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SUB -AERIAL DENUDATION 
The action of rain, frost, and the weather, on the other hand, 
is like the action of the sandpaper in our experimental illustra- 
tion. The action is general over the entire surface ; not 
special along definite lines. It wastes away the surface over 
the area of the softer strata ; but having less effect on rocks of 
greater resisting power, leaves them to stand out as bold ridges. 
Its action, moreover, is not limited in any way by the depth to 
which it can waste the harder bands. That which limits the 
depth of its action is the level of the river into which the detritus, 
which results from the general waste, is carried. For unless the 
detritus be removed, further waste is impossible. And thus we 
come to see that a band of hard rock, in limiting the depth to 
which a river can cut its way down into the rocks which lie 
further up stream, determines at the same time the level to 
which general denudation can reduce the whole surface of the 
land which lies up stream. 
3 . — The Physical Features of the Area under consideration. 
The physical features of the area under consideration ( see 
map ) are in some respects remarkable. The Avon which has, 
above Bristol, emerged from a wmody gorge, passes, near Bristol, 
through more open country, extending right and left from either 
bank, and then, below Bristol, makes its way between the steep 
slopes, and, in places, precipitous rock faces of the Clifton 
Gorge, emerging thence into more open country ere it finally 
passes through a band of Severn flats into the Bristol Channel. 
Opening out into the main ravine are several lateral valleys or 
gullies, some of them having now dry beds, others occupied by 
brooks or “ pills.” 
A bird’s-eye view of this part of the country will show on 
the Somersetshire side of the Avon a well-marked ridge of 
Downs, trending to the S.W. On the Gloucestershire side the 
ridge is split into two portions, of which one soon ends off, as 
