to Ground Water along Coasts. 281 



low tide, because of the extreme porosity of the sand, that 

 at high tide there is a distinct influx landward of salt 

 water. High storm waves sometimes completely overtop 

 the bars and spits, thus undoubtedly saturating them with 

 salt water. On unusually high and wide spits a small 

 amount of usable water might be obtained by shallow 

 wells located as far as possible from the sea. However, 

 the ground water doubtless becomes exceedingly saline 

 at no great depth below the water table on most of these 

 areas. 



Islands. — Conditions on islands do not appear to differ 

 materially from those on the mainland except where the 

 available intake for ground water is exceedingly small. 

 The nature of the land surface and of the rock composing 

 the island are factors of importance in influencing the 

 character and quantity of the ground-water supply. A 

 covering of till or of stratified drift promotes absorption 

 of rainfall and equalizes the supply of ground water 

 reaching the water table, whereas bare rock surfaces pro- 

 mote run-off. It seems from all the available evidence 

 that good water, in small quantities, may be obtained even 

 on very small islands where there is a cover of till or 

 stratified drift. An island having an area of an acre, or 

 about 250 feet in diameter, usually will supply enough 

 water for an ordinary household. The supply increases 

 with the size of the island, though probably in consider- 

 ably greater ratio, as the relative losses about the 

 perimeter become smaller. 



Effect of bedrock fractures. — Sea water enters wells 

 that are dug or drilled in bedrock near the shore through 

 fractures. Clapp 7 suggests that in certain contaminated 

 wells on a peninsula in Maine the sea water enters at 

 the side of the peninsula farthest from the wells, because 

 the cleavage of the rocks dips from that side toward the 

 wells. From the evidence on the New Haven coast it 

 seems reasonably safe to assume that a high degree of 

 fracturing in the bedrock on the shore increases the 

 chances of sea water entering wells near by, and that the 

 trend or inclination of fractures toward a well may also 

 be of importance, but only as indicating the greater like- 



7 Clapp, Frederick G. : Underground waters of southern Maine, U. S. Geol. 

 Survey Water-Supply Paper 223, p. 67, 1909. 



