THE CRETACEOUS PERIOD. 



151 



climate. Foraminiferal shells may accumulate as well on the bottom 

 of a shallow sea as on the bottom of a deep one. The purity of chalk 

 depends not on the depth of the water, but on the absence of clastic 

 sediments. 



The Montana series. — Following the Colorado epoch, there were 

 changes in the sedimentation and in the life of the western interior sea. 

 The sediments of the Montana series are chiefly clastic, and the area 

 of sedimentation was somewhat contracted. The beds are, for 



Fig. 395. — Fossil-bearing concretion in the Fox Hills sandstone, Carbon Co., Wyo. 

 The concretions are of lime-iron-carbonate and contain many molluscan fossils. 



the most part, marine, but the water shallowed as the epoch progressed, 

 for the Ft. Pierre beds contain fossils referable to deeper water 

 than those of the Fox Hills beds. Local beds of coal give evidence 

 of local marshy conditions. Like other parts of the Cretaceous sys- 

 tem of the west, the Montana series abounds in concretions, some of 

 which attain great size (Fig. 395). 



The thickness of the Montana series is variable, and its maximum 

 is great. From 8700 feet (7700 being Pierre) in Colorado, it is reduced 

 to 200 feet in some parts of the Black Hills, though it is much thicker 

 in others. 1 



1 Darton, New Castle, Wvo.-S. D. folio, U. S. Geol. Surv. 



