable fauna has been secured in the lead region about Galena, in Illinois, Iowa, and Wisconsin. The collectors and describers of this were not careful to designate the localities, and in some cases these can not at present be determined. These collections are discussed on page [343], in the account of the geology of Wisconsin.
An interesting list of Late Wisconsin mammals has been secured near Whitewillow, Kendall County. From Dr. E. S. Riggs, of Field Museum of Natural History, and from Netta C. Anderson’s list, the writer learns that at least six skulls of the common mastodon, together with many other parts of the skeleton, has been taken from a well 10 feet in diameter (p. [109]). Above, there were bones of bison (p. [269]), deer (p. [229]), and elk (p. [240]). It is stated that a layer of these about 2 feet thick was encountered at a depth of about 5 feet.