JACOB SANDS 379 



finally the Montauk drift, would make plausible the supposition that it 

 was the supply of new materials by the advancing ice that brought about 

 the change. Elevation of the land, however, is often thought to have 

 accompanied ice invasions, and it may be that such was the case in 

 the present instance. Nothing in the way of an unconformity which 

 could be referred to stream erosion has been noted at this horizon. The 

 unconformities cutting through Herod gravels, Jacob sands, Gardiner 

 clays, etcetera, are such as might readily have been produced by the 

 ice of the Montauk invasion ; hence it is improbable that the land was 

 uplifted above sealevel. On the contrary, the thickness of the series of 

 deposits, including the Jacob sands, the Herod gravels, and the Mon- 

 tauk drift, all of which accumulated beneath sealevel, seems to indicate 

 a progressive subsidence, since the Gardiner clays at the start were 

 nearly at sealevel. It is not unreasonable to suppose that the change 

 of material marking the beginning of Jacob deposition took place as 

 soon as the advancing ice-front reached the headwaters of the Connecti- 



Figube 9.— Section at Headland Three-quarters of a Mile Northeast of North End of Isabella Beach, 

 a, till ; 6, gray sand ; c, clayey gravels ; d, greenish-gray clay. 



cut and discharged a portion of its drainage down its valley. Under 

 such conditions it would only be the light materials which would be 

 borne to the mouth and out into the sea. The amount of materials, if the 

 supposition as to origin is correct, would indicate a very slow advance, 

 since the Jacob sands reach a thickness of at least 40 feet on Fishers 

 island. 



Character of the sands. — The Jacob sands are not sharply separated 

 from the underlying Gardiner clay, although the transition from one to 

 the other usually occupies but a few feet. The change begins with the 

 introduction of a few sandy laminse into the clay, these rapidly becom- 

 ing more numerous and of greater thickness until they take on the 

 character of beds of exceedingly fine sands, almost quartz flour, but in 

 which numerous grains of white mica and other particles of darker 

 minerals may be seen. In color they commonly vary from a very light 

 gray to yellowish and buff tints, but where lamina? of true clay are 

 present they may be stained externally to a reddish tinge. They are 

 always clearly stratified, although individual beds several feet in thick- 

 ness, appearing structureless to the eye, are sometimes encountered. 

 When wet they are often somewhat plastic, but lack the toughness of 



