236 



PHYSICAL GEOLOGY 



belt covers an area of 62,000 square miles. The seaward limit is 

 seldom sharply differentiated, since the deposits grade into those 

 of the shallow sea, although sand, gravel, and shingle usually mark 

 the outer surface of the beach. 



Character of Littoral Deposits. — The deposits of this zone vary 

 greatly on different parts of the same coast. Muds are most common 

 in lagoons and in sheltered spots, and bowlders and shingle prevail 

 along rocky shores. The sand of beaches is usually composed of 

 quartz grains, since this is the most common mineral of the rocks of 

 the earth's crust. Another reason for the predominance of quartz 

 is the fact that when rock fragments are rolled about by the waves, 

 the softer minerals of which they are composed are soon ground to 

 fine powder and carried away by even slight currents, and finally 

 deposited as clay, the harder constituents only being left as sand. 



Locally, sands of other composi- 

 tions than quartz occur. Occa- 

 sionally garnet or magnetite 

 grains constitute the chief ma- 

 terial of beaches ; in the Bay of 

 Naples the sand is made up of 

 the olivine and feldspar derived 

 from volcanic rocks ; the sand of 

 the Bermuda Islands is composed 

 of minute shell fragments. 



Distinguishing Characteristics 

 of Littoral Deposits. — Certain 

 features are characteristic of 

 littoral deposits and are due to 

 the fact that these are alternately 

 covered by water and exposed to 

 the sun and wind. Ripple marks 

 (Fig. 229), made by the wind or 

 water; rill marks (Fig. 230), 

 formed by the water as it flowed 

 back down the beach at low tide ; 

 rarely sun cracks (Fig. 231), 

 formed by the drying of the mud ; raindrop impressions, footprints of 

 animals, 1 and fossils of land and sea animals and plants characterize 



1 Sun cracks, raindrop impressions, and footprints are more common on flood plains 

 :ni(l playas. 



Pig. 229. — Wave ripples on sandstone. 

 (Photo. H. L. Fairchild.) 



