[Annals N. Y. Acad. Sci., Vol. XVIII, No. 10, Part III, pp. 425-429. 



16 December, 1908.] 



OUTLINE OF THE GEOLOGY OF LONG ISLAND, N. Y.^ 



By W. O. Crosby. 



{Read before the Academy 5 October, 1908.) 



The crystalline rocks (chiefly granitic and gneissic) outcropping in 

 Astoria and Long Island City are the foundation or true bed-rock of Long 

 Island geology. Their origin need not be considered here; for the geological 

 history of Long Island begins with the development on this crystalline bed- 

 rock of the Cretaceous peneplain, with its heavy load of sediments. The 

 Cretaceous was a period of slow subsidence, the land sinking beneath the 

 sea slowly enough to permit its almost perfect planation by marine erosion. 

 In other words, this peneplain has a dual origin, — subaerial and marine ; 

 true peneplanation obtaining above sea level, and still more approximate 

 planation below sea level. This seaward plain, in further contrast with the 

 landward peneplain, was covered by the Cretaceous sediments by which it 

 is still, in large part, protected. It is clearly indicated, where recently un- 

 covered, in the straight crest line of the Palisades. 



The progressive subsidence was favorable to the progressive landward 

 overlap of the Cretaceous sediments, by virtue of which only the later divi- 

 sions are exposed to observation, the true lower Cretaceous being confined 

 to the continental shelf, beyond the existing shore line. The conditions were 

 undoubtedly favorable, also, to the extensive subaerial decay of the crystal- 

 line rocks, thus furnishing in abundance the variegated clays and musco- 

 vitic or fluffy sands so characteristic of the Raritan formation. Marine 

 planation was clearly favorable, too, to the elimination from the mechanical 

 detritus of all decomposable materials, leaving a residuum of clean quartz 

 sand and gravel, thus accounting for the Lloyd Sand and other highly 

 quartzose members of the formation. The increasing remoteness and 

 degradation of the land finally made possible the deposition of the clay 



^ Published by permission of the Chief Engineer of the Board of Water Supply, City of 

 New York. 



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