PLEISTOCENE GEOLOGY OF NASSAU CO. AND BOROUGH OF QUEENS 623 



the limits of this part of the island naturally precludes any observa- 

 tion of glacial striae indicating the direction of local ice movement. 

 The ledges of gneiss in Long Island City bear striae whose direc- 

 tion is s. s. e. and presumably a similar course was followed over this 

 tract. 



Glacial formations 



The glacial formations of this area are divisible into two great 

 groups : those of an unassorted, unstratified structure, composed of 

 mixtures of boulders, pebbles, sand and clay, frequently, when 

 covering the surface, witli a knob and basin topography, forming in 

 general terms till, or boulder clay when boulders are mixed with 

 clay ; and those composed of gravels and sand with a stratified struc- 

 ture showing their evident deposition by running water. 



Till constitutes the larger part of the ridges or moraines already 

 described. Boulder clay occurs as a thin laver in the bluffs on 

 either side of Hempstead liarbor and in the area between Searing- 

 ton and the main ridge near Lake Surprise. Ordinary till, largely 

 in tlie form of scattei'ed boulders, covers the terraced j^lains and 

 the ridges and valleys north of the main moraine. The rest of the 

 area is largely composed of gravel and sand with local deposits of 

 blackish or bluish black clay not certainly of glacial origin and per- 

 haps to be regarded as of Tertiary or older age. Gravel and sand 

 constitute by far the greater portion of the glacial deposits both as 

 regards the surficial extent and cubic contents of the Pleistocene. 



Since these deposits appear by their structure and relations to 

 have been deposited in succession, some till having been made under 

 the ice or at the ice front while gravels and sands were being laid 

 down by water running through or pouring out of the ice, it will be 

 necessary to consider them in the order of their development in 

 time. In the chronologic succession, the glacial deposits exhibit 

 three marked phases of Pleistocene history in this area : 1) a group 

 of older gravels and sands with an intercalated till bed, the evident 

 equivalent of the Columbia formation ; 2) the moraines and their 

 attendant stratified gravels and sands, forming the topographic 

 details of the surface ; 3) between these deposits in the order of 

 time, evidences of erosion by other than glacial action, which 

 demand separate treatment. 



