AND THE AVON GOKGE. 
189 
an almost impregnable camp. I think this is due to a disloca- 
tion of the strata along the line of Nightingale Valley. This is 
seen also on the Clifton side of the river in the precipitous face 
of limestone which overlooks the little recess in which lies the 
Clifton Station. Such a dislocation, probably following a joint 
plane, is scarcely to be w'ondered at in the immediate neighbour- 
hood of so well-marked a fault. 
The question naturally arises, Can w^e trace the fault further 
W. ? Between Hill Farm and the Lower Limestone Shale 
depression there is no evidence of its existence. Nor should we 
expect to find such evidence. For since the fault cuts the 
strata obliquely, beyond Hill Farm no Upper Limestone Shales 
would be faulted down to the surface ; but the upper beds of 
Mountain Limestone would be faulted down against the lower 
beds of the same rock. There would be no difference in hardness 
of the beds to produce differential denudation. But when we 
reach the depression of the Lower Limestone Shales, w^e have, I 
think, evidence of the existence of the fault. There is, in this 
line of depression, just where the road from Hill Farm descends 
into it through a little notch in the Down, a wooded island of 
Mountain Limestone. This I believe to be part of a wedge of 
limestone let into the Lower Limestone Shales, just as the w^edge 
of Upper Limestone Shales is let into the Mountain Limestone 
further E. And if this be so there must be a corresponding 
w'edge of Lower Limestone Shales let into the Old Led Sand- 
stone ; of the existence of which I have some evidence, from the 
nature of the stones ploughed up in the fields at this spot. 
Owing to the overlying cover of Mesozoic rocks it is impossible 
to trace the eastward extension of the fault. 
8 . — The Gloucestershire Tributaries. 
Our object being to trace the influence of geological structure 
on the contouring of the land, we must not omit to notice even 
