the surface materials, and decay of granite bowlders that have taken place in the Iowan drift up to the present time, is insignificant. While the loess was forming, Iowa was occupied by herds of reindeer and musk oxen, and it is possible that these arctic forms were present in parts of the state even while the Iowan ice was at its maximum. Portions of the skeleton of a musk ox (Ovibos cavifrons), including a fairly perfect skull now in the museum of the State University, were some years ago taken from loess beds at Council Bluffs, and Professor Witter has found bones of the reindeer in the loess at Muscatine. The loess also contains as fossils numerous species of land snails that have been carefully collected and studied by Professor Shimek of Iowa City. The most common and characteristic belong to the genus Succinea.

10. The Wisconsin Stage. The last glacial invasion of Iowa is known as the Wisconsin stage. The Wisconsin glaciers, however, covered only a small part of the state. When the Wisconsin glaciation was at its height, a lobe of ice crossed the northern boundary of the state with a width reaching from Worth to Osceola county, and with a somewhat attenuated extremity resting upon the present site of Des Moines. The Wisconsin drift overlaps the Iowan area. In the city of Des Moines and for some distance north, Wisconsin drift rests upon the fossil-bearing loess laid down at the beginning of the fourth interglacial stage. Along its southwestern margin the newer drift sheet rests in places upon the Kansan.

The Wisconsin drift is largely a pale buff, very pebbly clay. The bowlders are granitic, but they are finer grained as a rule, and the average size is smaller than those of the Iowan drift. The ledges whence they were derived were intersected with numerous veins of trap. Very generally the smaller pebbles are fragments of limestone.

The Wisconsin glaciers, more than those of any other glacial stage in America, heaped up the drift around their margins in the form of moraines. These moraines are usually very conspicuous topographic features. In some instances they form ranges of hills rising from 50 to 150 feet above the adjacent plains. The largest moraines belonging to this

stage are found in Wisconsin and the Dakotas, but irregular ridges of drift, more or less conspicuous, may be traced along the eastern margin of the Wisconsin lobe, through Worth, Cerro Gordo, Franklin, Hardin, and Story counties in Iowa. At its extreme southern limit the drift of this stage thins out without forming any terminal ridge. The western margin is marked in many places by morainic ridges.

The retreat of the Wisconsin ice is so recent, geologically speaking, that the drift surface remains almost as the glaciers left it. Drainage has not been completely established. Numerous lakes occupy depressions in the irregular surface, particularly in the moraines. Basin-like marshes are still numerous. Many square miles are still in a condition to be flooded after any unusual rainfall. The channels of even the largest streams traversing the area are cut only a few feet below the level of the general surface. The beautiful rolling country traversed by the Great Western railway southwest of Des Moines, where every acre is thoroughly drained and the stream valleys are wide and frequently more than 100 feet in depth, may be contrasted with the level lands of Winnebago, Hancock, Kossuth, Emmet and Palo Alto. Southwest of Des Moines the topographic forms are the result of erosion acting continuously since the close of the Kansan stage. In the northern counties mentioned we see how inappreciable have been the effects of erosion during the relatively short period since the close of the Wisconsin.

11. Warren Stage (?). Mr. Upham uses the term Warren stage for the time immediately following the melting of the Wisconsin ice; but no records of deposits made during this stage, and no facts throwing light on its duration or climatic conditions, have been recognized in Iowa. With the disappearance of the Wisconsin ice lobe the state was freed from its latest glacial invasion so far as known. For a long time, however, the ice fields must have lingered north and northeast of Iowa. Upham notes a number of advances and recessions of the ice that took place after the close of the Wisconsin stage, but none of these movements affected the state except so far as they may have produced fluctuations in the annual

temperature. The general climate of the state, however, must have felt the influence of great bodies of ice so long as they lingered very far outside of the limits to which the Greenland and other northern glaciers are now confined. When that limit was essentially reached, when modern conditions as to climate were established, the recent stage was introduced and the geological history of the globe was practically ended.

During the Warren stage, or at least while the temperature of Iowa was still affected by the retreating glaciers to the north, Iowa became populated with a mixed fauna, part of which persists among our modern species, part of it has become extinct. Among the more conspicuous animals were three species of elephant, or probably three varieties of the same species. Remains of the elephants are not uncommon, and some may be found in almost every museum collection in the state. The Historical Department has its share, and some are interesting as showing a very close relation to the typical Mammoth or hairy elephant of the eastern continent (Elephas primigenius). The larger number of elephant remains found in Iowa are referable to De Kay's species, Elephas americanus.

Entering the state later than the elephant, but apparently contemporaneous with it for some time, was the Mastodon, another elephantine creature differing from its great congener principally in the structure of the molar teeth. Only one species is indicated in Iowa, Mastodon americanus. There are reasons for believing that the Mastodon survived later than the elephant and continued to inhabit Iowa until comparatively recent times.

There were horses, too, in our Pleistocene fauna as demonstrated by a number of discoveries, but the horse, like the Mastodon and the elephant, became extinct before the Columbian discovery of America. But the extinction of the older types of life, the retreat of glaciers to the fields they now occupy in high latitudes or at high altitudes, the coming of man, and the introduction of modern faunas and floras, mark the close of geologic history. Pleistocene Iowa becomes