THE PLEISTOCENE OR GLACIAL PERIOD 903 



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 numerous trifling uncon- 

 formities, and the absence of fossils, all are consistent with this in- 

 terpretation. 



The second factor contributed to the same end. The climate 



Fig. 595. Unconformable contact between the Columbia formation and 

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



of the period was changeable, and at least periodically cold, as the 

 recurrent ice-sheets show. Under these conditions a larger pro- 

 portion of the precipitation than now was doubtless in the form 

 of snow, and this was favorable to the flooding 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 erosion, and therefore 



