Relative agency of glaciers and sub-glacial streams in the erosion of valleys

Abstract

IN some remarkst which I made at a meeting of this Society in April, 1873, I stated that my observations among the glaciers of the Alps during the previous summer had led me to the conclusion "that glaciers were not the principal agents in the excavation of valleys." I have since had the opportunity of spending two summers more among those glaciers, and the observations which I made have not only confirmed my previous conclusion, but they have also furnished me additional evidences of the excavating power of sub-glacial streams. This time I was more successful in getting underneath the ice than before, particularly upon the right side of the Great Aletsch Glacier where it passes the cliff near the Bell Alp Hotel. The way glaciers usually move over the OI'dinary roches moutonnees, bridging the hollows between them without conforming to all the inequalities of surface, has been macle so well known that additional description is unnecessary here. Under these conditions the glacier does not act upon the lowest surfaces of rock beneath it, and these show by their roughness and irregularity that they were not shaped by its action. It, therefore, becomes evident that in snch places some power must have acted or is now at work lower than the surfaces upon which the glacier moves. Under the edge of tbe Great Aletsch Glacier I observed in a few places, that pieces were being broken from the lee edges of the roches moutonnees by the pressure concentrated upon certain stones or bowlders which had reached these edges in their progress under the ice, but I was not successful in my search for like phenomena in connection with other glaciers. But this action, even if we could suppose it to be sufficiently common, would serve to break away only the same prominent portions of the rock which the glacier abrades. The ice of the glacier, however, is sufficiently plastic to conform to certain kinds of irregularities of surface, and of one of these there are good examples at the above-mentioned locality. There are long, narrow ridges, the trends of which are the same as the strike of the rock and nearly parallel with the direction of the motion of the glacier. A. longitudinal section of one of these ridges gave an outline like that of an elongated roche moutonne, while a transverse section showed quite a regularly