Geology of Long Inland. 343 



below the surface of this plain, it would necessarily be filled by 

 these subterranean waters, which, by their constant percolation 

 through the sand, would remain pure and clear, without material 

 or sudden change of level under average conditions. The largest 

 of these ponds is Lake Ronkonkoma, which is three miles in cir- 

 cumference, and has a maximum depth of 83 feet. 



The coast-line of Long Island is strikingly irregular. Along 

 the north shore are eight deep and extensive bays, which form 

 excellent harbors, and also a large number of inlets, most of 

 which are navigable. At the heads of these bays, numerous 

 springs of pure water issue from the hillsides, indicating the 

 presence of an impervious stratum within the hills. The east 

 end of the island is penetrated by Great and Tattle Peconic Bays 

 to a depth of 22 miles, while the south shore west of Southhamp- 

 ton for about 95 miles, consists of an intricate series of shallow 

 creeks partly surrounded by salt marsh, tributary to Shinnecock, 

 Moritches, Great South, Hempstead, and other bays, which are 

 divided from the ocean by long sand beaches, or reefs intersected 

 in places by narrow inlets. Shelter, Bobbin's, Plum, Gull and 

 Gardiner's Islands, which form part of Suffolk County, New 

 York, do not differ from Long Island essentially in physical or 

 geological characteristics. 



The lithology of the island is comparatively simple, the crys- 

 talline rocks being confined to quite a limited area. The greater 

 part of the region consists of gravel, sand and clay, overlain 

 along the north shore and for some distance southward, by gla- 

 cial drift. This material forms an important element of the 

 surface formation, and though it has been already described by 

 Mather and Upham, I shall devote a short space to its discussion. 

 For the sake of clearness, we may describe the drift as of two 

 kinds : 1st, the till or drift proper, a heterogeneous mixture 

 of gravel, sand and clay, with boulders, and 2d, the gravel 

 drift, a deposit of coarse yellow gravel and sand, brought to its 

 present place by glacial and alluvial action, but existing near 

 by in a stratified condition, before the arrival of the glacier. 

 This .yellow gravel drift, which in a comparatively unaltered 

 condition forms the soil of the pine barrens of southern and 

 eastern Long Island, and is exposed in section at Crossman's 

 brickyard in Huntington, is equivalent to and indeed identical 



