GEOLOGY OF THE NEW YORK CITY AQUEDUCT 35 



have advanced and retreated repeatedly, how many times in this 

 region is not clear. With each time of advance and retreat, the 

 work done by it -partly destroyed, or disturbed or modified or cov- 

 ered the earlier ones in what appears now to be a most arbitrary 

 way (in reality, of course, in a very consistent way for the condi- 

 tions that then existed) . So one frequently finds a till beneath a de- 

 posit of stratified drift, or modified drift beneath till, or a succession 

 of a still greater number of changes in almost hopeless confusion. 

 In New York city, for example, at Manhattanville cross valley, 

 the exposed drift above street level includes (a) at the bottom, 

 water-marked stony till and assorted gravels, (b) in the middle per- 

 fectly horizontal, stratified rock flour and the finest sand, (c) top, 

 wholly unassorted bouldery till, covered by thin soil. It is evident 

 that the most careful and accurate identification of the surface type 

 without subsurface investigation would give, for such uses as are 

 now being considered, thoroughly unreliable evidence as to the 

 behavior of the whole body at this point. Therefore, a determina- 

 tion of the changes and quality forms an essential record. All of 

 these types are to be found in the region, but the different grades of 

 till and roughly modified material belonging to the kame type are 

 more common inland. 



On Long Island the development of marginal modified types is 

 extensive and more or less obscured by the advance and retreat 

 noted above. The larger divisions recognized in deposits are (a) 

 an early accumulation of sands and gravels, strongly developed near 

 the western end of the island, known as the " Jameco " gravel, 

 (b) an interglacial (retreatal) deposit of blue clays known as the 

 ''Sankaty" beds, (c) a later series of deposits, sands, clays, gravels 

 and till, belonging to the closing stages of the ice period correspond- 

 ing to the surface deposits of the larger portion of the whole region 

 (Tisbury and Wisconsin advances). Some of these sands and 

 gravels are in^portant water-bearing sources for the new Brooklyn 

 additional supply. 



The whole Long Island series according to Veatch^ includes : 



^Glacial two lines of ter-^ ^^ , , .,1 

 ,xr- • . -1 • -^1 ^ I Harbor hill morame 



Wisconsm stageJ mmal morames with out- f^ , , 



1 wash plains J Ronkonkoma moraine 



{Great deposit of outwash sand and gravel (de- 

 pression) 

 Gardiner interval with erosion (interglacial) 



1 After PP 44, U. S. Geological Survey, p. 33. 

 2 



