The evidences of such interglacial stages, and the means by which the different drift sheets are told apart, are illustrated in Figure 361.  Here the country from N to S is wholly covered by drift, but the drift from N to m is so unlike that from m to S that we may believe it the product of a distinct ice invasion and deposited during another and far later glacial stage.  The former drift is very young, for its drainage is as yet immature, and there are many lakes and marshes upon its surface; the latter is far older, for its surface has been thoroughly dissected by its streams.  The former is but slightly weathered, while the latter is so old that it is deeply reddened by oxidation and is leached of its soluble ingredients such as lime.  The younger drift is bordered by a distinct terminal moraine, while the margin of the older drift is not thus marked.  Moreover, the two drift sheets are somewhat unlike in composition, and the different proportion of pebbles of the various kinds of rocks which they contain shows that their respective glaciers followed different tracks and gathered their loads from different regions.  Again, in places beneath the younger drift there is found the buried land surface of an older drift with old soils, forest grounds, and vegetable deposits, containing the remains of animals and plants, which tell of the climate of the interglacial stage in which they lived.

By such differences as these the following drift sheets have been made out in America, and similar subdivisions have been recognized in Europe.

5 The Wisconsin formation 4 The Iowan formation 3 The Illinoian formation 2 The Kansan formation 1 The pre-Kansan or Jerseyan formation

In New Jersey and Pennsylvania the edge of a deeply weathered and eroded drift sheet, the Jerseyan, extends beyond the limits of a much younger overlying drift.  It may be the equivalent of a deep-buried basal drift sheet found in the Mississippi valley beneath the Kansan and parted from it by peat, old soil, and gravel beds.

The two succeeding stages mark the greatest snowfall of the Glacial epoch.  In Kansan times the Keewatin ice field slowly grew southward until it reached fifteen hundred miles from its center of dispersion and extended from the Arctic Ocean to northeastern Kansas.  In the Illinoian stage the Labrador ice field stretched from Hudson Straits nearly to the Ohio River in Illinois.  In the Iowan and the Wisconsin, the closing stages of the Glacial epoch, the readvancing ice fields fell far short of their former limits in the Mississippi valley, but in the eastern states the Labrador ice field during Wisconsin times overrode for the most part all earlier deposits, and, covering New England, probably met the ocean in a continuous wall of ice which set its bergs afloat from Massachusetts to northern Labrador.

We select for detailed description the Kansan and the Wisconsin formations as representatives, the one of the older and the other of the younger drift sheets.