NON-GLACIAL FORMATIONS 655 



to the same end. The climate of the period was changeable, and at 

 periodically cold, as the recurrent ice-sheets show. Under 

 tlu-M- conditons a larger proportion of the precipitation than now 

 was doubtless in the form of snow, and this was favorable to the 

 Hooding of streams during the melting seasons. Floating ice helped 

 to transport the bowlders of the formation, and so to give it the 

 heterogeneity which is one of its distinctive features, especially in 

 proximity to the glacial drift. The cold climate probably affected 

 t -rosion, and therefore deposition, in another way, for the reduction 

 of temperature probably was attended by a reduction of vegetation, 

 and this by an increase of erosion. The reduction of vegetation pre- 

 sumably was greatest just where erosion was stimulated most readily, 

 namely, in the higher altitudes. 



It is conceived, therefore, that the deposition of the principal 

 subdivisions of the Quaternary series of the Coastal Plain resulted 

 from the combined effect of slight surface warpings and climatic 

 changes; that epochs of notable deposition alternated with epochs 

 when erosion was dominant in the same regions; and that the ma- 

 terials of each principal stage of deposition were deposited, shifted, 

 and re-deposited repeatedly. The youngest division of the series 

 was essentially contemporaneous with the last glacial epoch, and it 

 seems not improbable that the earlier members were deposited 

 during earlier glacial epochs. 



In recent times, dunes have been developed at numerous points 

 along the coast, and their development arid destruction is still in 

 progress. 1 Humus deposits also have somewhat extensive develop- 

 ment in the tidal marshes, and to a less extent elsewhere. 



Interior. Some of the non-glacial Pleistocene formations of the 

 interior, notably the loess, the valley trains, etc., have been referred 

 to. Apart from such formations, there are others which seem to be 

 measurably or wholly independent of the ice. The widespread 

 gravels of the western plains have been referred to (p. 599), but 

 their deposition continued through the Pleistocene, and is indeed 

 still in progress. There are numerous tracts and belts of dunes 

 where conditions favor their development, as in central and western 

 Nebraska, and Kansas. Dunes are of common occurrence locally 

 even east of the Great Plains, as about the head of Lake Michigan 

 and along its eastern shore. Even where dunes are wanting, wind- 

 blown sand and dust in small quantities are widespread. 



1 See for example, the Norfolk, Va.-N. C., folio, U. S. Geol. Surv. 



