academy of sciences] GILBERT'S LAST STUDY 299 



graphic characters have been adduced in support of the view that the [Wasatch] escarpment was created by 

 faulting. In my judgment the physiographic evidence is adequate, but not all are prepared to accept it, and 

 for this reason I shall begin the discussion of the subject by presenting evidence other than physiographic. 

 Afterward several classes of physiographic evidence will be taken up. 



STRUCTURAL EVIDENCE OF FAULTING 



After a preparatory statement concerning the nomenclature of faults and their physical 

 effects, a number of localities are given where such inferred effects are duplicated in facts, 

 such as slickensided surfaces and sheets of sheared rock fragments, in a plane on the basal 

 slope of the dissected mountain front. Some of the observed effects appear to have been pro- 

 duced under heavy pressure at a great depth, others at a small depth. 



Despite the great contrast between the two groups of facts, they are really harmonious — under the hypothesis 

 that faulting on this plane created the entire escarpment. If the faulting began when the tops of the valley 

 block and mountain block were at about the same level, that part of the footwall which is now visible at the base 

 of the escarpment lay initially at a depth beneath the surface roughly equivalent to the present height of the 

 escarpment, plus the amount of degradation of the mountain block, and the frictional effects in that part of 

 the footwall were conditioned by the corresponding pressure. As faulting progressed the pressure on that part 

 of the footwall gradually diminished, with the result that deformation within the wall rock ceased after a time. 

 . . . That part of the fault rock near what is now the base of the escarpment was worked over, with gradually 

 diminishing pressure. . . . The phenomena are all explicable on the hypothesis that they were produced by 

 faulting and that the western fault block is concealed by valley deposits. So far as I know there is no alterna- 

 tive hypothesis to be discussed. 



An important modification of earlier statements regarding the simple pattern of the fault 

 line along the mountain base is then introduced: 



In characterizing ranges which have a bold front due to faulting, I wrote in 1890, "the line separating the 

 rock of the mountains from the alluvium of the valley is simple and direct." The statement is perhaps defensible 

 because the context compares such a line with the corresponding line on the back slope of a range consisting of a 

 tilted block; but it is fair to say that my conception of the flexuosity which may affect fault outlines has been 

 materially enlarged by more recent studies of the Wasatch front. . . . The line of the fault is crooked. It not 

 only curves through considerable arcs, but in two places turns sharply at right angles; and in three other places 

 its course runs for miles athwart the general direction of the fault as a whole. Some of the deflections, but not 

 all, are evidently associated with preexistent structural features. . . . One of its greatest deviations is in passing 

 around the granite body north of Alpine. ... It is evident that great simplicity of basal outline is not an essen- 

 tial characteristic of faulted range fronts. On the other hand, the phrase "simple and direct" may properly be 

 used in a relative sense when basal outlines of faulted fronts are compared with rock basins [bases?] shaped 

 chiefly by erosion. The erosive outline ... is greatly influenced by the arrangement of drainage lines and by 

 the distribution of strong and weak rocks; the fault-block outline is independent of drainage lines and its adjust- 

 ment to rock structure is partial and local; and these differences serve to make the erosive outline comparatively 

 tortuous, and the fault outline comparatively direct. 



The bearing of these facts and inferences on the basin-range problem is then summarized. 



The structural data set forth in the preceding sections as proof that a fault created the western face of the 

 range are mainly new, receiving here their first publication. They were unknown to those who early recognized 

 the fault by means of its physiographic features, and to those who afterward discussed the physiographic evi- 

 dence. So as a matter of history they constitute supplementary evidence, and their function has been to con- 

 firm a conclusion already reached. They are here given first place because they are supposed to appeal to the 

 understanding of those geologists whose opinions are little affected by physiographic evidence and the physi- 

 ographer's reasoning. In the following pages I propose to present and discuss the physiographic data, not 

 primarily for the purpose of adding evidence of the existence of the fault, but in order to determine if possible the 

 value and also the limitations of physiographic characters as criteria for the discrimination of fault-block 

 mountains. 



A number of mountain salients, lower than the main mass of the range, from 5 to 7 miles in 

 length along the range front and 3 or 4 miles in breadth, are then discussed, described under the 

 name of "fault-block spurs," and explained as "the upper parts of crustal blocks that are 

 separated by normal faults from the Wasatch block on one side and from the valley block on the 

 other, so that they stand lower than the one and higher than the other." Three of these spurs 

 " are capped in whole or part by alluvium from the Wasatch range, alluvium that was once part 

 of the piedmont apron of the range. . . . The presence of this alluvium and its dislocation 

 from the general modern alluvial apron of the range show that the spur blocks were once so 

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