THE UPPER CRETACEOUS GROUPS. • 183 



taceous. The low angle of dip, the similarity of the different groups in 

 lithological character, their too frequent concealment under grassy slopes, 

 and the absence of fossils for long stretches, rendered a true estimation of 

 their thickness and extent a matter of difficulty. 



From the point where the Dakota sandstone dips under the river and 

 gives place to the overlying Fort Benton, to the acute northern bend of the 

 river the following beds are exposed. The section is given in descending 

 order. 



Feet. 



4. Gray clay, with many calcareous iron nodules ; capping the bluffs at the bend 

 of the river. (The summit of the bluff carries many water- worn fragments 

 of sandstone, limestone, and granitic rocks) 35 



3. Dark gray shales, weathering light gray; stratified in distinct layers, with 



bauds of white sandstone at the base 200 



2. Black or very dark plastic clays . 100 



1. Dark sandstone — 



In the absence of fossil remains a true reference is not easily made. 

 The bed numbered "2" is evidently the Fort Benton clay; the one 

 numbered "3" is probably the representative of the Niobrara; while the 

 capping gray clays with calcareous nodules may indicate the base of the 

 Fort Pierre. 



The river is cut through these beds for a long distance below the bend, 

 and for some ten miles southeast of the bend "3" of the above section forms 

 the immediate banks of the river, the beds being nearly horizontal. By the 

 southward course of the river the valley east of the bend is cut back upon 

 the dip, so that at a point about ten miles below the bend the Fort Benton 

 clays again appear and form the banks for a considerable distance. 



About half way between the north bend and the mouth of the Red- 

 water a local change of dip brings the Dakota sandstone above the surface 

 of the river, and for a short distance it shows a cliff of 75 feet. This soon 

 disappears, however, and gives place again to the Fort Benton and Nio- 

 brara. These dip slightly toward the east and north, and for some distance 

 above the mouth of the Redwater the valley is cut in the gray clay of the 

 above section (Fort Pierre group ?). 



Judging from the efflorescence on its surface, this clay contains con- 



