Page:The Journal of geology (IA journalofgeology21894univers).pdf/374
of erosion. Now ordinary valleys, whether occupied by streams or glaciers, are of progressively diminishing declivity from source to terminus; ordinary glacial valleys exhibit successive zones of active erosion, feeble erosion, slight deposition, and abundant deposition in passing from their upper reaches to the broader valleys into which they embouch or upon the plains with which they merge; and in such cases the down-stream impulse must wane to practically nothing at the extremities of the glaciers, and must hence greatly fail of the weight. It follows that at some point (or at diverse points) in every extended glacier-course the components weight and impulse are equal at the centre of the glacier.
Since glacier ice but slightly approaches perfect fluidity and the flow of the center is greatly retarded by the sides, the ratio of impulse to weight (and with it the effectiveness) continually and largely increases from center to sides: if the central effectiveness be just zero, that at the sides will nevertheless remain important; if it be minus centrally, it may still be considerable laterally; and however great may be its value at the center, it must have far greater value at the sides. The disposition, then, will ever be to protect the bottom and equally to attack the sides of the valley; and since the down-stream impulse of the several parallel laminæ forms a curve when platted, so will the disposition also be to form concave valley-sides.
Of the unknown factor in the effectiveness term, the first component (rock-crushing) can be but trivial in the ordinary case, while the second (pressure-liquefaction) exercises antagonistic influences. It may, accordingly, be safely neglected.
Collectively, the tendencies of the third element of glacial erosion are (1) to effectually protect the valley-bottom throughout a considerable portion of the glacier course, (2) to develop the U form of canon, and (3) to materially increase the relative width of the gorge.
The fifth feature of glacial cañons is explained by the operation of this element, and in turn establishes the importance of the element.