Page:The Journal of geology (IA journalofgeology11893univ).pdf/271

In an article in the first number of this journal on the nature of the englacial drift of the Mississippi basin, I endeavored to show by evidence drawn from a wide area of the interior that erratics dislodged from the summits of the hills of crystalline rock in the northern region by the Pleistocene ice-sheet were borne south within the ice in such a way as to be kept separate from the basal material throughout the whole course of their transportation, and that they were at length let down upon the surface of the basal drift at the margin of the ice as a separate deposit. The evidence seemed to force the view that the basal material was not carried upward by transverse ice currents even into the heart of the glacier much less to its surface. The facts there cited seemed to make it clear that there is not only a theoretical but a practical horizon of demarcation between the englacial drift and the basal drift, and that under circumstances of this kind—and they seem to have wide prevalence—there is little or no confusion of the two. Very possibly this conclusion does not hold equally good in very hilly or mountainous regions.

In carrying out into further application this distinction, it seems well to specify the precise sense in which the term englacial is used. It may be applied to any erratic material that, at any time during its transportation, may be enclosed within the ice even though it be essentially at the bottom of the glacier and may have been actually at the bottom a little before and may again be at the base a little later on; or it may be applied, less technically but more significantly, to that only which is embedded in the heart of the ice and borne passively along with it free from basal influences until it is at length brought out to the surface of the terminal slope by the agency of ablation.