208 DOUGLASS — CRETACEOUS AXD LOWER TERTIARY. [Aprils, 



the foothills. The strata are, as a rule, not horizontal, but have 

 been affected by the disturbances which have elevated the moun- 

 tains farther to the west or north. In restricted localities the beds 

 are horizontal and in others nearly vertical, and there are all inter- 

 mediate grades. The average dip is probably not more than fifteen 

 or twenty degrees. 



The relief beautifully expresses the geological character. Through 

 the whole section there are alternations of sandstones and shales 

 and all grades between the two. Sometimes, as in the Fort Benton 

 and Fort Pierre, the shales predominate and attain a considerable 

 thickness. Again, as in the Dakota (?), Niobrara, Fox Hills, etc., 

 sandstones predominate — at least there is enough indurated sand- 

 stone to retard erosion and to produce prominent ridges which can 

 be followed for long distances — fifty miles or more. In all the 

 formations there is considerable sandstone, and in all there is much 

 shale j but I have seen but very little limestone in the whole section, 

 though it sometimes occurs in concretions or in thin layers. 



It does not appear that during the whole period of deposition the 

 sea ever attained any great depth. Probably it was deepest at times 

 during the Benton epoch, yet even here the great amount of sand 

 in the shales indicates near-shore deposition. The erosion features 

 will be given in the descriptions of the different formations. 



So far as I am aware this particular region has been described 

 only by the writer (see Science^ January 3, 1902, p. 31, and Febru- 

 ary 14, 1902, p. 272). A little to the west is the area mapped in 

 the Little Belt Folio (No. 56) of the U. S. Geol. Survey, and some 

 work was done to the eastward on Swimming Woman and Careless 

 Creeks by W. Lindgren and George H. Eldrege, in connection 

 with the Northern Transcontinental Survey.^ 



Of course there is no single section where all the features here 

 described can be seen, and the depressions or ridges into which the 

 different strata weather have frequently to be followed for a few 

 miles to obtain good exposures. Fortunately this is easily done. 



The Lake Basin, to which reference will frequently be made, is a 

 large, depressed area nearly fifty miles long east and west and 

 twenty-five miles north and south in the widest portion. The 

 former represents the greatest east and west extension. The east- 

 ern portion extends northeastward. This portion I have not ex- 



^ Tenth Census of the United States, Vol. XV, p. 243. 



