The till is presumably made in part of preglacial soils, but it is more largely composed of rock waste mechanically comminuted by the crccpiiig ice sheets; although the crystalline rocks from Canada and some of the more resistant stratified rocks south of the Great Lakes occur as boulders and stones, a great part of the till has been crushed and ground to a clayey texture.
In this central region, however, it is only by way of exception that the cirques were so far enlarged by retrogressive glacial erosion as to sharpen the preglacial dome-like summits into acute peaks; and in no case did glacial action here extend down to the plains at the eastern base of the mountains; but the widened, trough-like glaciated valleys frequently descend to the level of the elevated intermont basins, where moraines were deployed forward on the basin floor.
This peculiar feature is explained as the result of displacement of the river from a better graded preglacial valley by the Pleistocene ice-sheet, which here overspread the plains from the moderately elevated Canadian highlands far on the north-east, instead of from the much higher mountains near by on the west.
Glaciation has strongly scoured away the deeply-weathered soils that presumably existed here in preglacial time, revealing firm and rugged ledges in the low hills and swells of the ground, and spreading an irregular drift cover over the lower parts, whereby the drainage is often much disordered; here being detained in lakes and swamps (muskegs) and there rushing down rocky rapids.
They are usually fine-textured limestones and shales, lying horizontal; the moderate or small relief that they were given by mature preglacial erosion is now buried under the drift, but is known by numerous borings for oil, gas and water.
By preglacial valleys filled with glacial drift.