230 



CAPE COD GEOLOGY 



beach, now forms an excellent harbor of refuge for coastwise steamers and is 

 resorted to by naval vessels engaged in manoeuvers in the neighboring sea. 

 Although the northward drift of the sand along the beach threatens to close the 

 canal, its value will justify the occasional expense required to maintain it. 1 



At the north end of Great Salt Pond a small island is so tied in by crescentic 

 beaches as to cut off from the main body of water a small lagoon. On its north- 

 east side this lagoon has in turn thrown a crescent barrier beach across a yet 

 smaller indentation of its shore line. Both these barrier beaches were doubtless 

 produced by similar processes, but they came into existence in succession, the 

 earlier having been formed at the original head of Great Salt Pond. Both are 

 examples of what Gulliver has called ' 'bay-bars." 2 



ECONOMIC GEOLOGY 



The black-iron sand of the island was gathered and shipped at one time 

 for commercial use, as already noted. The white Cretaceous clay or ' 'kaolinite " 

 at Clay Head is not favorably located for commercial exploitation, and the 

 quantity available is not certainly known. 



The water resources of the island are adequate to supply the usual popu- 

 lation and passing vessels. Some of the high-lying ponds on the main part of 

 the island furnish supplies of potable water. These ponds appear to lie in hollows 

 over the Montauk till, the clay of which forms an impervious bottom for water 

 derived from the gravel and sand that overlies the boulder clay. 



A few wells have been driven on the east side of Block Island to obtain water 

 for the use of the summer hotels. Boulders may be encountered at three horizons. 

 Beneath the upper boulder clay (Montauk) a well should encounter beds of water- 

 bearing gravel (the Herod), which is underlain by beds of sand (Jacob) and 

 fine gravel. These beds occur at all levels from the surface to and below sea 

 level, and as they are in places folded and contorted no rule can be given to 

 determine the depth at which water-bearing gravel or sand may be found at 

 any given place on the island. 



1 C. T. Jackson's map of Block Island, the result of a reconnaissance survey made in October, 1839, 

 shows an entrance into Great Salt Pond which lay north of the present canal and was known as "Old 

 Breach." See report on the Geology of Rhode Island, 1840. 



2 See Shore-line topography, pp. 201-206. 



