PLEISTOCENE GEOLOGY OF NASSAU CO. AND BOROUGH OF QUEENS 621 



The main topograpliic features thus set forth are traceable east- 

 ward for 200 miles, one or more elements appearing either on the 

 islands off the south coast of New England or on the adjacent main- 

 land, the essential elements of the topography being two ridges each 

 one of which rises rather abruptly above a plain sloping southward 

 from it toward the sea. If the plain is absent, the sea covers the 

 space where we should expect it to occur. Westward, the main 

 ridge here described abuts on lower Kew York bay at the Narrows, 

 reappearing on Staten Island and continuing to be recognizable far 

 inland over the continent as a topographic feature, often imposed 

 on the rocky profile of valleys and high ridges alike. 



GEOLOGY 



The topographic features above described have long been known 

 to constitute a group of drift materials laid down along the margin 

 of an ice sheet or a successive series of such glaciers in the Quater- 

 nary or, as it is now usually denominated, the Pleistocene period, 

 a time defined as beginning in this hemisphere with the first of 

 these ice invasions on the coast plain and closing with the final dis- 

 appearance of the ice from eastern America. The time since this 

 disappearance of the ice, variously estimated at from 7000 to 10,000 

 years, is frequently denominated the post-glacial epoch. With the 

 deposits made during this Pleistocene period, the present report has 

 mainly to do. 



Pre-Pleistocene formations 



The basement on which these Pleistocene drift materials repose in 

 this part of Long Island has but a small exposure above sealevel, 

 and that is mainly limited to the northern coast north of the main 

 ridge, or moraine. These older materials are clays and sands, evi- 

 dently an eastward extension of the nearly horizontal clays and sands 

 largely of Cretaceous and late Jurassic age which constitute a large 

 part of the coastal plain from New Jersey southward. Little is 

 known of the attitude of these beds in this region, prior to the 

 earliest ice invasion, farther than the reasonable presumption that 

 they lapped over on the underlying gneisses and igneous rocks of 

 the mainland and the extreme western end of the island, gently 

 sloping from their inner margin seaward, as they still do in the 

 coastal plain soutli of the glacial district. 



