EFFECTS OF MARINE CURRENTS 29 



its base. Gradually the undertow of the breaker dra<''s the 

 debris to seaward, and the varying- currents produced by 

 the tides and storms remove it from the j)recipitous shores 

 to the pocket beaches, where, as we shall see hereafter, it is 

 ground to povv'der. The result of these causes is one of the 

 many beautiful adjustments of activities which the study of the 

 shore brings to our attention. The waves excavate only what 

 the currents can take away ; if at any time the\' cut out more 

 debris than is removed, their energy is diminished by the 

 shoaling of the water next the shelf ; if the currents clear 

 away more of the waste, the surges are for a time free to 

 act and deliver more sand and g^ravel to the sea. Thus the 

 processes of excavation and of carriage become accurately 

 balanced with each other. 



It is on these soft-rock shores, where strong currents oper- 

 ate, that we find the swiftest conquests of the sea over the' 

 land. On the hard-rock cliffs the erosion rarely forces the 

 cliffs inward at a o-reater average rate than a fraction of an 

 inch a year, while on gravelly or sandy shores the rate often 

 exceeds a yard per annum. Thus, on the coast of Cape Cod, 

 near Chatham, the shore is retreatino- into the land at the rate 

 of at least a foot each year. On the southern shore of Martha's 

 Vineyard, the recession of cliffs which are about one hundred 

 feet high, has been, on an average of forty years, about three 

 feet, and on the southern face of Nantucket, near Surfside, 

 the retreat of the escarpment has been as much as six feet, 

 in a single year. Although composed of somewhat harder 

 materials, the island of Heligoland, in the north of German}-, 

 near the mouth of the Elbe, exhibits a similarly rapid process 

 of destruction ; though within the historic period it was a 

 tolerably extensive land, it has shrunk before the surges of the 



