GEOLOGIC FORMATIONS TO BE MAPPED 



starting with the basement rocks and proceeding upwards, the following formations are to 

 be mapped : 



Solid rock surface. 



Surface of disintegrated rock is not to be mapped. 



Lloyd gravel surface — These gravels are an important aquifer. 



Raritan clay surface. 



Magothy surface — Much of this formation is water bearing. In Nassau county three Mago- 

 thy aquifers occur in the lower part of the stratum which are so well known and appear to be so 

 continuous over considerable distances that they are known as Magothy I, II and III. More study 

 will be required before mapping of these aquifers will be justified. To some extent they may be 

 indicated on the profiles. In places where the top of the Magothy beds is gravel and Glacial gravel 

 rests on them, the two formations may be a sirgle aquifer. 



Jameco gravel surface — Glacial — This is an important aquifer in the western part of the 

 island. 



Gardiners clay surface — Interglacial — Not water bearing, but more or less impervious. 



The uppermost complex of beds is the Upper Glacial and Recent, comprising every deposit 

 of age later than the Gardiners interglacial period. The surface of this formation is the present 

 ground surface which has been mapped by various Government agencies and of which topogra- 

 phic maps of various sorts are available (Coast Survey, Geological Survey and Army) . This for- 

 mation contains sands, gravels, clays and hardpans, but usually it will yield water to wells of 

 suitable depth, frequently in large quantities. These aquifers have irregular depth, boundaries 

 and thickness. Most are more or less interconn3cted, but many are "perched". No attempt at 

 mapping these has been made, but the free wa'er surface (water table) is under continuous 

 observation and results are published by the Giological Survey. This free water surface has 

 been mapped a number of times. 



All of these surfaces have been subjected to sub-aerial erosion except the Lloyd where 

 covered by Raritan and perhaps parts of the Jameco and Gardiners. Such erosion should have 

 resulted in a more or less normal topography of stream valleys, hills and ridges, but as each 

 major erosion cut down the rock in places some of the surfaces bear traces of two or more drain- 

 age systems superposed on each other. This particularly is true of the Raritan when not pres- 

 ently covered by Magothy. The final erosion of the Cretaceous or pre-glacial surface is 

 complicated by changes in sea level, cutting by ice and greatly altered by glacial streams during 

 several different advances and retreats of the ice including deposition, erosion, re-deposition and 

 re-erosion of glacial material. 



Raritan formations suffered the most from these multiple erosion effects and as mapped 

 constitute a species of topographic nightmare. It should be studied as two entities with an indefi- 

 nite connecting zone. That to the eastward of the limit of the Magothy was chiefly eroded dur- 

 ing one major period and was then submerged by the ocean. The westerly end was eroded during 

 three major periods and a number of minor periods and was submerged and re-elevated several 

 times. The two sections are topographically unrelated to each other, but the western end forms 

 part of the Cretaceous surface which is also the upper surface of the Magothy, as reshaped dur- 

 ing the Glacial period. 



165 



