PLEISTOCENE GEOLOGY OF NASSAU CO. AND BOROUGH OF QUEENS 625 



Qaartz pebbles predominate in this formation, particularly those 

 stained yellow by the oxid of iron ; hence the term, " yellow 

 gravel," which has been sometimes given to it. This discoloration 

 will be treated more at length later. White quartz pebbles are not 

 uncommon, pebbles which appear never to have been stained. The 

 silicified fossils and cherts are relatively rare, but search carried 

 over a few square yards of surface in any gravel pit in the forma- 

 tion north of the moraine will usually reveal two or three of these 

 erratics. The gneissic and granitic pebbles are at least in the mass 

 of the formation not much decayed. On the whole, the materials 

 are like those in the moraine and in the gravel and sand terraces on 

 the mainland except for local staining by iron oxids. The com- 

 parison with the moraines is perhaps hardly just, because the 

 moraines are locally largely composed of rearranged drift from 

 these same beds, as in Harbor hill. The inference from the sands 

 and gravels is that they are of glacial origin, modified by the work 

 of running water, either ice-born streams or extraglacial waters. 

 This conclusion as to their glacial origin amounts to a certainty 

 when the intermediate boulder clay bed is taken into the account. 



That the beds extend southward beyond the Harbor hill ridge 

 or moraine can hardly be questioned ; but it is difficult to distin- 

 guish the formation in front of the moraines from the later gravels 

 and sands washed out from the ice front. At one point in the 

 mounded drift southwest of Koslyn an exposure by the roadside of 

 a coarse cobble bed with yellow pebbles contains also iron stone con- 

 cretions which have evidently not been rolled, showing that they 

 are probably in place, though loosened by exposure to surface 

 actions from the surrounding pebbles. Beds of this character are 

 found at the base of the Pleistocene on Marthas Vineyard in the 

 Gay head section,^ wliere the origin of the concretions is clear. The 

 concretions arise from the erosion of light colored clays of the 

 underlying Cretaceous or Potomac beds and their deposition with 

 the coarse gravels as pebbles permeable to percolating water charged 

 with iron salts. Cementation takes place by deposition of iron 

 oxids around all of the pebbles, involving the outer part of the clay 



1 Woodworth, J. B. Geol. soc. Am. Bui. 1897. 8 : 205-6. 



