Physical features of the Des Plaines Valley . thickness over Joliet and Chicago of over12,000 feet. There is reason to believe, however, that the Greenlandice sheet covers a high mountainous region from which there is a rathersteep descent to the sea; and that the slope of the surface of the icesheet is steeper on that account than it would be over a flat region likeIllinois. So far as this is true it should lead us to reduce the estimateof thickness accordingly. We may reasonably believe, however, thatduring the Illinoian ice invasion the ice sheet was several thousandfeet thick over northeas
Physical features of the Des Plaines Valley . thickness over Joliet and Chicago of over12,000 feet. There is reason to believe, however, that the Greenlandice sheet covers a high mountainous region from which there is a rathersteep descent to the sea; and that the slope of the surface of the icesheet is steeper on that account than it would be over a flat region likeIllinois. So far as this is true it should lead us to reduce the estimateof thickness accordingly. We may reasonably believe, however, thatduring the Illinoian ice invasion the ice sheet was several thousandfeet thick over northeastern Illinois. During the last glacial epoch, how-ever, the ice advanced only a short distance southwest of the Des Plainesvalley, and its thickness over that area was probably not more than a veryfew thousand feet. For a long period, probably a hundred thousand years, this great capof ice was a powerful agency in modifying and reshaping the surfacefeatures of this district. When it finally melted away, on the establish- (a) ? lb) Cot ,;f. J \ i L_ Fig. 7. Section showing (a) residual soil, passing downward into rock, and(b) glacial drift overlying a glaciated rock surface unconformably. ment of the present climate, a wholly new topography appeared. Intwo distinct ways the ice sheet had changed the topography of the sur-face. It had carried away the residual soils and most of the loose rockwithin the zone of surface decay and disintegration, and as a rule itground, scraped, and scoured the firm rock below until its surface wasreduced, in manv places at least, several feet below its former position(See Fig. 7). The smoothing and striating of the roclc surface.—The appearance ofa strongly glaciated surface of rock is so characteristic that it deservesmore than mention. The drift-shod ice, moving slowly along over therock floor and pressing down upon it with tremendous weight, rasps 30 THE DES PLAINES VALLEY. [bull. no. li and scours it. The finely comminuted clay or rock flour
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