900 GEOLOGY 



the area of glaciation, the commoner phases of erosion and deposi- 

 tion were in progress, and non-glacial Pleistocene format ions arc 

 wide-spread. Under the varied conditions of the period, various 

 classes of deposits were made, among which were the following: 

 (1) Eolian deposits, conspicuous along many shores and rivers, 

 and in sundry arid regions, and inconspicuous as dust over much 

 larger areas. (2) Fluviatile deposits, made by streams (a) with 

 and (b) without connection with the ice. These deposits occur 

 along most streams of low gradient, and along many others. Kin- 

 dred deposits were made by sheet-floods and temporary streams, 

 even far from the courses of permanent streams. (3) Lacustrine 

 deposits of both the glacial and non-glacial types, made in existing 

 lakes and about their borders, and also over the sites of the numen >us 

 lakes which have become extinct since the beginning of the period. 

 (4) Deposits made by springs. (5) Terrestrial organic deposit* 

 (peat, calcareous marl, etc.) occur outside the area directly affected 

 by the ice, but are more common in the ponds and marshes to which 

 glaciation gave rise. (6) Marine deposits, on lands submerged 

 during the Pleistocene period, and doubtless over essentially all of 

 the ocean bottom. (7) Volcanic rocks of Pleistocene age are found 

 in our continent, chiefly west of the Rocky Mountains, though 

 volcanic dust is widely distributed on the Great Plains. All tl 

 kinds of deposits were doubtless made at other periods, but have 

 not been so generally preserved. 



The average thickness of the Pleistocene deposits is not great. 

 Glacial drift and Pleistocene accumulations of debris at the b. 

 of mountains are sometimes several hundreds of feet thick; but 

 otherwise the thickness of non-glacial Pleistocene deposits rarely 

 exceeds a few score feet. 



On the Atlantic and Gulf Coasts 



On the Coastal Plain of the Atlantic and the Gulf of M 

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

 referred to the Pleistocene period. It ranges from sea-level 1 1 j 

 altitudes of several hundred feet, though most of it lies below Jnu 

 feet. All of the non-glacial post-Tertiary deposits of 1 lie Atlantic 

 and Gulf plains were formerly grouped together under the n. 



