THKUST-FAULTS OF THE NORTHERN ROCKIES 269 



In the Cottonwood district, Utah, the Alta overthrust has been recog- 

 nized by Hintze, Butler, and Loughlin and more recently it has. been 

 studied by Calkins. This fault dips eastward but steepens downward and 

 probably passes the vertical in places. Calkins^^ believes, mainly from 

 the relations of drag folds, that the upper block moved eastward and that 

 the original dip of the thrust-plane was westward. 



Along the north base of the Uinta Mountains two important zones of 

 thrusting have been observed. As described by Schultz,^^ the more west- 

 erly of these zones is exposed for 20 miles and the more easterly, the great 

 Uinta fault, first recognized by geologists of the King and Powell sur- 

 veys, is 80 miles or more long. In both instances the forces which pro- 

 duced the dislocation appear to have acted from the south or southeast. 

 The stratigraphic interval produced by these faults is not less than 20,000 

 feet. Most of this seems to be vertical, though there may be also a hori- 

 zontal displacement of considerable magnitude. 



Eelative Magnitude and Importance of Overthrusts 



The overthrusts above described vary in length from a few miles to 

 nearly 300 miles. The breadth of their respective areas of dislocation 

 varies from 2 or 3 miles to nearly 40 miles. They differ also in the strati- 

 graphic intervals which they produce. Some of these apparent differences 

 are probably due to lack of present knowledge, but without doubt some 

 of the faults, such as the Lewis, Bannock, and Absaroka overthrusts, are 

 displacements of a larger order than some of the others. 



Relative Ages of Overthrusts 



It has not been possible thus far to fix the geologic age of many of the 

 overthrusts mentioned above more closely than to state that they are pre- 

 Tertiary (Philipsburg quadrangle), late Cretaceous, or early Tertiary, or 

 post-Cretaceous. For a few the information is a little more definite. For 

 example, Willis, on the basis of both structural and physiographic evi- 

 dence, ascribes mid-Tertiary age to the Lewis overthrust. Later, mostly 

 unpublished work by geologists of the Federal Survey, in regions east of 

 the mountains not visited by Willis, shows that this fault is either of 

 late Eocene or early Oligocene age. The Beartooth fault is indicated as 

 of post-Fort Union age. Hewett shows that the Heart Mountain over- 



" Personal communication. 



i« A. R. Schultz : A geological reconnaissance of the Uinta Mountains', northern Utah, 

 with special reference to phosphate. U. S. Geol. Survey Bull. 690, 1919, pp. 69-74 ; see- 

 also U. S. Geol. Survey Bull. 702, 1920, pp. 43, 46, and pi. i. 



