20 SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
Mesozoic strata, and the old land surface on which they rested, a 
prevalent gentle easterly dip. And we have good reasons for con- 
cluding that the limestone ridges round Bristol were still covered by 
Secondary deposits when the district once more formed dry land. 
The beginnings of our present system of river drainage were then 
outlined, and the fourth chapter in the Geological history of the 
district exhibits the action of these denuding forces, first on the 
Secondary rocks, and secondly on the old pre-Mesozoic land surface, 
gradually laid bare. 
The Severn and its tributaries, especially the Bristol Avon, play 
the leading parts. The depression through which the Severn now 
passes is a very old physical feature. Great faults such as that 
which, near Cattybrook, has brought the Coal Measures against the 
more resisting Lower Carboniferous rocks, and those which disturb 
the strata near Portiahead, may have helped to outline its southern 
shore. Prolonged denudation scoured it out ; and in Triassic times 
it seems to have formed an arm of a lake, narrowing westwards. 
On the re-elevation of the land in Tertiary times it began to be 
what it now is, a dominant line of drainage, and poured its waters, 
as now, westwards into the Atlantic. Tributary streams bore down 
to this ancient Severn the debris removed from the Secondary rocks, 
wearing them back to the escarpment which continues the line of the 
Cotteswolds southwards towards Bath, and leaving outliers (of which 
Dundry is a conspicuous example) standing out as hills of circum- 
denudation, surrounded by the shrunken streams which have carried 
off the detritus from their weathered flanks. 
If we endeavour to picture the scene when, in early Tertiary 
times, the tributaries of the Severn began their work of denudation, 
and when heavy rains swept the gentle slopes of their valleys, we 
see as yet no sign of the ridges of Carboniferous Limestone or Old 
Bed Sandstone which now form such marked features of the land- 
scape. The pre-Mesozoic land-surface, in the immediate neighbourhood 
of Bristol, was still buried beneath a gently undulating sheet of 
Secondary strata, in the hollows of which the streams had already 
established their course — a course thus necessarily wholly independent 
of the ancient physical features still enveloped in the sediments of 
the Secondary age. Unless we realise that the Avon and its tributary 
the Trym, had their course determined long before the Downs 
emerged into view through subsequent denudation, it is quite im- 
possible to understand how the gorges of Clifton and Combe Dingle 
came into existence. One may liken the effects of sub-aerial denuda- 
tion to the concurrent action of two processes ; first, the file-like 
fretting of the streams in their beds ; and secondly, the wasting of 
the whole surface by means of the sand-paper action of rain and 
the crumbling disintegration of the weather. But the stream has a 
double office ; not only does it deepen its channel, but it bears sea- 
wards all that rain and the weather wear off the valley slopes. 
Where the rocks are soft and yielding the sand-paper and the file 
act at nearly equal rates : the valley is wide with gentle slopes. 
Such were probably the conditions when the Avon and the Trym at 
