432 STUDIES FOR STUDENTS 



Approaching the shore, the material of the shallow bottom be- 

 comes coarser and ends in the undertow slope of the beach. This 

 is sometimes coarser than the material of the beach itself/ stones 

 of a certain size being swept here by the undertow and only carried 

 back to the beach by the heavier storms. It is to be concluded, 

 therefore, that one of the most striking characteristics of ancient 

 beach-action, a basal conglomerate, is hardly so much a mark of 

 littoral deposition as of marginal marine deposition bordering the 

 true littoral zone. 



As river deposits of delta surfaces, on the one hand, tend to extend 

 themselves both by building forward into the sea and by building 

 backward over the land, so, on the other hand, marine deposits, as 

 Chamberhn has pointed out, tend to extend themselves in both 

 directions.^ Toward the deep ocean basins the clays are swept and 

 deposited near their brink, doubtless building out submarine deltas, 

 as is suggested by the submarine platforms of the Caribbean Sea.^ 

 and by the hypsographic curve of the earth crust given by Penck.'^ 

 In the opposite direction the waves are always at work upon the 

 coast, and tend to cut the sea-chff landward, except where the supply 

 of material from the land is equal to that removed by the sea. As 

 the wave-beaten material is rolled backward and forward, it is 

 gradually reduced to a fineness where the undertow can sweep it 

 away from the beach-action and allow it to finally settle at some 

 little distance from the land. The result of these activities within 

 the upper portions of the ocean is to cut back all headlands, to fill 

 up recessions in the coast line, and to cut away all islands except 

 where these have been thrown up as barrier beaches by the sea. 



The rapidity with which the waves may cut into unconsolidated 

 material has been frequently illustrated by the destruction of recent 

 ash-cones, which in a few months or years have completely dis- 

 appeared. Barrier beaches, the only form of islands tolerated by 

 the sea, are thrown up where the waves drag and break in shallow 



1 Dana, Manual of Geology, p. 223. 



2 T. C. Chamberlin, "The Ulterior Basis of Time Divisions," Journal of Geology, 

 Vol. VI (1898), p. 454- 



3 Bailey Willis, "Conditions of Sedimentary Deposition," Journal of Geology, 

 Vol, I (1893), pp. 496, 497- 



4 Morphologie der Erdoberfldche, Vol. I (1894), p. 136. 



