Page:The Journal of geology (IA journalofgeology21894univers).pdf/369

(ceteris paribus) measured by the ratio between weight and downstream impulse; for manifestly, if the weight be in excess, the predominant tendency must ever be to fix and retain in their places all bowlders, pebbles, sandgrains, and smaller particles; when the weight and impulse are as w and v in the diagram (fig. 1) their resultant will tend to retain rather than remove

Fig 1.

such fragments, and transportation will be limited to that due to friction and sub-glacial water; when the factors are equal, as are w′ and v′, their resultant will tend equally to retain and to remove particles, and the effects of friction and flowing water will be counteracted by the greater specific gravity of rock than ice; and when the ratio is as w″ to v″, the disposition will be to overturn and sweep forward all fragments. Also, the weight of ice tends to produce crushing of the rock in a degree probably increasing increasingly with its value. Finally, with increased weight will go increased pressure-liquefaction of the ice, and from this will result the antagonistic effects of reduced friction and augmented transportation. The last two agencies are variable, only very roughly determinate in the ordinary case, and generally of inconsiderable value. They may be thrown together as an unknown factor which, in conjunction with the predominant first agency, constitutes the effectiveness of glacial erosion at any point.

The three elements of intensity, friction, and effectiveness, therefore, determine the rate of glacial erosion. To more succintly express their relations, let—