144 GEOLOGY. 



The Western Interior. 



Before the Cretaceous period was far advanced, non-marine sedi- 

 mentation was in progress over an extensive area in the western interior. 

 Later, the sea entered this region from the Gulf, covering a wide belt 

 east of the Rocky mountains, and reaching perhaps to the Arctic ocean, 

 thus connecting the subtropical seas with the polar. 



The Cretaceous system of the western interior consists of the fol- 

 lowing subdivisions: 



4. Laramie. 

 3. Montana. 



Fox Hills. 



Fort Pierre. 

 2. Colorado. 



Niobrara. 



Benton. 

 1. Dakota. 



The Dakota formation. — The Dakota formation is mainly of non- 

 marine origin, being comparable in this respect to the oldest formations 

 of the Comanchean system, the Potomac, the Tuscaloosa, the lower part 

 of the Trinity, the Morrison, and the Kootenay. (See note, p. 190.) 



The Dakota formation is present over the Great plains generally, 

 though buried over the greater part of the area. It extends west- 

 ward beyond the eastern ranges of the western mountains, though in 

 the mountain region, the area of deposition was greatly interrupted 

 by elevations which rose above the lakes, marshes, or river flats where 

 the sedimentation took place. In northern Montana, it is not known 

 west of the Rocky Mountains. 1 The original eastern boundary of the 

 formation is not known, for erosion has removed it from considerable 

 areas which it once occupied. Remnants of the formation are now 

 exposed as far east as eastern Iowa 2 and Minnesota. It must origi- 

 nally have covered an area 1000 miles wide and 2000 miles long within 

 North America. Its outcrops are chiefly along the eastern and west- 

 ern borders of the plains, and in the mountains to the west. Here 

 it sometimes overlaps Paleozoic and earlier Mesozoic formations, and 

 rests on the Archean (Fig. 391). 



1 Willis, Bull. Geol. Soc. of Am., Vol. 13, p. 326. 



2 Calvin, Iowa Geol. Surv., Vol. I, 1892; Bain, Idem, Vol. Ill, p. 108 and Vol. V, 

 p. 267 — a good review of the Dakota of Iowa. 



