﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 65 



from its own weight by continuous accumulation spread out 

 (flowed) from great central areas toward the margins. There is 

 clear evidence of interruptions or advances and retreats of this 

 general movement many times. But the same type of work and 

 similar results were attained in each case. The chief features of 

 this work was the moving of rock material frozen in the ice to long 

 distances and the deposition of it again, more or less modified by 

 its contact with the ice or by the effect of water upon its release, 

 at other places and with entirely new associations. The tendency 

 to ice accumulation was finally overcome to sufficient extent for the 

 inauguration of the present condition of things. Whether it is a 

 permanent change or only an interglacial interval is not clear. 

 But the ice has withdrawn to the mountains and the polar north 

 at the present time. It has not occupied the surface of this region 

 probably within the last 40,000 years, and perhaps for a much 

 longer time. 



5 Outline of geographic history — physiography 



The surface features of a country are the result of the working 

 out of a long and complex series of processes with and upon the 

 materials of the rock floor or bed rock. The relationship of surface 

 features to the formations that occur in the rock floor and their 

 stages of development, in short, an interpretation of their origin and 

 meaning, constitutes geographic history or physiography. It differs 

 little in essential character from geologic history, of which it is only 

 a special branch, i. e. the history of surface configuration. And it 

 can not be appreciated or understood except in the light of a 

 thorough knowledge of stratigraphic and structural geology. In 

 individual cases or particular regions the geologic knowledge must 

 also be specific. 



a Early stages. Occasional glimpses of surface features, and 

 some scattered facts about their development are to be gathered of 

 older continental existence. Surface features characteristic of their 

 time were developed in the great intervals between each successive 

 period of continuous deposition. Traces of them are involved in the 

 unconformities of the geologic column already shown in the discus- 

 sion of geologic history. Hills, valleys, streams, shores and all the 

 appropriate assortment of forms must have existed. But they 

 could not have been like those of the present in many minor fea- 

 tures — especially in arrangement and distribution — because the 

 bed rock of those times had only in part reached the complexity of 



