Page:The Journal of geology (IA journalofgeology21894univers).pdf/380

reduced by increase of width and depth; though if (as is probable) erosion varies more nearly with the weight than the velocity, its amount will increase absolutely, and the expanded valley will tend in a stronger degree than that measured by the ratio of the inverse volumes to assume the general form characteristic of contracted glacial gorges. As in the contracted gorge, too, lateral effectiveness will remain predominant; but the effective energy of the glacier will be mainly concentrated upon the obstructive angles, spurs, and cusps of the irregular water-carved walls, and the removal of these and the rounding out of the amphitheatre will be in the first work of the glacier. Again, the partial rigidity of the ice-mass will lead to culmination of pressure about the distal extremity of the amphitheatre, and to consequent extension of its boundaries beyond the confluence of the tributary by which its water-fashioned prototype was originated.

It follows that glaciated amphitheatres may be merely water-carved valley expansions modified by temporary ice-action into regularity of contour (as are, for instance, those of the Faeröe Islands^[1]), and that they do not necessarily argue profound glacial erosion.

Summarizing the chief effects of the several agencies involved in the development or the characteristic features of glacial cañons, it appears that temporary occupancy of a typical water-cut cañon by glacier ice will (1) increase the width, (2) change the V to a U cross-profile, (3) cut off the terminal portions of tributary cañons, and thus relatively elevate their embouchures, (4) intensify certain irregularities of gradient in the cañon-bottom, (5) excavate rock-basins, (6) develop amphitheatres, and, in general, transform such cañon into an equally typical glacial cañon. It follows that these features do not necessarily imply extensive glacial excavation or indicate that glaciers are superlatively energetic engines of erosion.