Page:The Journal of geology (IA journalofgeology11893univ).pdf/282

It is probably safe to say that in selected instances at least 90 per cent. of the material was derived from the Paleozoic series and more than half of this from the vicinity. This is, however, too large an estimate for the average, but the local constituents were never seen to be other than pronounced if not predominant. Material of local derivation also enters into the constitution of the sand and clay as well as the coarser material showing that the hillocks are made up not only of the glacially ground rock-fragments, but of the glacial grindings. The whole aspect of the material of these kames is so strikingly in contrast with that of the superficial bowlder belt and points so definitely to their derivation from the common sheet of subglacial till as to seem to put beyond doubt the view that they are quite strictly basal in formation.

Osars of the typical variety have comparatively few representatives in the plain tracts of the interior, but several well characterized instances occur. It is notable that, in most of these instances, as pointed out by Mr. Leverett, who has perhaps carefully studied a larger number of them than anyone else, they often lie in river-like channels cut into the till sheet of the region. There are perhaps a dozen of these that have been studied, varying in length from a few miles to about fifty miles. These channels have the characteristics of river troughs, and usually stand so related to the margin of the ice as to seem to indicate that they were lines of subglacial drainage during the same glacial stage as that in which the osars were formed. These channels are so related to the surface slopes that they could not have been formed by free open-air streams. The restraining aid of ice seems necessary. While no demonstration of the history of their formation can be claimed, the most plausible explanation appears to be that the river-like channels were cut by subglacial streams at a time when the urgency of the ice was such as to compel basal cutting, and that, subsequently, when the pressure of the ice was less insistent, and its motion feebler, the draining stream was permitted to fix its channel in a tunnel cut in the under surface of the ice, which