76 
[ASSEMBLT 
Marine Alluvial Detritus. 
Beaches^ Shoals, Spits, 8fc, 
The destroying action of the sea upon the headlands and cliffy 
where currents and a heavy surf beat against the coast, has been 
considered. Another effect of the sea, and as important to the 
.community as that just discussed, is the formation of marine allu* 
vion. It results from the deposition of the materials transported 
coastwise by tidal and marine currents, and by the action of the 
waves in the direction of the prevailing winds and storms. The 
winds which produce the greatest transport of alluvial matter on the 
coast of Long Island, (with the exception of particular parts, 
where there are local exceptions in consequence of the form of 
the shore, or direction of currents,) are from the northeast, dur- 
ing the heavy northeast storms. These storms bring in a heavy 
sea from the ocean, which, rolling obliquely along the shore, aid- 
ed by the powerful tidal current, sweeps the alluvia along in a 
westerly direction. The northwest winds are nearly as powerful 
as the northeast, and blow for a much longer period in the year; 
but do not bring in an ocean swell, and the waves which they 
raise, fall upon the shore in a line nearly perpendicular to the 
trend of the coast, so that their effect is to grind the pebbles and 
gravel to sand, by the action of the surf, rather than to transport 
them coastwise. In this way, outlets of small bays are frequently 
more or less obstructed by bars, shoals and spits, formed by the 
tidal currents sweeping past their mouths, and depositing the ma- 
terials in the eddy formed by the meeting of the currents. If the 
strongest currents and prevailing winds be coincident in direction, 
the outlet of the harbor is on the leeward side. 
When rivers and small streams do not project deltas into the 
sea, the surf frequently throws up a bank of shingle or sand, so 
as to block up their mouths; and then, a fresh water pond or lake 
is the result. This effect, however, is much modified by the size 
of the materials, forming the beach across their outlet. If the ma- 
terials be coarse gravel and pebbles, or shingle, the water filters 
through at ebb tide, so as to retain it near the tide level; but if they 
be fine sand, the water accumulates until it overflows the obstacle, 
or has a sufficient head to excavate a channel through the barrier, 
and escape. 
Almost every bay, inlet and marsh, on the north coast of Long 
Island, and also, on the south coast, where they are not protected 
