WATER AS A MECHANICAL AGENT. 223 



for long lines of seaweeds, which the wash of the waters carries up the beach 

 and has to leave because the sands of the upper and drier part take most of 

 the waters off by absorption. Here and just below are often found accumu- 

 lations of magnetic iron sand and garnet sand, which the return-flow was not 

 strong enough to carry back down the beach with the other lighter sands. 

 (See page 170.) (2) The heach-slope, the outer surface of the beach-formation, 

 the stratification being parallel to it. When sand-made, its surface is marked 

 with faint channelings of rills from the return-flow, and more faintly with 

 wave-like outlines of the upward wash. (3) The under-water slope — the 

 continuation of the beach-slope downward beneath the water made by the 

 undertow and perhaps coarser in material than the part above. It is the 

 place for boring Mollusks, Sea-worms, and Crustaceans. Stones and coarse 

 shells that may be dropped by the flinging breakers on the beach-slope are 

 pretty sure to be carried back by the return-flow for another chance of trans- 

 port, because the plane of rest is underneath them and not through their 

 centers of gravity ; and for the same reason the stones of experimenters on 

 beach-action usually go the unexpected way — seaward. 



The grinding carried on over the beach reduces the sand to finer sand, 

 and especially the grains of feldspar and of all minerals softer than quartz. 

 The undertow carries these seaward, where the current distributes them over 

 the shallow bottom. In this way deposits of fine earth, clay, or mud are 

 forming near those of coarse sand or gravel. Tidal flats of mud or sand in 

 estuaries, when lying exposed above low water, are likely to receive ripple- 

 marks, foot-prints of passing animals, raindrop prints, mud-cracks, and to 

 secure, when the tide turns, their burial beneath other sands and thus their 

 preservation. Under the tearing action of the heavier seas, the summit- 

 ground may be put temporarily into the beach-slope, or large portions of a 

 beach may be torn away and reconstructed; and, since the volume of the 

 return-flow would be at the same time augmented, the beach may become 

 temporarily steeper and coarser. Along most windy shores it requires only 

 one of the extraordinary storms that come at long intervals to destroy much 

 of the work of a century. 



5. Extension of beaches into points or spits, and harriers. — A beach is, in 

 the long run, essentially permanent in form and structure, unless a coast is 

 undergoing change in level or in other respects. But in regions of frequent 

 storms, the storm-made waves and currents give the sands a set or drift 

 to leeward. When, in this way, the line of a beach reaches in its leeward 

 extension a shallow bay, the drift of sands, still continuing, will build out a 

 point where the current loses velocity against the stiller water of the bay ; 

 or, if the water is not too deep, it will extend a barrier of beach-sand across 

 the bay, cutting off an inner shallow portion from the ocean, leaving only a 

 single oblique entrance, which the tides had kept open. By such means the 

 south side of Long Island, and a large part of the Atlantic coast south of 

 New York, has been supplied with its beach-sand barriers, and also, inside of 

 the barriers, with a long range of sounds. 



