68 GEOLOGICAL SUEVEY OF THE TERRITORIES. 

 THE GREAT FAULT FOLD OF THE ELK RANGE. 



On September 5th we reached the northern limit of our last year's 

 work, and little remained to be done but to examine a few complicated 

 spots along the main fold of the Elk range. Most of the difficult prob- 

 lems occur along this fold, between station 24 on the north and Cascade 

 Creek on the* south, and as the axis of the fold is west of the crest of the 

 range, the complicated parts are cut by the deep transverse valleys of 

 the western slope and many good sections are exposed. Six of these, 

 D, E, F, G, H, and I, are given in the main sheet of sections accompany- 

 ing the map. 



It will be observed by reference to the map that the granite, which 

 is represented by heavy horizontal lines and marked A A, occurs in 

 two great masses, and that in these masses are the culminating sum- 

 mits of the range. The northern, which is cut by sections D and E, is 

 the Snow Mass group, and the southern, cut by sections G. H, and I, is 

 the White Rock group. At first glance it might seem that these were 

 separate centers of elevation or upheaval, or at least that they were not 

 intimately related, but closer examination develops the fact that there 

 is a line of disturbance of a very marked and extraordinary character 

 connecting them. Section F cuts this fold at e e, and gives one of its 

 peculiar phases. But I found that a very large number of sections, 

 even, could not be made to give a connected idea of so complex a fold. 

 I have, therefore, prepared the accompanying illustration (Fig. 11), in 

 which the entire fold is given in relief and so placed upon an outline 

 map that the location of the various parts may be easily recognized. 

 I have carefully kept in view the idea of showing simply the peculiar 

 foldings of the broken edges of the strata. The granite areas have been 

 shaded down and the effects of erosion partially ignored in order to 

 develop the one idea, and a single convenient horizon, the base of the 

 Cretaceous, is taken, entirely disincumbered, for the sake of greater 

 simplicity. 



It should be remembered that this representation is highly artificial ; 

 that in reality the fold is very obscure, and has but little apparent effect 

 upon the topography; that it is cut into fragments by ten immense 

 valleys ; and that its anatomy can only be studied on the steep faces of 

 the ridges between these valleys. 



It will be seen by reference to the figure and the sections opposite, 

 that the conditions all along the east side are simple, there being a 

 gradual and gentle dip from the crest of the range toward the valley of 

 Roaring Fork, while on the west side there has been a general depres- 

 sion or downthrow, so to speak, amounting in many places to 7,000 or 

 8,000 feet; at the same time a combination of movements, principally 

 lateral, have produced along the axis an immense wrinkle, a fold so 

 abrupt that the beds are crushed and shattered and the severed edges 

 shoved past each other, as shown in the drawing and sections be- 

 tween e and n. It will not be difficult to imagine that while this was 

 going on, the plastic mass beneath was assisting the movements and 

 shaping the results, and that during the process it forced itself, or was 

 forced, through the fractured line in the two great masses of the Snow 

 Mass and White Rock groups. 



Beginning at the north, I shall give a detailed description. All along 

 the north face of the Snow Mass group the sedimentary rocks lie in the 

 usual order upon the granites, with a slight dip toward the north. (See 

 axis-section, large sheet.) Between Station 24 and Aspen Creek the 

 granite begins to fault up past the broken edges of the sedimentary 



