SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 25 
The way in which it is possible to reconstruct some of the physical 
features of the ancient land surface, which the later erosion has 
revealed to us after so great a lapse of time, is peculiarly interesting. 
No doubt the uplands, before they were planed down by the waves of 
the Jurassic sea, may have been higher and more rugged than they now 
appear ; but since, as they sank beneath the waters of the Triassic 
lake, their valleys formed creeks in which were formed marginal 
deposits of a tough and resisting nature, the tongues of these rocks, 
which run up into the limestone uplands, enable us to trace the old 
valley lines, and also to see their independence of the more recent 
gorges, which have been formed during the later denudation. This is 
•exemplified in Fig. 5, representing the neighbourhood of Back well on 
the northern slopes of the upland of Broadfield Down. It also in- 
troduces one of the patches of Lias, which have been already referred 
to as resting on the broad back of the Down. The manner in which 
the marginal deposits run up into the limestone along the lines of old 
creeks is clearly seen ; and the comparative independence of the 
gorge of Brockley Combe, the product of later denudation, is also 
■evident. 
Fig. 5.— SKETCH MAP OF THE NEIGHBOURHOOD OF 
BACKWELL HILL. 
Scale inch = 1 mile. 
Keiiper and alluvium 
Dolomitic | o°o°o 
Conglomerate o^o 
Carboniferous Limestone 
