242 ECONOMIC GEOLOGY 
from 50-100 fathoms, the sea-bed begins its long descent 
to the oceanic floors. This shelf has been regarded as either 
a great plain of marine erosion, or a submerged peneplane ; 
but it is too wide and deep to have been cut by the tide, 
and is more regular and extensive than any peneplane. 
The explanation has been given by Nansen, who attributes 
the shelf to waves spreading loose material evenly over the 
sea-floor, filling up depressions, and wearing down rock 
ridges. 
RATE oF MARINE ABrAsioNn—The rate of marine abrasion 
varies with the strength of the rocks and their exposure. 
The loss of land on the southern coast of Yorkshire is 
estimated at 3 yards per annum for the past century, and 
a total of 3} miles since Roman times. A British Association 
Committee on Coast Erosion reported in 1895 that the York- 
shire coast during the 37 years, 1852-89, had lost 5 feet 
10 inches per annum. The recession is also rapid at exposed 
positions in Norfolk and Kent. The question whether this 
process is a national danger was investigated by a Royal 
Commission on Coast Erosion (1906-11); the evidence 
proved that the British Isles gain from the sea more than 
they lose. In the 35 years, 1848-93, 774 acres were lost 
on the Yorkshire coast, but 2171 acres were reclaimed within 
the Humber. The cliffs in Southern-eastern Yorkshire are 
so high that for every square yard lost 3 square yards are 
gained by the redeposition of the material elsewhere; as 
the new land is Crown property the gain is national. The 
total loss to the United Kingdom between 1848 and 1893 
was 6640 acres; but over 49,000 acres were gained. 
Subaerial erosion, though less spectacular than marine 
abrasion, has greater effects. Subaerial lowering of the land 
has been estimated at about 1 foot in every 10,000 years ; 
the British coasts undergoing abrasion probably do not lose 
on an average more than 5 feet a century. The average 
height of England is about 650 feet. Hence the whole land 
would be reduced to sea-level by subaerial denudation in 
6% million years, during which marine abrasion would have 
removed a strip 65 miles wide, from part only of the coast. 
Coastar Accrerion—The land may be extended seaward 
by the formation of beaches, bars, and deltas, and the filling 
of bays and estuaries. The first stage in the filling of a bay