CHAPTER X. 



POST-LAHONTAN OROGRAPHIC MOVEMENTS. 



In our sketch of the origin of the Lahontan basin {ante, pf^ge 24), a 

 brief account of the pre-Quaternary faiilts of the region was given. As 

 there stated, the area we are studying has been broken by profound frac- 

 tures, which resulted in the division of the rocks into a great number of 

 orographic blocks. The unequal displacement of these gave origin to the 

 various valleys that were occupied by the Quaternary lake. In the present 

 chapter we wish to direct attention to similar movements of the earth's crust 

 which have taken place since the evaporation of Lake Lahontan. 



The traveler in the Great Basin frequently sees low escarpments in 

 lacustral beds and alluvial slopes, which form irregular lines along the bases 

 of the mountains, and at times cross the valleys. In profile, these scarps 

 present various appearances, as illustrated by the following sections. Where 

 they cross alluvial slopes they usually exhibit a profile similar to that shown 

 at a. In the open valleys they form a small cliff or steep ascent, joining a 



Fig. 36.— Ideal cross-profiles of faulted beds. 



horizontal plain below with a similar plain above, as indicated in section 

 at h. On a mountain side tlio scarp is usvially partially in I'ock and partially 

 in alluvium, as represented at c. The course of tlie scarps is always irreg- 

 ular, and sometimes forms zigzag lines that may be followed for many miles. 



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