THE ECONOMIC ASPECTS OF SOIL EROSION 
333 
be reckoned at less than two to three millions of years ago, the 
recession of the shore, due to the action of the waves, may safely 
be estimated at several miles. Taking all the coastline on the 
eastern border of the United States into consideration, we see 
that the sand beaches, owing to their singular endurance of wave- 
action (a feature I have discussed elsewhere), have an important’ 
restraining efiect on the process of marine erosion. Making al- 
lowance for this protective work, it remains clear that the effect 
of ocean waves is to wear back the shorelines into the land, and 
this at a rate which in a geological sense may be termed rapid. 
As geologists find but few shores bordered b}" distinct benches 
cut in the hard rocks, the.y have generally underestimated the 
value of wave-work, but in forming their opinion they have 
neglected the important fact that the continents are continually 
changing their positions in relation to the sea level. Every step 
in the advancement of our knowledge of the problem shows that 
the shore lands are ceaselessly and at times suddenl}^ moving 
upward or downward. Even those coasts which now appear to 
be steadfast have in very recent times changed their positions by 
sinking or rising. The result of these perpetual swayings of the 
coastlines is to distribute the benching action of the waves over 
a wide zone, extending along the most of the great lands from 
a level much below that of the present shores to a position far 
higher than that which they now occupy. In some instances, 
where the sea has chanced to remain for a long time in contact 
with the land on one horizontal plane, we note the existence of 
broad shelves of rock extending outward from the sea-cliffs, 
sometimes to the distance of a mile or more. Thus, on the coast 
of Yorkshire, from Whitby southward, a sea-cut bench, with its 
surface just above low tide, stretches seaward from the foot of 
the towering cliffs for an average distance of more than a mile, 
attesting in the j)lainest possilde manner the cutting power of 
the sea. In general, we may say of the eastern coasts of North 
America that indications of marine Avork are visible to a height 
of several hundred feet above the i)lane of the ocean, and that 
there is good reason to believe that such cutting Avork has been 
done on much of the slope Avhich noAV lies beloAV the sea. \\’hen 
by the uplifting of the land ancient sea bases are carried above 
the limits of Avave-action they are quickly Avorn aAvay hy the 
})roce.sses of erosion Avhich are proper to the land. When such 
benches are loAvered beneath the ocean they are soon covered b}” 
sediments, and thus brought into j)ositions Avhere even subse- 
22 
