44 DESCRIPTIVE GEOLOGY. 



The following- section was not made at any one point, but will serve to 

 show the character of the entire group : 



1. Interstratifled beds.of clay and sands, in many localities the clays ^ 



predominating, and in others the sandy beds I 



2. Dark arenaceous clays . [ 



3. Grayish-black carbonaceous clays and shales 3 



4. Bluish-gray, soft, earthy beds 



5. Yellow marls and limestone . . 



6. Whitish-gray marl , 



7. Yellowish-white and cream-colored marls and limestone, with 



gypsum 



8. Light-colored marls , 



9. Blue argillaceous limestones \ 



10. Clays, with tine calcareous beds 



11. Dark plastic clays, with arenaceous and ferruginous layers 



250- 350 feet. 



100- 200 feet. 



250- 450 feet. 



600-1000 feet. 



Although the beds show considerable variety, it is evident that they 

 possess, in general, simple features; the lower members being clay-beds, 

 more or less arenaceous, passing up into calcareous beds, and, still higher in 

 the series, again becoming clays, with sands near the top. 



It would be difficult in most places to separate the Fort Benton from 

 the Fort Pierre group by lithological distinctions, but for the striking pecu- 

 liarities of the brilliant-colored marls of the Niobrara'; where the latter are 

 concealed, and the beds are not defined by their organic remains, it becomes 

 almost impossible to say to which series certain beds may belong. The 

 Fort Benton clays, lying next the rigid sandstones of the Dakota, are easily 

 eroded into broad shallow depressions, with the gray and yellow marls 

 of the Niobrara forming low ridges to the eastward. These marl-ridges, 

 although very insignificant in proportions, form one of the most striking 

 features along the foot-hills, affording excellent geological horizons in trav- 

 elling rapidly over the country. In many places, they show great persist- 

 ency, and may be followed without any break in their continuity for many 

 miles. They rise usually but a few feet above the surrounding- level, stand- 

 ing out in a most marked manner above the dull earthy clays, presenting 



