MID CRETACEOUS MOVEMENT. 27 



harder calcareous and arenaceous layers, is such that, in the absence of 

 any discrepancy of angle, unconformity by erosion would not be readily 

 distinguishable. Since the discovery in this field of unmistakable evi- 

 dences of considerable erosion following- the deposition of the Niobrara 

 Cretaceous, attention has been called to facts indicating that, throughout 

 the area of the Great Plains at least, there was a general elevation produc- 

 ing a temporary recession of the ocean waters, which was followed in 

 places by considerable erosion before the area was again depressed below 

 ocean-level. 



In the Denver Basin area the principal evidence of this movement is 

 found in the Golden arch, which, according to Mr. Eldridge's observations 

 and inductions, must have been elevated by it some 9,500 feet, so that its 

 cross-section widened to 21 miles from north to south and the involved 

 strata up to the top of the Niobrara were crumpled into minor folds. In 

 the erosion which followed, the whole thickness of the Niobrara, Fort 

 Benton, Dakota, and whatever may have been left from previous denuda- 

 tions of the earlier Morrison and Wyoming formations, was entirely removed 

 from the crest of the arch. The thickness of material thus removed from 

 the crest of the arch Mr. Eldridge estimates at something over 1,000 feet. 

 Over the Boulder arch the present disposition of the strata indicates 

 the probability of an earlier movement at the close "I the Dakota, this 

 formation apparently having been eroded, to a certain extent, before the 

 deposition of the Benton (days. The evidence of this movement is of less 

 conclusive character than that of the post-Niobrara movement, and its 

 effects were at best very local in their character, so that it max be passed 

 over without further consideration. 



General depression followed the erosion of the post-Niobrara elevation. 

 It has been suggested that this erosion may have been submarine and due 

 to the action of strong long-shore currents in not yet consolidated material 

 recently deposited on the ocean bottom. This depression lasted during 

 a large part of Montana time and was probably followed bv a gradual 

 elevation and shallowing of the waters during Fox Hills and Laramie 

 time. 



