Page:The Journal of geology (IA journalofgeology21894univers).pdf/367

of the subject in this paper is analytic, the work was primarily synthetic and based directly on field observations and inferences in the magnificent field of the southern Sierra.

Glacial cañons are characterized by several peculiar features: 1. They are U shaped rather than V shaped in cross-profile; 2. Small tributary gorges usually enter at levels considerably above the cañon-bottoms; 3. In longitudinal profile the cañon-bottoms are irregularly terraced—i. e., made up of a series of rude steps of variable form and dimensions,—and some of the terraces are so deeply excavated as to form rock-basins occupied by lakelets; 4. The cañons are sometimes locally expanded into amphitheatres; 5. The cañon-bottom is not always obdurate rock, but may consist of coarse fragmental debris in which individual blocks are as deeply striated and as smoothly polished as are the most solid ledges, though they may rest so insecurely in their positions that a hand can overthrow them; and 6. The volume of glacial debris in moraine and valley deposits is but a small fraction of the cubic content of the cañon from which it was derived.

Of these features the first four suggest that glaciers are most effective engines of erosion, while the last two indicate that glacial erosion is inconsiderable. The source of the apparent discrepance may be sought through analysis of the agencies involved in the development of the four features first enumerated.

Whatever be the physical cause of ice-flow, the motion of a glacier is unquestionably determined by (1) the weight of the ice, (2) the declivity of the channel, (3) the share of potential energy not expended in overcoming internal cohesion, and hence available in producing mass motion, and (4) the friction against bottom and sides of the channel; of which factors the last two (one of which is positive and the other negative) are indeterminate. The united effect of all—i. e., the total sum of potential