Page:The Journal of geology (IA journalofgeology21894univers).pdf/371
In such consideration let the ice be assumed to occupy a previously-formed gorge of the typical V form of water-cut cañons.
The weight of the ice varies directly with its thickness, and accordingly increases progressively from sides to center of the gorge. The tendency of this factor is hence to continually deepen the cañon and to perpetuate the V form.
Three of the four factors into which down-stream impulse may be resolved are of unequal value in different portions of the width of the glacier, and from such inequality the differential flow of ice-streams results; for from sides to center the weight increases uniformly, the available energy increases increasingly, and the friction probably increases less rapidly than the thickness; whence the impulse at the center must ever remain predominant. But if the ice-stream be conceived to consist of a parallel series of longitudinal vertical laminæ (for in the present discussion the vertical variation of flow is immaterial), it is evident that those at the edges will be retarded by the valley-sides, that the mediolateral laminæ will be equally retarded and accelerated by their unequally flowing neighbors, and that the central lamina will be retarded by the more slowly moving ice on either side; and if the mutual interaction of the various laminæ be considered, that the platted ordinates of flow will form a curved figure, and not a triangle homologous with the cross-section of the gorge (fig. 2). Such indeed is the case of differential ice-flow, as empirically established by Forbes, Agassiz, Tyndall, and other observers; though in the V gorge the curve would unquestionably be less flattened than in the U gorges within which the measured glaciers lie. On the whole, the disposition of the second factor must be to most energetically attack the valley-bottom, but at the same time to develop concavity of the valley-sides.
Summarizing, it appears that the general tendency of the intensity element is preëminently to deepen the cañon and slightly to transform the V to a U profile.