84 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



CRETACEOUS FORMATIONS OF THE GREAT PLAINS AT THE FORTY- 

 NINTH PARALLEL. 



The writer has made no special study of the Cretaceous formations over 



which the eastern part, if not all, of the Lewis range and the eastern part of 



the Clarke range have been thrust. These rocks crop out nowhere within the 



area covered by the detailed survey and they were only cursorily examined 



during a rapid traverse to Chief mountain. The reconnaissance of Willis and 



Weller has yielded useful results, which may be described by full quotation 



from Willis' paper:* 



' Cretaceous strata are but poorly exposed along the eastern base of 

 Lewis range, although they form the subterrane beneath hundreds of 

 square miles of the plains. The mantle of drift is widespread and often 

 thick, and outcrops of rock in place are limited to occasional freshly 

 scoured gullies or ledges- of sandstone along hilltops. Such outcrops were 

 noted, however, in traversing the plains from Cutbank river to Saint Mary 

 lake, and others were found about the mountain slopes west of Saint Mary 

 lakes, up Swift Current valley, on Kennedy creek, about Chief mountain, 

 and on Belly river. Weller collected fossils sufficient to determine three 

 horizons, namely, Dakota, Benton, and Laramie, and through the light 

 thrown by fossils on their relations these occasional Cretaceous outcrops 

 become interesting as elements of a structure which they do not suffice 

 to make clear. Their distribution is such that the Dakota and Benton, 

 while occupying normal relations one to another, are apparently above 

 the Laramie. The significance of this from the point of view of structure 

 is discussed under that head. 



' No occurrences of rocks of Cretaceous age were observed west of the 

 Front range of the Rockies, and it is probable that there are none south 

 of the Crowsnest coalfields. 



' Dakota. — Arenaceous and argillaceous shales and sandstones of 

 Dakota age occur on North fork of Kennedy creek near its junction with 

 South fork, 5£ miles east by south from Chief mountain, at an elevation 

 of 4,800 feet. The exposures constitute a bluff 30 feet high, near the top 

 of which are layers bearing fossil plants and freshwater shells. A collec- 

 tion of leaves, though badly broken up in transit, was examined by Mr. 

 Knowlton, who reports Fleas proteoides ( ?) Lesq., Magnolia botdayana 

 Lesq., Liquidamba integrifolius Lesq., Liquidamba obtusilobatum Lesq., 

 Diospyro rotundifolia Lesq., Phyllites rhomboideus Lesq. " The above 

 species," says Knowlton, " are all characteristic Dakota group forms, and 

 the beds at this locality are referred without hesitation to this age." The 

 strike of these Dakota beds is nearly north and south and they dip at a 

 low angle, — 10 degrees westward. 



'Bentpn. — Dark bluish-black to leaden gray shales constitute the mass 

 of Cretaceous rocks west of Saint Mary lakes. With them are associated 



* B. Willis, Stratigraphy and Structure, Lewis and Livingston Ranges, Montana; 

 Bull. Geol. Soc. America, Vol. 13, 1902, pp. 315 and 326. 



