654 THE PLEISTOCENE PERIOD 



shells of fresh-water mollusks have been found and at a few points 

 marine shells, all within a few feet of sea-level. 



The origin of the Columbia formation presents much the same 

 problems as that of the Lafayette, and is probably to be explained 

 in much the same way. The series is looked upon as largely sub- 

 aerial (pluvial and fluvial), the result of land aggradation. The 



Fig. 537. Unconformable contact between the Columbia formation and the 

 Potomac, Washington, D. C. (Barton, U. S. Geol. Surv.) 



occasion for repeated intervals of deposition on the Coastal Plain, 

 separated by epochs of erosion, probably lay partly in changes of 

 gradient incident to surface warpings, and partly in the changes of 

 climate of the period, (i) Slight further upward bowing of the 

 highlands west of, the coast probably stimulated the streams descend- 

 ing from them to increased erosion, and the deposition of a part of 

 their loads on the plain below was a natural result. The poor 

 assortment of the material, the common cross-bedding, the numer- 

 ous trifling unconformities, and the absence of fossils, all are con- 

 sistent with this interpretation. (2) The second factor contributed 



