THE PLEISTOCENE OR GLACIAL PERIOD. 453 



the constitution of the material deposited. One of its common con- 

 stituents is crystalline rock, now generally thoroughly decayed. This 

 material points to conditions when erosion and transportation exceeded 

 rock decay, as might be the case after the development of increased 

 declivity. 



The second factor, the climate, contributed to the same end. The 

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

 as the recurrent ice-sheets show. Under these conditions a larger 

 proportion than now of the precipitation of the Appalachians was 

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

 of streams during the melting seasons. At the north, the deposition 

 of the Columbia material was probably partly by water coming directly 

 from the ice of the early glacial epochs. Floating ice helped to trans- 

 port 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. In this way also, the presence of large bowlders of soft 

 shale, scores of miles from the nearest outcrop of similar rock, may 

 be explained. 



The cold climate probably affected erosion, and therefore deposi- 

 tion in another way, for the reduction of temperature was probably 

 attended by a reduction of vegetation, and any diminution of vegeta- 

 tion must have reflected itself in .ncreased erosion. The reduction 

 of vegetation was probably greatest just where erosion was most readily 

 stimulated, namely in the higher altitudes. The importance of this 

 consideration has perhaps not been duly recognized. 



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

 divisions of the Quaternary series of the Coastal Plain resulted from 

 the combined effect of surface warping and climatic change; that 

 epochs of notable deposition alternated with epochs when erosion was 

 dominant in the same regions; and that the materials of each principal 

 stage of deposition were deposited, shifted, and re-deposited repeatedly, 

 so that the Bridgeton (High-level Columbia), the Pensauken (Low- 

 level Columbia), and the Cape May formation, are each really com- 

 plex series, though they nowhere attain great thickness. 



While the Cape May division of the Quaternary was being deposited, 

 the sea transgressed some parts of the present coast to a slight extent 

 at the same time that deposition was taking place in the valleys scores 

 of miles inland, and in some cases hundreds of feet above sea-level. 



