266 The Origin and Mode of Formation of the Vale of Wardour. 
and at a much higher level. These lines of drainage are clear along 
the north side, though they are less clear on the south side, being 
obliterated by subsequent rain action. While this river action at 
the base of the escarpment was going on, landslips no doubt often 
took place, and thus the hills retreated to their present position. 
But the actual recession was not great. The summit ridge is 
nearly where it was originally, it is only the lower parts that have 
been searped back to the outerop of the Chalk rock. 
There is another agent which has had a powerful effect in wearing 
away the calcareous rocks, and that is chemical action. We have 
now in the Vale of Wardour about 36in. of rain annually, and it is 
possible that there may have been formerly much more. This rain, 
charged with carbonic acid which it has absorbed from the air, and 
taking up a much larger portion, as well as humic and crenic acids, 
from the decaying vegetation on the ground on which it falls, 
immediately attacks all calcareous rocks with which it comes into 
contact. 
The Portland and the Purbeck beds, when they in their turn were 
exposed by the denuding power of the stream, were in some degree 
worn away by this chemical action, which removed some of their 
calcareous matter and left as a residuum Sand and Clay on the 
surface. 
But, if the Portland and Purbeck beds have suffered some waste, 
the Chalk has been very largely removed by chemical action. 
Although the central ridge of the Downs, as we have seen, was 
never much higher than the plain marked out by the Chert Gravels, 
or by the “ Clay-with-Flints,” still, the parts now lower than the 
ridge have been much eroded, and a large portion of the material 
that has been removed has been taken away in solution by the acids 
taken up by the rain-water. 
When the rain falls upon the Chalk some small portion of it 
evaporates again, whilst another small portion flows over the surface, 
especially when the rainfall is heavy, washing away the fragments 
loosened by the frosts—for frost disintegrates the surface, as the 
water contained between the particles freezes—and carrying away 
the earth raised in the worm casts. But by far the largest part of 
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Ch 
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. L © eres. Pew eeys 4 
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