Page:The Journal of geology (IA journalofgeology11893univ).pdf/422

Former outlets should bear a definite relation to neighboring shore lines and to sedimentary deposits. The channels leading from former points of discharge merit examination, as here again changes of level may perhaps be detected in the gradients of stream terraces.

Most of the ancient outlets thus far recognized lead southward, but as previously mentioned, a former channel of discharge north of Lake Superior has recently been reported. If this observation is confirmed, it will have an important bearing on questions relating to changes of level and to the position of the ice front during the later stages in the retreat of the glaciers.

9. Probable effects of a retreating ice sheet on drainage. The generally accepted conclusion that glaciers advanced southward and occupied the Laurentian basin during the Glacial epoch and retreated northward toward the close of the epoch, is sustained by a vast body of evidence. As the ice sheet withdrew it left a superficial deposit frequently one or two hundred feet thick over nearly all of the region it abandoned, and pre-glacial drainage lines were obstructed and mostly obliterated. As long as the slope in front of the ice was southward, the drainage from it found ready means of escape, but when the slope was northward towards the ice front, the drainage was obstructed and lakes were formed.

We have good reasons for believing that the topography of the Laurentian region was essentially the same at the close of the Glacial epoch as it is now, but the broader question of continental elevation is less definite. The inequalities of the surface being essentially as we now find them, it would follow that the first lake formed when the ice retreated to the north of the divide running through central Ohio and central New York, would be small and dependent on minor features in the relief of the land, and would discharge southward. As the ice retreated, the lakes would expand and become united one with another and the larger lakes thus formed would still find outlet across the southern rim of the basin. As the glaciers continued to retreat lower and lower, passes would become free of ice and the lakes