GEOLOGY OF THE NEW YORK CITY AQUEDUCT 67 



Therefore, when long periods of denudation are referred to, it is 

 well to appreciate that this is especially applicable to the interior, 

 that the sea margins are comparatively new, and that certain of the 

 inland areas were suffering erosion long before the rock forma- 

 tions that lie beneath and form the rock floor of the sea border 

 districts were in existence. 



Cretaceous peneplain. It appears from studies of these problems 

 in a broad way, and, drawing upon generalizations from continental 

 features of a much larger field than that of the present study, that 

 the continental region of which this forms a part must, in the 

 earlier periods, have remained in comparatively stable equilibrium 

 for an extraordinarily long time. So long a time elapsed that most 

 of the area was reduced by erosion to a monotonous plain (pene- 

 plain) at a very low altitude, probably not much above the sea 

 (base level). Only here and there were there areas resistant enough 

 or remote enough to withstand the denuding forces and stand out 

 upon the general plain as remnants of mountain groups (Monad- 

 nocks). Possibly the Catskill mountains of that day had such 

 relation. 



This reduction of surface feature it is believed was reached in 

 late Cretaceous time. The continent stood much lower than now. 

 Portions that are now mountain tops and the crests of ridges were 

 then constituent parts of the rock floor of the peneplain not much 

 above sea level. This rock floor was probably thickly covered with 

 alluvial deposits (flood plain) not very different in character from 

 the alluvial matter of portions of the lower Mississippi valley of 

 today. 



Upon such a surface the principal rivers of that time flowed, 

 sluggishly meandering over alluvial sands and taking their courses 

 toward the sea (the Atlantic) in large part free from influence by 

 the underlying rock structure. The ridges and valleys, the hills, 

 mountains and gorges of the present were not in existence, except 

 potentially in the hidden differences of hardness or rock structure. 

 Such conditions prevailed over a very large region — certainly all 

 of the eastern portion of the United States. This so called Creta- 

 ceous peneplain is the starting point in development of the geo- 

 graphic features of the present. 



Continental elevation. Following upon this period of stability 

 and extensive denudation came one of continental elevation. How 

 much above sea level this raised the areas under present discussion 

 may not be determined, but that it was a suflicient amount to 



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