The Post-Cretaceous Movement. 



The post-Cretaceous movement, as has been almost universally recognized, 

 was that which produced the main plication and faulting and played the 

 most important part in determining the present orographic features of the 

 Rocky Mountain region. But, as it is evidenl that these features had been 

 in a great extent already outlined in the movements that went before, it is 

 also more than probable that the post-Cretaceous folds and faults have been 

 further emphasized along the principal lines of disturbance in the less violent 

 movements that have affected the region since, even into very recent times. 

 It is therefore manifestly impossible to determine with absolute accuracy how- 

 much of the present displacement of Cretaceous beds in folds and faults was 

 produced in the first post-Cretaceous movement and how much in those that 

 have supervened in Tertiary and Recent times. That during this movement 

 the tangential thrust or force of compression was very intense is proved by 

 the fact that in very disturbed regions the upper beds of a series, upturned 

 against the flanks of an ancient island, often stand at steeper angle than the 

 lower beds of the same series, producing thus something similar to the fan 

 structure observed in the Swiss Alps. 



The character of the sediments deposited during the periods immediately 

 preceding this movement, which show gradually shallowing waters during 

 the Fox Hills period, culminating during the Laramie in an entire change 

 of its fauna through brackish-water into fresh-water forms, indicates a 

 gradual elevation of the land until barriers similar to and perhaps more or 

 less corresponding with those formed during the Jurassic movement cut off 

 the whole interior region from the ocean. It might naturally be expected 

 that during such elevation the shore-lines of succeeding stages would recede 

 somewhat, and such Dr. White : slates to have probably been the case with 

 the eastern shore-line of the Cretaceous ocean in the < rreal Plains depression, 

 which, he considers, alter reaching its greatest extension during the Niobrara 

 was carried westward during late Cretaceous times. In the Rocky Mount- 

 ain region, where erosion and denudation have naturally been greater than 

 in the plain regions, it is more difficult to determine the original extent of 

 the beds last deposited previous to the orographic movement, since these 

 were necessarily the lirsl to suffer abrasion and denudation, which would 

 have carried their outcrops further hack from the original Bhore-line of the 

 continental islands than those of the Bubjacent beds. Still, some idea of the 

 probable extent of the Laramie deposits can be formed by considering to 



what extent they -till occupy the great valley depressions formerly covered 



by the < Iretaceous -■ as, since there denudation would have been less uniform 



and thorough than on the mountain Blopes and ridges. 



* Hayden'a Eleventh Repot I (for 1877), p. 

 (280) 



