academy of sciences] GILBERT'S LAST STUDY 301 



Every canyon broadens downstream, flaring widely as its stream leaves the strong bed. The head of each 

 minor stream is in a reentrant angle of the escarpment. The outline or ground plan of each escarpment is a, 

 series of convex scallops, large and small, the larger scallops being spaced by the major drainage and the smaller 

 by the minor drainage. The canyons and gullies of the drainage correspond to the canyons of the Wasatch front, 

 and instead of being truncated at the basal outlines of the escarpments they determine reentrants of the escarp- 

 ments. The ridges between lines of drainage correspond to the ribs of the Wasatch, and instead of being trun- 

 cated along the bases of the escarpments they determine the salient scallops of the escarpments and their 

 bases. . . . Such structures as faults and folds, instead of being truncated by the escarpments, serve as important 

 conditions for the control of the courses of the escarpments. 



It is therefore concluded that — 



the forms of escarpments created by stream erosion are largely controlled by the structure of the eroded terrane 

 and by the pattern of its drainage. Because flowing water is the dominant agency in the sculpture of the land, 

 the form types it produces constitute the norm, and departures from its types call for explanation. When 

 therefore so important a form element as the basal outline of an escarpment is found to be independent of the 

 rock structure, independent of the drainage pattern, and independent of the distribution of inter-canyon ridges 

 (which distribution is of course a function of drainage pattern) , so that the basal outline appears to cut off or 

 truncate these features, the phenomenon is one requiring explanation. 



In the case of the Wasatch escarpment the explanation is at hand, for the base line of the escarpment is 

 also the line of a great fault, characterized by an abundance of the phenomena created by fault friction and 

 followed through long distances by [alluvial] piedmont scarps. The escarpment is evidently the face of an 

 uplifted tectonic block, and the adjacent depressed block is buried under waste from the sculpturing of the 

 uplifted block. The structure elements of the uplifted block end at the fault line because actually cut off by 

 faulting. The ribs end where the block from which they were carved ends. The canyons also end where the 

 block ends, and they end without flaring because the upfaulting is too rapid to give their streams time for 

 lateral corrosion. 



The evanescent character of some of the physiographic evidence for block faulting is clearly- 

 stated : 



The frontal facets to which attention has thus far been directed are all, in a topographic sense, young. 

 Because the Wasatch block is actively rising, they have not been long exposed to wasting agencies; and because 

 their rocks are strong they have wasted little. The Wasatch facets in weak rocks are comparatively worn and 

 indefinite: and the original facets of [other] block mountains, now old and stagnant, have been worn out and 

 replaced by rib ends of a different type. . . . The Wasatch escarpment is characterized by topographic youth. 

 The fault that has produced it is an active fault, and the fault's activity is vigorous. . . . The youth and 

 strength of the escarpment give it special distinction as the representative of a type. ... If the activity of 

 the Wasatch fault should cease, the processes of erosion would gradually mutilate and eventually obliterate the 

 characteristic features of the escarpment. 



CONFIRMATION OF PHYSIOGRAPHIC BY STRUCTURAL EVIDENCE 



It is probable that those who have expressed the most pronounced opposition to the fault- 

 block theory of the basin ranges proper would not take exception to the faulting of the Wasatch 

 Range, as thus demonstrated; and they might indeed fairly enough argue that the faulting of the 

 Wasatch did not prove the faulting of the ranges farther west. But that the faulting of one 

 proved the faulting of the others was not the least Gilbert idea. His idea was that if physio- 

 graphic evidence of faulting in the Wasatch is shown to be trustworthy by its systematic 

 association with structural evidence of faulting, then physiographic evidence may be accepted 

 as proof of faulting when found in other ranges where structural evidence is wanting. 



Because the physiographic characters of the Wasatch did not prove misleading it seems proper to infer 

 that similar characters may be used as criteria for the recognition of faults at the margin of other ranges. 



It was evidently his intention to bring forward in later chapters of his report the physio- 

 graphic evidence that he had found in 1901 for the block faulting of other ranges; but those 

 later chapters were never written. 



THE WASATCH FAULT BLOCK 



The faulting of the Wasatch being thus doubly assured, important conclusions are an- 

 nounced as to the attitude of the fault surface and the measure of the displacement upon it. 

 The dip of the fault surface near the mountain base at seven localities varies between 29° and 

 45°, and averages 35°. The throw, or vertical component of the displacement, near Twin 



