188 
SUB-AERIAL DENUDATION 
Shales. The new Zigzag path lies in a notch cut in the 
Limestone, from the point of junction at the edge of Durdham 
Down of the Mountain Limestone and the Upper Limestone 
Shales. 
If, on the other hand, we look across from Observatory Hill, 
towards the Leigh Woods, we shall see to our left the beauti- 
fully-wooded cleft of Nightingale Valley ; then follows a second 
face of limestone, which abuts against the Upper Limestone 
Shales brought down by the fault. The slope then becomes 
more gentle, for here we are in the softer shaley beds. The 
solid limestone is seen rising from the river bed near the railway 
tunnel ; after which there are bold quarried limestone faces. 
If we cross to the Somersetshire shore and ascend Nightin- 
gale Valley, we shall probably notice that, about half way up, 
the path becomes decidedly less steep. The valley also opens 
out and becomes wider. Here it is, I believe, that the fault 
crosses the valley. If, at this point we ascend the right (N.) 
side of the valley into Leigh Woods, we find the end of a vallum 
thrown up by the Britons in making their Leigh Woods entrench- 
ment. And if we follow this camp vallum we find that it leads 
to the edge of the limestone bluff, which abuts against the 
Upper Limestone Shales faulted down. The vallum, in fact, 
marks the line of the fault. 
Beturning to Nightingale Valley and following it up, we find 
that it passes into a line of depression which forms a well-marked 
dip in the Abbot’s Leigh Road, crosses Ashton Park and Beggar’s 
Bush Lane, and so leads up to Hill Farm. 
It is interesting to notice that the line of drainage, after 
following the line of the Upper Limestone Shales for awhile, 
deserts that line and passes down Nightingale Valley, thus 
cutting off a triangular wedge of limestone between the line of 
fault and the line of Nightingale Valley— a wedge which the 
Britons had only to fortify on its faulted side to convert it into 
