132 GEOLOGICAL SURVEY OF THE TERRITORIES. 



south of one another as they actually occur, while 1 and 2 are offset a 

 few inches to the left. The latter, to be represented in their true relative 

 position with respect to the others, therefore, should be considered 

 moved about 5f inches (representing 4.3 miles) to the right, as indicated 

 by the points at which each section is intersected by the meridian of 105'^ 

 15' west longitude. 



These sections are the result of rapid field- work, and can by no means 

 claim to be free from local errors, though they express truthfully all the 

 main features of the region. 



Thicknesses have in all cases been estimated o'r obtained by pacing, 

 and many local points not well fixed by the topographer's notes have 

 been filled in by the eye. 



In traveling from the north along the zone of hog-backs lying at the 

 base of the mountains southward, the traveler finds the mountain- slope 

 directly west of him falling lower and lower until it becomes an insig- 

 nificant ridge, and finally dies away in the plains, v 



Passing around the southern end of the diminishing ridge the main 

 mountain-slope is found lying several miles to the west, and separated 

 from the ridge by a bay-like valley extending northward behind it. 

 There are here several such offsets or jogs in the mountain-border, 

 caused by its component ridges being arranged en echelon, north and 

 south of each other. The trend of these spurs is somewhat west of 

 north, while their echelon arrangement is such that a line touching 

 their southern ends trends east of north and west of south, with a flat 

 concavity presented to the east. As is so often the case in the West, 

 these peculiar topographical features are but the surface exi^ression of 

 a similar and equally important geologic cause. These ridges, and the 

 included valleys, indicate that here the folding of the rocks have also 

 taken place en Sclielon, The ridges are uplifted or anticlinal folds, the 

 valleys depressed or synclinal folds, both dying away southward into 

 the flatness of the plains, though the west side of the westernmost 

 synclinal is always preserved in the normal uplift along the main 

 mountain-base. With such a structure, and since the sedimentary rocks 

 have been to a very great extent eroded from the summit of the ridges 

 and worn down to a pretty uniform level, it is necessary that the out- 

 cropping strata should be found bending around the southern ends of 

 the spurs, their strike first swinging westward, that of the lower beds 

 bending on still farther to the northwest to form the eastern side of 

 shallowing synclinal basins, which finally terminate to the north, the 

 reverse, in all respects, to the anticlinal ends, while the uppermost beds, 

 those farther out, do not necessarily bend around into the synclinals, 

 but after turning somewhat westward, again resume their southern 

 course with the others.* 



The most interesting feature of these folds, next to their general eche- 

 lon arrangement, is the fact that in the anticlinals the western side of 

 the fold is always more abrupt than the eastern side, and may become a 

 fault, the downthrow being upon the western side. That is to say, the 

 tendency of the forces forming the folds seems to have been to lift up 

 the eastern side relatively to, and push it over against, the western side; 

 and the expression of this tendency has been either an abrupt down- 

 ward bend of the west side, or a direct downward faulting of the west 

 side, or by both combined. And along the same fold these three forms 

 of arriving at the same result are interchangeable. Indeed, the type of 

 these anticlinals en echelon may be expressed thus : From the rather 



* These echelon ridges and folds were first observed, so far as I know, by Dr. Haydea 

 in 1869, and partially described in his report for that year. 



