186 
SUB-AEEIAL DENUDATION 
succession. On entering the gorge we have near Windsor 
'J errace, Millstone Grit.. Then follow the Upper Limestone 
Shales, dipping to the S.E. or up stream at an angle of about 
97°. After this comes the massive Mountain Limestone, on 
which the buttresses of the Suspension Bridge are built. As 
we walk through the gorge beneath the bridge, we pass through 
a considerable thickness of this rock. But just beyond the 
Clifton Station, there is a sudden change. Abutting against 
the Mountain Limestone are much contorted red grits and 
Limestone Shales. And if we ascend to the Observatory Hill, 
we pass, in a few paces, from Mountain Limestone on to Mill- 
stone Grit. 
This sudden change is the result of the Clifton Fault, which 
ci’osses the Avon at this spot. By it the strata which lie to the 
N. of it are shifted downwards. Taking as a datum point the 
intersection of the line of fault and that of high-water mark, 
the rocks relatively shifted downwards are : — 
On the Gloucester Side. 
Mountain Limestone ... ... 710 feet. 
Upper Limestone Shales ... ... 440 ,, 
Total 1150 „ 
The total thickness of the upper Limestone Shales is 
600 feet, so that above high-'water mark there are some 
160 feet, in vertical thickness, of these beds, which will 
extend to a height of some 180 feet or so above the river, 
and above them the Millstone Grit comes in near the top 
of Observatory Hill. 
Since the fault cuts across the strata somewhat obliquely 
(namely at an angle of about 5° with the strike of the beds), 
lower beds of Mountain Limestone are intersected on the Leigh 
side than on the Clifton side. Hence the thicknesses of 
