GENEEAL STBUCTOEE OF THE FOOTHILLS. 81 



fn the foothills themselves the sedimentary beds usually maintaiu an 

 easterly dip of 35 to 50 . varying locally between 15 and 90 . Passing 

 outward from the range, the < 1 1 j » gradually increases until at 1 .', or 2 miles 

 from the Dakota, along the outcrops of the Laramie and Arapahoe, it is 

 rarely under 80 and often vertical or slightly overturned. Mast of the 

 basal sandstones of the Arapahoe the dip rapidly shallows to the gentle 

 degree (3 to 5' ) uormal for the prairie. In the northwestern corner of 

 the field, where tin 1 Laramie and overlying beds have been eroded, the 

 change from steep to gentle inclination appears in the Montana, at the 

 usual distance of 1.1 to 2 miles from the foothills. 



The flexure suggested in the foregoing change of dip is that general 

 along the foothills, a part of the great fold involved in the uplift of the 

 range. 



Folds en Echelon. — Besides the fold resulting from the general uplift, and 

 a considerable amount of minor crumpling along lines parallel with this, 

 there occurs in front of the Colorado Range, from the northern to the 

 southern end, a series of folds which, from the successive offsets they cause 

 in the face of the range, have been named by Dr. F. V. Hayden "folds 

 en echelon." Their general outline is represented in the accompanying 

 Plate IX,' a reproduction of a series occurring between St. Vrains and 

 Cache la Foudre creeks, 12 to !•» miles north of the limits of the Denver 

 field as mapped. 



The folds are anticlines, springing from the main range at angles of 5° 

 to 25 with its axis; they gradually sink beneath the prairie, and finally 

 die out altogether. They usually point to the south, and the reentrant 

 angle between them and the range is occupied by a syncline opening in 

 the same direction. The folds of greatest size <>ccm- bevond the limits of 

 the Denver field: some, 12, 17, and 25 miles to the north, in the vicinity 

 of St. Vrains, Little Tl ipson, and Big Thompson creeks; others, still 



larger, in the southern part of the State, near the Arkansas and Huerfano 

 rivers. The tacts that these folds, with one or two minor exceptions in the 

 last-mentioned region, all point south, and that to their south generally 

 occur streams of considerable importance, are worthy of remark. 



t'r Sheet XII. Hayden's Atlas of Colorado. 



MOM XXVII G 



