332 
THE ECONOMIC ASPECTS OF SOIL EROSION 
Whenever the level of sea and land remains for a considerable 
time constant and the shore is not protected by sand beaches, 
the sea cuts a distinct bench into the rocks. Even a few cen- 
turies will suffice to make this bench a noticeable feature in the 
sea front. A single geological period may serve to bring it to a 
Avidth of one or many miles. In general, howcA^er, the frequent, 
we may say the incessant, changes of level place the shoreline 
noAV here and noAV there on the land surface and so distribute 
the effect of the marine benching over an area the Avidth of 
Avhich varies Avith the stee])iiess of the slope from the interior to 
the ocean. 
It is not as yet possible for us to estimate the A'alue of this 
erosive Avork of the Avaves. Geologists and geographers have of 
late been disposed to give it less importance than they did in 
the earlier stages of the science. In my opinion they have seri- 
ously underestimated its importance. That it is of much A^alue 
is clearly shoAvn by the AVork that has been accomplished along 
the shores in very recent times. To limit ourselves to coasts 
that are at the moment steadfast and to areas within the limits of 
the United States, Ave may instance the southern borders of the 
islands of eastern Massachusetts, Avhich since the settlement of 
the country have been encroached upon by the sea at a very 
rapid rate. On the south side of Nantucket the Avaste in certain 
years has amounted to five or six feet. On the corresponding 
shore of Martha’s Vineyard the recession during the last forty 
years (as has been shown b}^ the surveys of Assistant H. L. 
Whiting, of the U. S. Coast Survey) has been at an average rate 
of three feet per annum. It is probable that the gain of the sea 
on this part of the coast during the three centuries since the 
land Avas first seen by Europeans has amounted to nearly a mile. 
On ordinary rock shores the rate of AA^earing is relatively slow 
and exceedingly variable in amount, but Avhere the waves have 
a fair chance to assault the land it is always considerable. Al- 
lowing the minimum results obtained in numerous observations, 
Ave must reckon the gain of the sea at a mean of tAvo feet per 
century. Computing at ten thousand years, the time that has 
elapsed since the ice-sheet of the last glacial period passed 
from these shores, the total amount of this coast erosion should 
average two hundred feet. During a period of one hundred 
thousand years — a very brief age in the history of the Avorld — 
the sea should have Avorked its Avay inunore than a third of a 
mile. Since the beginning of Tertiary time, Avhich cannot Avell 
