acadkmy op sciences] WHEELER SURVEY 39 



However, Gilbert was not the first observer to detect the mam features of the plateau 

 region; that had been done by Newberry, Gilbert's chief in Ohio, who, as geologist of the Ives 

 expedition to the Colorado River of the west in 1857-58, had crossed the plateaus south of the 

 Grand Canyon, and had reached well-grounded views on many points. He saw the funda- 

 mental crystalline rocks deep in the canyon, lying unconformably beneath their heavy sedi- 

 mentary cover; he understood the importance and efficacy of ordinary erosional processes, 

 not only in the excavation of narrow canyons beneath the plateau by larger or smaller streams 

 but also in thebroad recession of cliffs upon the plateau surface; indeed, he regarded the opening 

 of broad upland valleys, such as that of the Little Colorado above its canyon, as "a much 

 grander monument of the power of aqueous action than even the stupendous canon of the 

 Colorado." Great honor is due to one whose vision was so broad! Gilbert had also been 

 preceded by a few years in the plateau province north of the Grand Canyon by Powell, where 

 the senior explorer had gained an understanding of its extraordinary cliffs of erosion and cliffs 

 of fracture somewhat earlier than the junior, although their reports were published in the same 

 year. But there was never any question of priority between these two comrades in science; 

 they shared their facts and their fancies in perfect confidence, and each always felt that his 

 results were at the service of the other. Indeed, their results were so freely interchanged that 

 neither one knew or ever sought to claim just what he had contributed to the total, as will 

 appear later. 



PROCESSES AND PRODUCTS OF STREAM EROSION 



The processes of erosion are so intimately associated with the forms that they produce 

 that the two may be well considered together as we select the most significant items concerning 

 both topics from Gilbert's early reports. As to processes, emphasis was given to the importance 

 of suspended sediments in river scouring, the source of the sediments being beautifully analyzed 

 (71, 72). The contrast between the plateau province and the Great Basin with regard to stream 

 erosion was clearly brought out; in the plateaus the valleys have been formed by erosion and 

 are still being deepened, while the tables between the valleys are residual; in the Great Basin 

 the intermont valleys are residual between mountain ranges uplifted in parallel lines, and the 

 valleys, initially of greater depth, have for a long time been filling with detritus eroded in and 

 transported from the mountains (63). Among other items of interest is an appreciative but 

 brief account of piedmont detrital fans, features that, although familiar enough to-day as 

 characterizing the basin ranges, were little known to eastern geologists and geographers 50 

 years ago: 



The debris of the mountain is brought to its margin in gorges or cafions, from the mouths of which it is 

 spread in broad, low talus-cones, which make up the foot-slope. The stream that flows from the canon, whether 

 transient or perennial, distributes the detritus over the cone by shifting its bed from time to time as the sediments 

 clog it. As the canon wears deeper at its mouth, and the stream discharges at a lower level, the upper portion 

 of the cone is excavated and a new one is modeled with lower apex and lower grade (65). 



To-day, a writer might pass over such matters as familiar to the point of being trite, but 

 such was not the case 50 years ago. 



It was, however, chiefly from the plateau province that Gilbert took his examples of erosion. 

 He evidently reveled in the opportunity for the investigation of surface foims that this extraor- 

 dinary region afforded. He wrote of it: 



The canons of the Colorado and of its tributaries, and the country which they intersect, are unsurpassed 

 as a field for the study of river denudation. Not merely do they exhibit the grandest and most impressive results, 

 but they show the agent by which they have been wrought, still in vigorous activity (67). 



Hence, although the theory that profound canyons have been eroded by their rivers was 

 not regarded as a fully established truth by all geologists at the time of the Wheeler survey, 

 Gilbert, like his predecessors and associates in western exploration, found the field evidence 

 for this theory so convincing that it was illustrated rather than discussed in his reports. The 

 most cleftlike canyon that he examined was that of the Virgin River, now included in Zion 

 National Park, near the western border of the plateau province, about 100 miles north of the 

 Grand Canyon. While passing through this cleft, 2,000 feet deep in massive sandstones, "the 



