SUBTERRANEAN EROSION, AND SOME OF ITS EFFECTS. 
im Ithell’s Sandpit. 
the nearer the under- 
ground water approaches 
the point of escape, the 
greater becomes its 
power of subterranean 
‘erosion; and decreasing 
in its power, from the 
point of escape zmwards, 
in inverse ratio towards 
the underground water- 
shed. This results in that 
lateral subsidence which is 
so characteristic of sub- 
terranean erosion. Inthe 
Sandgate disaster /afera/ 
subsidence was a very 
marked feature. 
By way of a local illus- 
tration we may take the 
following section from 
Ithell’s sandpit, Upton, 
Chester, to the Chester 
and Birkenhead road, a 
distance S,S.E. to N.N.W. 
of 480 yards, across two 
valleys cut, out of the 
Glacial Drift. 
Along the line of sec- 
tion at B-B the sands 
and gravels crop out from 
under the clay. At such 
points springs are of fre- 
quent occurrence, and 
if these are caught in 
troughs deposits of sand 
quickly accumulate in the 
receptacles. After heavy 
rains the quantity of sand 
deposited by even tiny 
springs is very consider- 
able. 
Where the drainage is 
insufficient to maintain 
permanent springs, a 
stratum of a soft puddly 
nature is produced by 
the constant oozing-out 
of underground water. 
There are other outlets 
which are active only 
during the continuance 
of very heavy tain, act- 
ing as storm-overflows for 
the ordinary underground 
drainage. 
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