FOOTHILL STRUCTURE. 



47 



The beds were probably deposited on ;i shelving shore; thai is, the 

 slope of the contact of sedimentary beds with the underlying complex of 

 crystalline rocks was not vertical, but decreased in angle with the distance 

 from the mountains. Hence a vertical upthrust of this complex could not 

 produce the abrupt transition from tin- vertical to the horizontal position in 

 the overlying beds thai is shown near D in diagram b. 



Tangential pressure, or a force of compression acting in a nearly 

 horizontal direction, seems to afford a more reasonable explanation of the 

 observed phenomena of deformation in the foothill region, and accounts 

 readily for most of them, though a certain amount of differential vertical 

 movement seems to be required for certain phases. 



Fio. 1— Upturning of strata along base of range. 



The observed phenomena to be accounted for may be briefly enumer- 

 ated as follows: 



I. Eastward dip of the beds away from the mountains. Tllis dip illClVaseS ill SteeplleSS 



as one ascends in geological horizon, or proceeds eastward across the 

 strike from the foothills toward the plains, from 25 to 30 ', as a rule, in 

 the lowest or Wyoming beds to 45° in the intermediate series, increasing 

 rapidly to a vertical or even beyond in the Laramie strata, with which 

 the overlying Arapahoe and Denver beds, where not eroded away, are 

 found to be involved; then changing in a few hundred yards of hori- 

 zontal distance to a practically horizontal position. The dip of the 

 upturned beds, if projected upward into the air, would produce a son of 

 partial fan structure. (See diagram l>, fig. 1.) The width of outcrop, 

 which nearly corresponds to the distance between the Arehean contact 

 and the vertical dip, varies with the thickness of the beds from 4 miles 

 On Turkey Creek, near Morrison, to about 1 mile at Golden. At the 

 latter point the vertical Laramie beds approach very close to the Archeau 



