GEOLOGY OF THE NEW YORK CITY AQUEDUCT 7I 



sidence, lie much beloAv sea level. The Hudson bed is of this 

 character throughout its course from Albany to the Atlantic, and 

 in the Highlands, 60 miles inland, the known rock bed at one point 

 is more than 700 feet below sea level. 



In late glacial time there was still greater subsidence (50-100 

 feet) than the present as is indicated by terraces above present 

 water level and the deltas formed at the mouths of tributary 

 streams. 



Such in general outline is the history of successive conditions 

 governing the topographic development of the rock floor. The suc- 

 cession of periods of stability, elevation, stability again, reelevation 

 and subsidence have had an effect on all sorts of formations, but 

 the extent of the impress and its permanence varies greatly in 

 the different districts. It is not possible to study these differences 

 in detail here. They are the minor and special local characters that 

 are in control at particular localities. In discussions of special 

 problems some of these are taken up in more detail. But in each 

 case the general history as outlined above, together with the modi- 

 fying influence of known local structure and stratigraphic char- 

 acter are the foundations of a working understanding [see Hudson 

 River crossings, Moodna creek, Rondout valley, etc., pt 2]. 



Pleistocene glaciation. An additional modification and one largely 

 independent of and largely inconsistent with the distribution of the 

 smaller features of the rock floor is introduced by the glacial drift. 

 It covers almost everything, but so unevenly as to largely destroy 

 some of the detail. It is in places more than 350 feet thick (as 

 in the Moodna and Rondout valleys) and in others it amounts to 

 nothing. It covers the narrow ravines and gorges heaviest and 

 has altered the courses of many of the smaller streams, the original 

 channels being hopelessly buried. The result has been chiefly one 

 of reducing the ruggedness of outline that prevailed along the 

 newer gorges of late preglacial time. 



Besides this the usual surface forms characteristic of glacial de- 

 posits, occur — the kame, the drumlin, the esker, the hill and ket- 

 tle topography of the terminal moraine, the overwash plain, the 

 delta, the lake deposit and the gentle undulations of the ground 

 moraine. These are superimposed on the rock floor features. Both 

 are equally important to understand in the problems that have been 

 encountered. Which set of factors is to be most regarded in a 

 given case depends wholly upon the locality and the kind of en- 

 terprise or work it is proposed to undertake. 



