120 Crefaceous. 



4, Graj'ish-black clays, rather hard and very homogeneous, breaking- 

 into small angular fragments on weathering, and forming earthy 

 banks. This division belongs to the upper part of the Fox Hills Group, 

 and only about 40 feet of it is exposed at this place. 



The sombre clays of the Fox Hills Group may be traced almost 

 continuously for a distance of about ten miles west, on the 49th par- 

 allel, where lower beds are exposed. Near the crossing of the 49th 

 parallel and trail to Fort N. J. Turne}^ where the Wood Mountain As- 

 tronomical Station was established, good exposure.s of the Fort Pierre 

 Group occur in the banks of the valley of a large brook. Taking into 

 consideration the difference of level between this locality and that of 

 the section above, it appears that the Fort Pierre Group must be at 

 least 200 feet below the Fort Union or Lignitic Group. 



Westward from these sections the continuity of the Cretaceous ckys 

 in the vicinity of the boundary line is indicated by occasional small 

 exposures, and at a distance of 13 miles a tolerably good exhibition of 

 the Fort Pierre Group occurs. Where the boundary line crosses 

 White Mud river, or Frenchman's creek, numerous and very fine ex- 

 posures occur. The stream flows in the bottom of a great trough, cut 

 out of the soft Cretaceous strata, over 300 feet deep, and in some places 

 fully three miles wide. The tops of the banks, on both sides of the 

 valleys are formed of yellowish ferruginous sands referable to the base 

 of the Fort Union Group. Below this the sombre cla3'S of the Fox 

 Hills and Fort Pierre Groups have a thickness of 273 feet to the water 

 level of the river. A similar section occurs on the main trail going 

 west from Wood Mountain in the Valley of the White Mud river, 16 

 miles north of the 49th parallel, and 23 miles northwest of the last 

 described exposure. 



On the western side of White Mud river, hilly ground occurs, and at 

 about the 505 mile point from Red river, the prairie makes a very 

 definite rise and forms a plateau, which extends along the 49th par- 

 allel to the 534 mile point. The plateau is composed of the Fort 

 Union Group. On coming to the western edge of this plateau, a great 

 area of barren and arid prairie, at a lower level, and based on the Fort 

 Pierre Group is seen stretching westward toward Milk river. An in- 

 teresting section of the Fort Pierre Group and lower strata occurs, in a 

 deep valley, about six miles west of the East Fork of Milk river, on 

 and near the 49th parallel. The thickness exposed is 893 feet. The 

 Valley of the Milk river offers continuous and magnificent sections. of 

 the Fort Union Group. The country, on both sides of it, is seamed 

 with tributar}^ ravines and gorges, the banks of which are often nearly 



