BEACH AND COVE GEAVELS. 43 



steeply up from the sea to a height determined by the waves, while at the 

 same time the undertow has taken a portion of the beach matter out into the 

 sea, as shown in fig. 4. 



The distance the finer matter is drawn back into the sea depends on 

 many circumstances, such as the height of the tides, the outline of the 

 coast, the slope of the shore, the depth of the water, etc. When the slope 

 is sufficiently gentle, the forward push of the breakers is greater than tlie 

 backward pull of the undertow, and a ridge of shingle is formed across 

 the bays, as shown in fig. 5. Such ridges are named sea walls in Maine, 

 and are common on the exposed coasts. The material is derived from the 

 erosion of the projecting headlands or is driven up from the sea bottom 

 when the slope is very gradual. Indeed, there is always a sort of shelf or 

 terrace near low tide, where the force of the tmdertow is checked by the 

 sea, even in the steeper coves. If, now, the slope should become more 

 gentle, the forward push of the waves would soon change the ten-ace into a 



Sealeye/. 



ridge rising above the land back of it. Such are the beaches of the glacial 

 Lake Agassiz, as described by Mr. Warren Upham, and the old beaches 

 of Lake Ontario observed by me in central New York. Occasionally 

 such a sea wall was formed in Maine in the period when the sea stood 

 above its present level, though the ones examined by me were neither so 

 high nor so long as those of the coast to-day. Having the foi-m of an 

 artificial embankment across a valley, which they are likely to dam, pro- 

 ducing a lake, they have sometimes been supposed to be prehistoric, built 

 by the inevitable Indians. 



It is important to note the action of the sea waves upon projecting 

 capes. As the waves strike a point of land they are divided, and the water 

 is forced obliquely or laterally along the coast toward reentrant parts. 

 Loose ddbris is at the same time driven obliquely away from the projecting 

 capes and collects in the bays as beaches or as sea walls. So, also, the 

 waves are constantly changing their direction under the action of varying 

 winds, and beach matter is transported laterally along the coast whenever 



