Reference has already been made to the fact that the western part of the area here described is driftless, and the line marking the limit of ice advance has been defined. Beyond this line, gravel and sand, carried beyond the ice by water, extends some distance to the west. But a large area in the southwestern part of the state is essentially free from drift, though it is crossed by two belts of valley drift (valley trains) along the Wisconsin and Mississippi rivers. The "driftless area" includes, besides the southwestern portion of Wisconsin, the adjoining corners of Minnesota, Iowa and Illinois. In the earlier epochs of the glacial period this area was completely surrounded by the ice, but in the last or Wisconsin epoch it was not surrounded, since the lobes did not come together south of it as in earlier times. (Compare Plate XXXIII and Fig. 36.) Various suggestions have been made in the attempt to explain the driftless area. The following is perhaps the most satisfactory:[12] [12] Chamberlin and Irving. Geology of Wisconsin, Vols. I and II. The adjacent highlands of the upper peninsula of Michigan, are bordered on the north by the capacious valley of Lake Superior leading off to the west, while to the east lies the valley of Lake Michigan leading to the south. These lake valleys were presumably not so broad and deep in preglacial times as now, though perhaps even then considerable valleys. When the ice sheet, moving in a general southward direction from the Canadian territory, reached these valleys, they led off two great tongues or lobes of ice, the one to the south through the Lake Michigan depression, the other to the south of west through the Lake Superior trough. (Fig. 36.) The highland between the lake valleys conspired with the valleys to the same end. It acted as a wedge, diverting the ice to either side. It offered such resistance to the ice, that the thin and relatively feeble sheet which succeeded in surmounting it, did not advance far to the south before it was exhausted. On the other hand, the ice following the valleys of Lakes Superior and Michigan respectively, failed to come together south of the highland until the latitude of northern Iowa and Illinois was reached. The driftless area therefore lies south of the highlands, beyond the limit of the ice which surmounted it, and between the Superior and Michigan glacial lobes above their point of union. The great depressions, together with the intervening highland, are therefore believed to be responsible for the absence of glaciation in the driftless area. _Contrast Between Glaciated and Unglaciated Areas._ The glaciated and unglaciated areas differ notably in (1) topography, (2) drainage, and (3) mantle rock.