THE PLEISTOCENE OR GLACIAL PERIOD. 447 



to which glaciation gave origin, and also, though less commonly, 

 outside the area directly affected by the ice. (6) Marine deposits 

 were made on lands submerged during the Pleistocene period, and 

 doubtless over essentially all of the ocean bottom. The areas where 

 such deposits have since emerged are chiefly confined to narrow belts 

 along the coasts. (7) Volcanic rocks of Pleistocene age are found in 

 our continent, chiefly west of the Rockies, though volcanic dust is 

 widely distributed on the Great Plains. 



These non-glacial deposits probably appear at the surface over a 

 larger area than the formations of any earlier period. In the aggre- 

 gate, they are more extensive and more readily identified than deposits 

 of like origin referable to any earlier period. If the subaerial deposits 

 of other periods were equally extensive, they have been largely buried, 

 destroyed, or so modified as to lose their distinctive characteristics. 



The average thickness of the Pleistocene deposits is not great. 

 Glacial drift and Pleistocene accumulations of debris at the bases of 

 mountains are sometimes several hundreds of feet thick, and in rare 

 cases even more ; but otherwise the thickness of non-glacial Pleistocene 

 deposits rarely exceeds a few score feet. 



On the Atlantic and Gulf Coast*. 



On the Coastal Plain of the Atlantic and the Gulf of Mexico, there 

 is a wide-spread but thin body of gravel, sand, loam, and clay, referred 

 to the Pleistocene period. In altitude it ranges from sea-level up 

 to several hundred feet, though most of it lies below 200 feet. All of 

 the non-glacial post-Tertiary deposits of the Atlantic and Gulf plains 

 were formerly grouped together under the name Columbia. 



Soon after the Columbia formation was differentiated 1 it was 

 found to be bipartite, and the terms " High-level Columbia " and 

 " Low-level Columbia " were applied to the two divisions in the type 

 area, the District of Columbia. 2 Further study has disclosed the 

 fact that the materials formerly grouped under the one name represent 

 at least three somewhat distinct stages of deposition. 3 Physically 



1 McGee, Am. Jour. Sci., Vol. XXXV, 1888, p. 367. 



2 McGee, 7th Ann. Rept,, U. S. Geol. Sun-., 1885-86. 



3 Reports of the State Geologist of New Jersey, 1897-1900. The Bridgeton, Pen- 

 sauken, and Cape May Formations. 



