240 HISTORICAL GEOLOGY 



later (Cenozoic) deposits and they there underlie much, if not all, 

 of the Coastal Plain (Fig. 155). 



A large part of the western interior shows Upper Cretaceous 

 rocks at the surface. Because of the usual only slightly deformed 

 character of the strata just west of the main axis of the Rocky 

 Mountains, there is a large, practically solid outcropping area, 

 while within the Rockies, where much deformation has affected 

 the rocks, the outcrops are much more patchy in distribution. 

 Large areas of Upper Cretaceous strata are also concealed under 

 later rocks in this western interior region (Fig. 149) . 



On the Pacific Coast and in Alaska only small areas of Upper 

 Cretaceous strata show at the surface, but since the rocks there 

 are usually in a highly deformed condition, they are really con- 

 siderably more extensive than the surface exposures seem to 

 indicate. 



Character of the Rocks. — Atlantic Coastal Plain. Three well- 

 known Lower Cretaceous formations (Patuxent, Arundel, and 

 Patapsco) of the Atlantic Coast have long been called the Potomac 

 series. They are all of continental origin. Describing these forma- 

 tions in Maryland and Delaware, W. B. Clark says: "The Patux- 

 ent formation consists mainly of sand, generally arkosic and in 

 many places crossbedded, with small masses of clay scattered 

 through it. The formation attains a thickness of 350 feet. It has 

 a well-defined early Cretaceous flora. The Arundel formation 

 consists chiefly of clays, which in places carry iron ore and are 

 commonly lignitic. It has a thickness of about 125 feet. It is 

 unconformable to the Patuxent. . . . The Patapsco formation 

 consists of highly colored and variegated clays and lighter colored 

 sandy clays and sands. It has a thickness of 200 feet and is un- 

 conformable to the Arundel." 1 



The Upper Cretaceous deposits rest unconformably upon the 

 Lower Cretaceous. According to Clark the Upper Cretaceous 

 formations of New Jersey are typical of the North Atlantic Coastal 

 Plain and are constituted as follows : " The Raritan formation . . . 

 consists of clays, sands, and gravels. It has been estimated to 

 have a maximum thickness of about 300 feet at the outcrop. . . . 

 The Magothy formation unconformably overlies the Raritan forma- 

 tion and consists of dark clays and light sands. The clays are 



1 W. B. Clark: in Index to The Stratigraphy of North America, U. S. G, S., 

 Professional Paper 71, p. 610. 



