308 B.WILLIS LEWIS AND LIVINGSTON RANGES, MONTANA 



from no degrees to ten south westward and strikes variously from north 

 to North 60° West. Thus it is warped, and this warping is found to de- 

 termine the general outline of the eastern face of the Rocky mountains, 

 particularly the prominence of Chief mountain, and the relative position 

 of the Lewis range, en echelon to the Livingston. 



Under the subject of structural antecedents the writer discusses hypo- 

 thetical conditions from which the o'verthrust fault may have resulted. 

 The physical history of the region is traced from the Dakota epoch to 

 Miocene time. Observed facts are arranged in sequence, interpreted^ 

 and supplemented by inferences. Deposition, deformation, erosion to a 

 peneplain, and later deformation are considered as successive stages in 

 development of the present geologic and physiographic relations. It is 

 concluded that the Lewis range owes its present elevation above the 

 Great Plains largely to upward movement on the overthrust : that this 

 uplift was preceded by a peneplain stage which came to a close in early 

 or mid Tertiary, and that the elevation of the Front ranges dates from 

 that time. Subsequently they were isolated along their western margin 

 by normal faulting, which determined new drainage lines. 



Introduction 



During the summer of 1901 the writer visited that portion of northwest 

 Montana lying west of longitude 113 degrees 30 minutes and north of lati- 

 tude 48 degrees 30 minutes, and examined especially the stratigraphy 

 and structure of that part of the Rocky mountains between the Great 

 plains on the northeast and Flathead valley on the southwest. The dis- 

 trict lies in Teton and Flathead counties, Montana, and in the adjoining 

 divisions of British America. It comprises the Front ranges, which con- 

 sist of two heights, the Lewis and Livingston crests. Streams flowing 

 from it enter the Saskatchewan, the Missouri, and the Columbia, and it 

 thus contains the main continental divide between the Atlantic and 

 Pacific oceans, as well as that between Hudsons bay and the gulf of 

 Mexico. . . 



The purposes of the expedition were those of general reconnaissance. 

 The work laid out for the season extended to an investigation of a strip 

 180 miles long, south of the international boundary, as far west as lon- 

 gitude 116 degrees 30 minutes, and therefore detailed work in any specific 

 district was impracticable. Nevertheless, two months were passed in 

 actual field work in the Front ranges of the Rocky mountains, and suffi- 

 cient data were gathered to add materially to our knowledge of them. 



In this trip the writer was associated with Mr Stuart Weller, paleon- 

 tologist, and Mr George I. Finlay, assistant geologist, and he is indebted 



