220 



PHYSICAL GEOLOGY 



is carried along it does not conform closely to the shore unless the 

 indentations are comparatively slight. When it is swept into a 

 shallow, sheltered bay or cove, it may form a bayhead beach (Fig. 210). 

 When such a beach is attacked by storm waves a ridge is sometimes 

 thrown up on the seaward edge, forming a dam behind which a shallow 

 lake or marsh is formed (Fig. 211). An interesting fact in connection 

 with these pebble beaches is that sometimes during a single gale an 

 entire ridge may be moved as much as 30 feet. 



Bars and Spits. — When littoral drift reaches an abrupt bend in a 

 shore, as, for example, at the entrance of a bay which extends some 



distance inland, it does not 

 follow the bend, but usually 

 continues in the direction in 

 which it has been moving. 

 It therefore passes from shal- 

 low water to deep, where it 

 drops its load. Since the 

 portion of the littoral cur- 

 rent that carries the sand is 

 usually narrow, the material 

 dropped into the deep water 

 is gradually built up in the 

 form of an embankment, like 

 a railroad fill, which may, in 

 time, extend entirely across 

 the bay. Currents do not 

 build bars above the level of 

 the water, but waves may do 

 so by washing the sand and 

 gravel from the slopes of the bar to the top. As soon as a portion of 

 the sand is exposed above the water, it may be blown into dunes 

 by the wind. Since dune topography is rough, such sandy stretches 

 often have an uneven surface. 



Often a bar is never completed (Fig. 212), since the rivers flowing 

 into the bays have sufficient volume and current to keep a channel 

 open. The scouring action of tidal currents may also be able to re- 

 move sediment to deep water as rapidly as it is brought in by the 

 shore currents. Incomplete bars, when built above the surface 

 of the water by waves, are called spits, and when curved by the 

 force of the tidal current at right angles to the drift are called hooks 



Contour interval 10 feet 



Fig. 212. — Map showing an incomplete bar 

 almost shutting the larger lake from the sea, 

 and a complete bar across the smaller lake. 

 Delta filling is well shown. (Atwood.) 



