A PECULIAR BELT OF OBLIQUE FAULTING 6ii 



the system is poorly developed. This poorly developed tract lies 

 on the border of the eastward-trending portion of the Rocky 

 Mountain belt, which connects its overthrust tract in northern 

 Montana with its south-trending tract in Wyoming and Colorado. 

 Because of its general east-west trend, it was not in a position to be 

 much folded by a thrust from the Pacific, but on the contrary was 

 well placed to receive elongation from the greater eastward thrust 

 of the Rocky Mountains farther to the south. The Big Horn 

 Mountains seem to partake in some measure of both these attitudes, 

 for the southern portion trends with the Rockies in Wyoming and 

 Colorado, while the northern portion veers round to a more westerly 

 trend, and dies down in the Lake Basin district on the very border 

 of the fault belt which, in striking eastward from the Big Coulee- 

 Hailstone dome, is almost tangent to the northern flank of the 

 range. 



From these relations it will be seen that, though the deformation 

 on the border of the Lake Basin was limited and gentle, there 

 would naturally have been some eastward movement at the Big 

 Horn end. On the other hand, the Big Coulee-Hailstone dome 

 at the northwesterly end lay on the Great Plains side of the fault 

 belt, in the lee of the less-moved portion of the Rocky Mountain 

 belt, and appears also to be more nearly related to the group of 

 igneous intrusions than to the folded belts, and so should have 

 been less affected than the Big Horn end by the eastward move- 

 ment. Hence it is inferred that the southern portion of the Lake 

 Basin field, which was most influenced by the Big Horn deforma- 

 tion, was shifted eastward, while the northern portion partook 

 rather more of the relative fixity of the Great Plains province and 

 the area of igneous intrusions. The differential motion involved 

 in this adjustment is not unlike that which produces the oblique 

 crevasses along the margins of glaciers, except that the differen- 

 tial movement was distributed, instead of having a limit at a 

 sharp borderhne of slippage such as lies at the junction of the 

 moving glacier with the wall of the valley. No such line of sharp 

 differentiation is of course assignable to the rather broad and 

 gentle movements in the Lake Basin. There may of course have 

 been a concealed longitudinal shear plane deep below the fault 



