42 GROVE KARL GILBERT— DAVIS [M " M0IM [ ££ , §3ct 



" Explorations" of the river, must be regarded as the more explicit of the two. Gilbert's recog- 

 nition of the general relation between stratigraphy and topography is nicely shown in his numer- 

 ous columnar sections, referred to above, for they are indented on one side so as to distinguish 

 the cliff-making and the slope-making strata; an excellent device that might well be generally 

 followed. With regard to the Grand Canyon, the general statement, "In every profile of the 

 canon the positions of the hard-massive beds are marked by precipices, and of the soft by 

 slopes" (68), is followed by detailed descriptions and illustrated by true-scale cross sections 

 of the canyon in different parts of its length (69) ; and these sections include also several exam- 

 ples of a third element of canyon form, namely, cliff-top platforms or terraces, which are so 

 clearly represented that, although they receive no specific explanation in the text, it can hardly 

 be doubted that they were understood to have resulted from the faster recession of medium- 

 strong cliff-making strata that overlie the back of the platform as compared to the slower 

 recession of the stronger cliff-making strata that underlie its front. Regarding the intricate 

 pattern of the side canyons and of the cliffs in their separating spurs, as seen in plan, it is natural 

 that nothing was said in the early days of physiography. 



Another and less generally understood principle of valley erosion was also recognized: The 

 transformation of a narrow canyon into a well-opened valley where an eroding river, as it is 

 followed downstream, passes from a body of strong cliff-making strata into an underlying body of 

 weak slope-making strata. Thus the change from the cleftlike Zion Canyon of the Virgin, cut 

 in massive sandstones, to the following open and habitable stretch of its valley is explained as 

 occurring where the river cuts down into the underlying variegated marls before it again canyons 

 in the still lower limestones (79) ; similarly, the Colorado, crossing the same series of weak strata 

 where they are gently inclined against its flow, has a broadly opened valley at the mouth of 

 Paria Creek, between two narrow and deep canyons, the one upstream being cut in the massive 

 overlying sandstones, and the one downstream in heavy underlying limestones. 



RETREATING ESCARPMENTS 



The plateau province is well described as being "divided into a series of great terraces, by 

 lines of cliffs trending east and west, facing south, and composed severally of the harder strata 

 of the geological series" (44) from Tertiary to Carboniferous; the broadest terrace being the 

 lowest member of the series in which the Colorado Canyon is cut. Each line of cliffs is described 

 empirically in some detail. In view of the clear explanations already given for the erosion of 

 canyons and for the forms of their walls, it is surprising to find here a less explicit discussion of 

 the great terrace-edged cliffs than was given by Powell who, in an elaborate account of retreat- 

 ing escarpments, contrasted them with fault cliffs in the following suggestive terms : 



The cliffs of erosion are very irregular in direction, but somewhat constant in vertical outline; and the 

 cliffs of displacement are somewhat regular in direction, but very inconstant in vertical outline. 2 



A corresponding statement is made by Gilbert only in explaining a specific fault scarp, as 

 noted in the next section; but the context of his report leaves no doubt that the problem here 

 considered was well understood and that the plateau terraces were seen to be the result of a 

 vast denudation. Indeed, a sentence that is buried in the abstract of a paper on the "Recency 

 of certain volcanoes of the western United States," presented to the American Association for 

 the Advancement of Science in 1874, suffices to show that, had not Gilbert chosen for some 

 reason to make his report brief, he could have been as explicit as Powell; for while explaining 

 the long duration of volcanic activity in the West, as indicated by the amount of erosion that 

 had taken place between the earliest and the latest eruptions, he introduced an excellent phrase 

 regarding the Triassic cliffs that face southward across the Carboniferous plateau: 



An erosion of infinite slowness is carrying these cliffs back toward the north and thus increasing the Car- 

 boniferous area at the expense of the Triassic. 



This subject will be further considered below in the analysis of Gilbert's views on the 

 degradation of highlands to lowlands, and on the occurrence of more than one cycle of erosion 

 in the plateau region. A single but highly significant passage may be quoted here, as it shows 



1 Explorations of the Colorado Eiver of the West, 1875, 191 



