194 
THE AGE OF THE WYE, 
The comparative rate of the disintegration of the rock under 
these two conditions we have no positive means of determining 
with accuracy. At the present rate of the progression of the 
bends” downwards already proved, of 52 yards in 100 years, 
the 110 yards would be exposed to the air for 212 years, and the 
1580 yards would then be covered by alluvium for 3039 years. 
After the foot of the cliff had been covered, the face of the 
cliff above would be subject to all atmospheric influences ; but 
that below the alluvial level would only be subject to a modified 
surface softening ” process — modified because the rock would 
have been already fully exposed to the air for 212 years before it 
was covered, and afterwards because that covering of alluvium 
would probably be more pervious to those atmospheric influences 
than the more compact rock in situ, and because the rains, and 
with them some portion of air, might find their way down the 
face of the rock between it and the deposited alluvium, so that 
when the cliff was again exposed by the descent of the “ bend ” 
above its face it would be in some degree softened, and thus 
subject to more rapid wear than it otherwise wmuld. In the 
figures I am about to give I have therefore assumed that the 
face of the cliff which has been covered by the alluvium would 
be worn away at half the rate at which the exposed cliff is 
actually worn away by the river. This is, of course, only an 
assumption ; but if there is an error, the assumption will most 
probably be in excess, rather than in dimunition, of the propor- 
tional rate of wear from the protected cliff. 
Coming now to the last element required for our calculation 
— namely, the rate at which the old red sandstone of this 
district is worn away by the river in the manner already 
described — I find that the Wilton Bridge, near Boss, was built 
in the year 1599, or 267 years before my measurements were 
made. The western abutment of this bridge was founded upon 
