PLEISTOCENE GEOLOGY OF NASSAU CO. AND BOROUGH OF QUEENS 661 



report were made before the ice liad melted back 5 miles from its 

 extreme prolongation. Tliej clearly belong however to the period 

 of retreat ; but the mainland on the north was still actively glaciated. 



POST-GIiACIAL CHANGES AND PBOCESSBS NOW IN ACTION 



The disappearance of the ice from a glaciated district of itself 

 induces certain changes which are not wanting in this part of Long 

 Island. The melting out of remnants of the glacier or those parts 

 of its base which filled depressions has in many instances given rise 

 to small lakes and tarns. A number of these small lakes exist in 

 the moraine w^est of Roslyr. Of these, Lake Surprise is the best 

 and largest example. It lies at an elevation of about 200 feet above 

 the sea in a basin whose sides are gravelly till. Presumably the 

 bottom is clay rather than gravel, as the waters would escape 

 through the latter. Such lakelets depend on the percolation of the 

 ground waters through the relatively gravelly or sandy materials of 

 the superficial deposits, the water standing in the pond at the level 

 of the ground water in the gravels. Other small lakelets lie in 

 depressions in the outwash plain, as at Plattsdale. Westbury pond 

 is one of this class named on the map. 



The streams of the plain flow, as has been indicated, in courses 

 which were carved out by the once more vigorous glacial streams or 

 in still older channels on the north side of the moraine. Owing to 

 the porosity of the glacial gravels, much of the rainfall soaks into the 

 ground and issues near sealevel in the form of springs, hence, since 

 the run-off is small, Httle erosive work has been accomplished in the 

 post-glacial epoch. Yet the streams which converge into Oyster 

 bay have contributed enough gravel and sand to form a narrow 

 flat, modified by wave action where the village of that name 

 stands.^ 



Marine action at the present sealevel has cut back the outwash 

 plain on the south coast as well as the Far Rockaway ridge, so that 

 the outermost extent of both of these formations is now destroyed. 



* In June 1900 a well bored by means of a drill on the north side of Main st. 

 760 feet distant from the beach met at the depth of 45 feet^35 feet below sealevel) 

 a marl containing oyster shells (Ostrea sp). Above this l)ed were gravels, 

 below light yellowish sand. 



