310 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



to both gentlemen for their cordial assistance. Mr Weller especially, 

 through his persevering search for fossils, contributed to the definite 

 results of the work. Mr Finlay's notes on igneous rocks are appended to 

 this article. 



Physical Features 

 great plains 



The eastern portion of the area examined lies adjacent to the Rocky 

 mountains, in the Great plains, which were traversed from Blackfoot, a 

 station on the Great Northern railway, to the 49th parallel. Although 

 properly described as part of the plains which stretch eastward for a 

 thousand miles, the surface here has marked relief, there being differ- 

 ences of elevation which amount to 500 feet between summits and val- 

 leys. About Blackfoot and Browning the relief is partly built up by 

 moraine of the great continental glacier, and along the eastern base of 

 the mountains there are generally morainic accumulations from the 

 local glaciers which descended along the valleys.* The greater part of 

 the inequality of altitude is due, however, to the down cutting of the 

 streams. These are consequent on the general slope descending north- 

 easterly. The valleys, as a rule, are broad and defined by one, two, or 

 more terraces, of which the lower ones were built up and cut down by 

 the stream in recent times, but the higher ones are plains of erosion 

 across marine strata. 



The highest surfaces of the plains are limited in extent^constituting 

 according to field estimate not more than one-fiftieth of the total area. 

 Nevertheless, their profiles fall into a uniform line that represents an 

 ancient plain, due to erosion across Cretaceous shales and sandstones 

 of unequal hardness. The extent and uniformity of this plain are very 

 marked, and it is the initial physiographic fact of the region. It is 

 herewith designated the Blackfoot plain, after the Indian tribe whose 

 name is associated with the region. On this ancient surface there is a 

 widely distributed thin layer of gravel which is supposed to antedate 

 the Pleistocene deposits. 



FRONT RANGES 



Lewis range. — The Front ranges of the northern Rockies between lati- 

 tudes 48 degrees 40 minutes and 49 degrees 10 minutes present two par- 

 allel crests about 8 miles apart. The eastern one rises from the Plains 

 in Canada about latitude 49 degrees 10 minutes between Saint Marys 



*See summary of observations of F. H. H. Calhoun, by R. D. Salisbury. Journal of Geology, 

 University of Chicago, January, 1902. 



