256 MAN AND THE GLACIAL PERIOD. 



in connection with floating ice. The boulders in the 

 Columbia formation clearly belong to the individual river 

 valleys in which they are found, and doubtless are to be 

 connected with the flooded condition of those valleys 

 when, by means of a northerly subsidence, the gradient of 

 the streams was considerably less than now. 



There is some difference of opinion in respect to the 

 extent of this subsidence, and, indeed, respecting the 

 height attained by the Philadelphia brick-clay, or McGee's 

 Columbia deposit. Professor Lewis (whose residence was 

 at Philadelphia, and who had devoted much time to field 

 observations) insisted that the deposit could not be found 

 higher than from 180 to 200 feet above the immediate 

 flood-plain of the river valleys where they occur. But, 

 without entering upon this disputed question, it is suffi- 

 cient to consider the bearing of the facts that are accepted 

 by all — namely, that towards the close of the Glacial period 

 there was a marked subsidence of the land on the eastern 

 coast of North America, increasing towards the north. 



Fully to comprehend the situation, we need to bring 

 before the mind some of the indirect effects of the Glacial 

 period in this region. The most important of these was 

 the necessary projection of subglacial conditions over a 

 considerable belt of territory to the south of that actually 

 reached by glacial ice ; so that, while there are no clear 

 indications of the existence of local glaciers in the Appa- 

 lachian Mountains south of the central part of Pennsyl- 

 vania, there are many indications of increased snowfall 

 upon the mountains, connected with prolonged winters and 

 with a great increase of spring floods and ice-gorges uj)on 

 the annual breaking up of winter. 



These facts have been stated in detail by Mr. McGee,* 

 from whose report it aj)pears that, on the Potomac at 



* Seventh Annual Report of the United States Geological Survey 

 for 1885 and 1886, pp. 537-646. 



