12 THREE GEOLOGICAL PROVINCES. 



about twenty-five thousand feet. From flank to flank the flexure is about 

 fifty miles, but varies much in width. We find on either flank, many miles 

 from the axis, a line of maximum flexure, which line presents a subparallel- 

 ism with the meandering axis. These lines have the effect of two mono- 

 clinal flexures in opposite directions, separated by the broad table, diversified 

 "by elevated valleys and peaks of which the great mass of the Uinta Moun- 

 tains is composed. But the portion between these monoclinal flexures or 

 lines of greatest flexure is itself gently flexed. In many places that which 

 I have called the line of greatest flexure is indeed a fault, in one place 

 on the north side of the Uinta Mountains having a throw of twenty thousand 

 feet. On the south side the line of greatest flexure is very irregular, being- 

 complicated in some places by faults having uplifts opposed to the down- 

 throw of the flexure. On either side the great displacement is partly by 

 faulting, partly by flexing, and either flank is a zone of diverse displace- 

 ment where the strata are faulted, flexed, twisted and contorted in many 

 ways. 



The character of these displacements in the Uinta Mountains is illus- 

 trated in Plates 1, 2, and 3 of the Atlas, and in a subsequent chapter the 

 subject will be more fully discussed. 



The simplest topographic forms produced by such displacements under 

 conditions of erosion in general outline, are plateaus with gently rounded 

 summits and abrupt shoulders on the flanks; but such general outline is 

 often modified by the corrasion due to antecedent or superimposed drain- 

 age; that is, by the corrasion of streams that head in remote regions and 

 pass through these uplifts either longitudinally, transversely or obliquely, 

 as in the case of Simple Anticlinals.* 



There are other modifications which sometimes greatly obscure the 

 general topographic outline due to consequent drainage, i. e., the local 

 drainage which is due to the upheaval itself and which produces inter- 

 esting 



CONCOMITANT FOEMS. 



1. Subsidiary Plateaus. Sometimes the streams which head near the 

 axis of such an upheaval, as they meander to the flanks, excavate valleys 



* For an explanation of what is meant by antecedent and superimposed drainage, the reader is 

 referred to the Report on the Exploration of the Colorado River and its Tributaries, page 160, ct scq. 



