. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 312 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. comparatively short time swing away, after having altered its course more or less in a fruitless eflfort to wear back the obstacle. The ledge thus comes to determine a cusp in the terrace front. A salient of this kind may be called a defended cusp, in distinction from the accidental or free cusps described in the previous sections; the ter- race behind it cannot be destroyed by the sti'eam. Following the colloquial style often adopted for field descriptions, a terr
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 312 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. comparatively short time swing away, after having altered its course more or less in a fruitless eflfort to wear back the obstacle. The ledge thus comes to determine a cusp in the terrace front. A salient of this kind may be called a defended cusp, in distinction from the accidental or free cusps described in the previous sections; the ter- race behind it cannot be destroyed by the sti'eam. Following the colloquial style often adopted for field descriptions, a terrace of this kind is sometimes entered in my notes as a " cau't-be," in contrast to the low " uot-yet " terraces of Figure 22. It should be noted that the ledge here considered does not deter- mine the depth to which the river may work ; the rock is exposed. Fig. 23. only in the river bank and enters but a little distance into the channel. The slope of the river and the depth to which it has cut at this or any other point in its course are assumed to be determined in all cases thus far detailed by the maintenance of an essentially graded channel with respect to some controlling baselevel further down stream ; the sea at the river mouth, a larger river into which the smaller stream enters, or a broad sill of rock that stretches all across the channel, somewhere further down the valley. When the withdrawing stream swings back again at a lower level, as in Figure 24, it cannot often under-cut and destroy all of the terrace on whose back border the first ledge rises, because, as has been noted, the slope of the ledge is seldom so steep as that of the terrace scarp. A second encounter with the ledge will usually be. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of C
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Keywords: ., bookauthorha, bookcentury1800, bookdecade1860, booksubjectzoology
