The Sangamon interglacial deposits are of peat, old soil, etc. A fourth recrudescence of the glacier (Iowan stage) occasioned the deposit of another till-sheet, of an extent not yet determined, which is best displayed in northeastern Iowa, where it is intimately associated with the largest accumulations of loess in the Mississippi valley. The Iowan till-sheet is followed (Peorian stage) by interglacial deposits which are perhaps contemporaneous with those so well shown near Toronto on Lake Ontario. The latter beds form a succession of fine shales and sand that lie between two sheets of glacial drift and are divisible into two parts; the lower (Don formation) contains many fossils of which the plants, such as the Pawpaw and the Mock-orange, indicate a climate distinctly warmer than that of the region at present and about like that of the middle United States. The fossils of the upper part (Scarboro formation) indicate a cold temperate climate and herald the approach of a renewed glaciation, which in turn is recorded in the overlying till. Such facts are difficult to explain, except as the result of truly interglacial conditions.

The Wisconsin stages are much the most conspicuous and best known of all, and the sheets of till and drift are far thicker than those of the other Glacial stages. Especially conspicuous is the great terminal moraine, or rather morainic belt, which itself records many episodes in the history of the ice and which has been traced across the continent. Beginning at Nantucket, the moraine runs through Long Island and Staten Island to New Jersey, which it crosses into Pennsylvania; here it bends sharply to the northwest to the boundary of New York, but turns southwest almost at a right angle, reaching nearly to the Ohio River at Cincinnati. It crosses in an irregular, sinuous line the states of Indiana, Illinois, Iowa, and thence northwestward through the Dakotas into Montana, where it nearly follows the international boundary to the Pacific coast mountains (see Fig. 311).

North America in the time of maximum glaciation.

Fig. 311. - North America in the time of maximum glaciation. The letters indicate the centres of dispersal of the ice. C = Cordilleran Glacier; K = Kewa-tin Glacier; L = Laurentide Glacier; N.F. = Newfoundland Glacier.

In the Eastern States there is no such clear indication of several successive ice-invasions as in the Mississippi valley, the Wisconsin erosion and its thick mantle of drift removing or obscuring the records of earlier events. The remnants of very ancient till-sheets in New Jersey and Pennsylvania have been mentioned, and in New York and New England not more than two or three invasions can be identified. In part, this may be due to the later development of the Laurentide glacier. The geologists of the Canadian Survey believe that, "beginning at the west and going eastward, these three great glaciers [i.e. the Cordilleran, Keewatin, and Laurentide] reached their widest extent and retired in succession." (Tyrrell).

The final retreat of the ice was by slow stages with many halts. In the central West are preserved many lines of moraine, with kettle-holes, kames, and drumlins, which mark readvances and pauses in the retreat.

Probably every retreat of the ice was accompanied by the formation of barrier lakes held in by the ice-front, but only those of the final recession have left intelligible records of themselves. A comparatively simple case is that of Lake Agassiz, which covered Manitoba and Minnesota with a great sheet of water, 70a miles from north to south. The lake was formed when the Keewatin glacier in its retreat had freed nearly all of Manitoba from the ice and was joined by the Laurentide glacier from the east, making a great wall of ice which shut off the drainage toward Hudson's Bay, while to the south high land held back the lake in that direction. The water of the lake rose until it overflowed the lowest point in the southern barrier and formed a river (Warren River, now abandoned) which joined the Mississippi. The level of the lake was gradually lowered as Warren River deepened its bed, and was finally discharged when the retreat of the ice opened the course to Hudson's Bay.

The history of the great Laurentian lakes is extremely complex and is slowly being deciphered by the combine efforts of many workers. The changing positions of the lobes which projected-from the ice-front, the numerous basins now connected and now severed, as the water rose and fell, combmed with slow diastrophic movements, make up a very intricate succession of temporary lakes and shifting outlets. Considerations of space forbid more than a brief and simplified outline of this interesting story. When the ice had retreated so far as to uncover land to the north of the divide between the basins of the St. Lawrence and the Mississippi and eastward the Hudson, the waters produced by the melting of the ice were held in between those divides and the ice-front, forming a great number of small lakes from New York to Minnesota, three of which require mention as the earliest recorded stages of the Great Lakes at a time when most of their present basins was filled by the ice. Of these three lakes which embraced three prominent lobes of the ice-front, one was in the axis of Lake Superior, one at the southern end of Lake Michigan, and the third in a line with Lake Erie, but west of it, and each discharged by a separate outlet to the Mississippi.

Omitting several intermediate stages, and coming to a time when the basins of Lake Michigan and Lake Erie and part of that of Lake Ontario had been freed by the retreat of the ice, we find Lake Whittlesey, which filled the basin of Lake Erie, but was very much larger; it was connected along the ice-front to the north with the crescentic Lake Saginaw, that discharged westward into Lake Chicago, a larger Lake Michigan, which retained its original outlet to the Mississippi. Lake Whittlesey was succeeded by Lake Warren, which was formed by a junction with Lake Saginaw on the northwest, and by an extension along the ice-front, eastward into New York and northeastward into Ontario, but still discharging westward to Lake Chicago and the Mississippi. Later, however, Lake Warren extended into central New York and emptied by way of the Mohawk into the Hudson. The condition of the Erie basin now becomes obscure, for when, after several changes, Lake Iroquois was established, it was merely an enlarged Lake Ontario, and the three upper lakes, now clear of ice, had coalesced into the great, irregular Lake Algonquin, which had lost connection with the Mississippi and discharged into Lake Iroquois, at first probably by the line of the St. Clair and the Erie basin and later probably along the course of the present Trent across the province of Ontario. As the Mohawk outlet persisted, the entire discharge of the lakes was into the Hudson, but whether the Erie basin was filled with water as it certainly was somewhat later, has not been clearly made out.

Lake Algonquin was eventually cut off from its connection with Lake Iroquois and found an outlet by way of Lake Nipissing and the Ottawa River, thus severing the series of lakes into two independent systems, while the lowering of the water level in Lake Algonquin had divided it into three lakes which had very nearly the present outlines of Superior, Michigan, and Huron. The final stage in the history is connected with the Champlain subsidence and reelevation presently to be described.