THE OCEAN AND ITS WORK 217 



lachian Mountains presented an irregular front to the sea, upon which marine erosion 

 was concentrated. As a result, a plain of marine denudation many miles wide was 

 cut. Upon subsequent oscillatory elevation, with many halts, lower plains were cut. 

 Consequently, in traveling from western Massachusetts to Long Island Sound, instead 

 of a much-dissected, gently sloping peneplain, one finds first the high, rugged moun- 

 tains which were not attacked by the sea; then a deeply dissected, slightly sloping 



Fig. 209. — New England plains of marine denudation, according to Barrell. The 

 dotted lines A, B, C, D are the successive levels of the sea. 



high plain, now almost completely destroyed ; and, successively, lower plains, better 

 preserved, until the sea is reached. The highest plain if restored would reach an 

 elevation of from 2300 to 2400 feet in western Massachusetts; and a total of seven 

 originally well-developed plains may be recognized, the lowest at a height of 700 

 feet. Below this are four plains of fainter development. If this theory is correct, 

 the so-called New England peneplain (p. 114) is really a combination of several 

 surfaces of marine denudation (Fig. 209). 



Transportation 



Littoral or Shore Currents. — The sediment carried into the ocean 

 by streams, as well as that eroded from the shores by waves, is 

 usually soon carried away by currents produced by waves, wind, and 

 tides. When a wave strikes a shore at right angles to its trend, the 

 water thrown upon the shore returns as the undertow (p. 200) and 

 may carry the sediment to great depths. The debris at the foot of 

 the cliffs is, however, not immediately transported to the deep water, 

 but is moved back and forth by the waves and the undertow, and is 

 thus ground finer and finer with time. Since the velocity of the 

 undertow rapidly decreases with the depth of the water, only the 

 finer sand can be carried a considerable distance. Consequently, 

 one usually finds coarse pebbles (shingle) near shore, and progressively 

 finer sediment farther out. 



When a wave strikes a shore obliquely, a portion of the water 

 returns immediately as undertow (Fig. 186, p. 200), and a portion 

 moves along the shore and forms a littoral or shore current. The 

 zone of breaking waves is the road of shore drift, and it often happens 

 that it is the waves produced by storms rather than those of the 



