Page:The Journal of geology (IA journalofgeology11893univ).pdf/272

It is clear that all erratic material as it was brought to the front edge of the ice appeared either on the surface or at the base. There is here a sharp physical horizon of demarcation. If the material that had been basal some distance back from the edge was carried up to the surface, or carried up so far into the body of the ice that the surface was brought down to it by ablation near the border, it is evident that it must have become commingled with that which had been englacial or superglacial from the moment it was dislodged from its parent hills, and hence this horizon of demarcation would not distinguish between the two classes of material as such. The distinction would rest upon mode of transportation and deposition. But if the interpretation of the article referred to is correct and holds good generally in similar regions, the horizon becomes a plane of separation between the classes of material as well, and assumes much importance in practical glaciology. It was, however, obviously not an absolute plane of demarcation, even at the border of the ice, and when we attempt to apply it to sections lying farther back, it needs some qualification.

Without doubt material which was picked up by the ice along its base was thrust up into it to greater or less heights. As a particular instance, beds of rock which were inclined upward toward the oncoming ice were obviously disposed to thrust themselves into it as they were being tilted out of their positions. They appear to have rotated upon their lower edges, as upon a hinge, and were probably only removed from their places after they had been turned into a vertical position or perhaps somewhat beyond it. They were then almost wholly embedded in the ice, and so, in a limited sense, they were englacial. So also it is extremely probable that, in the case of undercut ledges, sharp ravines, narrow gorges, and similar very abrupt inequalities in which the surface was suddenly depressed, there was more or less overriding of the basal currents of the ice and consequent incorporation or overplacement of the material held in the bottom of the overrunning portion. But notwithstanding the fact that this material became englacial in a limited sense, because