430 ELIOT BLACKWELDER 
Cretaceous sedimentation and orogeny.—lf one may judge from 
the uniformity of the Cretaceous strata. and the prevailing fineness 
of the materials of which they are composed, they were deposited 
upon a nearly level surface distant from any rugged highlands. 
Through much of the period this surface was beneath the sea, as 
MESOZO/C = 
Fic. 1.—Probable condition of the Laramie district in late Cretaceous time. 
shown by marine fossils; but toward the close it appears to have 
become a broad flat river-built plaint with marshes and lakes. It 
seems clear, then, that up to this time the Rocky Mountains were 
not in existence, and that the Mesozoic strata lay like a horizontal 
blanket over all of the Laramie district. 
The events of the initial Rocky Mountain disturbance are not 
satisfactorily recorded in the Laramie district. It has long been 
known, however, that at or near the close of the Cretaceous period 
the flat-lying strata of the Rocky Mountain province were wrinkled 
into their present attitudes, and locally were broken by faults. A 
distinctive negative feature of the Laramie region is the absence of 
the volcanic activity which, in many parts of the West, was a con- 
spicuous accompaniment of this great disturbance. 
Neglecting for the moment the effects of denudation, it may be 
said that these movements resulted in the formation of a low broad 
arch on the site of the present Laramie range, and a more irregular 
uplift, with local faults facing eastward, where the Medicine Bow 
ranges now stand. 
Eocene erosion.—An immediate result of the mountain-building 
epoch must have been a period of active erosion of the rising arches. 
There is no reason to doubt that this accompanied the growth of the 
folds and carved them into rugged highlands from the very beginning 
of the uplift; but it also continued long after the structure of the ranges 
_ 1 The interpretation advocated by Chamberlin and Salisbury (Geology, 1904, No. 
III, p. 152) and others. 
