37b GEOLOGICAL EXCLUSION TO THE ROCKY MOUNTAINS. 



FAULT SOARPS. 48 

 Bj G. K. QlLBBRT. 



Tlie mountains of the Great basin arc. in large part, carved from 

 orogenic blocks uplifted along fault planes. The displacements, which 

 were probably initiated in Mesozoic time, wore continued during vari- 

 ous Oenozoic epochs, and arc now in progress. The steeper laces of 

 most of the mountain ranges are rugged escarpments primarily due 

 to faulting, and at their liases are frequently to be found Smaller 

 escarpments of so recent date that the traces of subsequent erosion are 

 scarcely perceptible. In 1872 the production of such a fault Scarp 

 along the base of the Sierra Nevada was accompanied by an earthquake. 

 In connection with the earthquake in Sonora, Mexico, in 1885, other 

 scarps were produced. In the Salt Lake basin there is no historical 

 record of their formation, but many of them intersect the beaches and 

 deltas of the Bonneville Shores, and some are so fresh that vegetation 

 docs not yet clothe them, and it is hard to believe their ant iquity is 

 measured by centuries rather than decades. They have been found 

 along the bases of a dozen ranges of the Salt Lake basin, but they are 

 most persistent and have greatest magnitude at the western base of 

 the main ridge of the Wasatch chain, where they have been traced 

 almost continuously for a hundred miles. 



As a great orogenic block, separated from another by a fault plane, 

 rises, the debris resulting from its sculpture is thrown upon the block 

 beyond the fault, and rests as an alluvial bank against t lie cliff produced 

 by the faulting. When subsequent movements occur on the same fault 

 plane they are superficially manifested either in the alluvium, or at its 

 plane of junction with the rock. The forms of the alluvium, being 

 determined by the laws of fluvial deposition, are regular, and the cliffs 

 produced by the faulting are thus rendered conspicuous and unmistak- 

 able. Sometimes a single scarp is seen to cross an alluvial slope, rising 

 and falling as the slope rises and falls; sometimes two or more scarps 

 are seen to run parallel to each other, and in such ease the intervening 

 surfaces of alluvium hav. '>e\ attitudes. their tendency being to incline 

 toward the mountain face; sometimes a wedge of alluvium has fallen 

 into the fissure due to faulting, so as to produce a trench on the alluvial 

 surface. 



All these special phenomena are illustrated in the localities to be 

 visited by the Excursion party, and the fault scarps of the Wasatch 

 can also he observed, at a distance, from the windows of the train. 



