14 SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
district was one of regular and gentle sedimentation. None of the 
present physical features had then any existence. 
But this was not to continue indefinitely. At the close of the 
Carboniferous period the Bristol district was involved in the great 
system of earth movements, variously known as the Armorican or 
Hercynian, which can be traced so widely over western and central 
Europe. The efiect of these movements was to throw the strata 
into two sets of ridges and accompanying troughs, one set exemplified 
in our district by the northern extension of the Bristol Coal field 
with its bounding limestone ridges passing through Tytherington to 
the west, and Chipping Sodbury to the east, having their axes 
running north and south, and a second set exemplified by the Mendips 
and Broadfield down having their axes running east and west. 
These two sets of uplifts intersect in the Radstock district and are 
no doubt responsible for the remarkably disturbed character of the 
Carboniferous strata there. 
In this way the Bristol district was upheaved above sea level, and 
became subjected to a prolonged period of sub-aerial denudation in 
all probability under desert conditions, lasting throughout the Permian 
and earlier part of the Triassic period, and constituting the second 
chapter in its geological history. During Keuper or late Triassic 
times the salt lakes which, during the Permian and Bunter periods, 
lay to the north and south of the Bristol area, gradually extended 
and crept over the low lying part of the district, and in their more 
open waters the red and grey marls with masses of gypsum seen at 
so many points near Bristol were laid down. The higher hills, 
however, which even the prolonged denudation had been unable to 
wear away, stood out of the salt and barren waters of the Keuper 
sea as an archipelago of islands of which Mendip was the largest.^ 
And as the waters crept up their sides and occupied their valleys 
as salt creeks, the stones and boulders which lay upon their flanks 
formed, with cementing materials deposited by the lake, the massiv^e 
deposit known as Dolomitic Conglomerate. An excellent example of 
such a boulder-filled creek, remarkable for the extreme size of the 
blocks, is seen in Bridge Valley Road, Clifton. When it was occupied 
by the Keuper waters the Avon Gorge had no existence. The 
hollowed sides of the creek are still traceable, rising on the one hand 
towards Durdham Down and on the other towards Observatory Hill. 
Eastwards, towards Cotham, lay the spreading salt lake ; westwards 
lay the head of the creek ; and on the opposite or Somersetshire side 
of the present Avon Gorge may be seen the flat lying deposits 
resting unconformably upon the older rock surface, and formed when 
the waters had crept yet further up the valley. Some observers 
impressed by the size of the blocks, have called in the aid of glacial 
action to explain their presence, but there is no evidence of glacial 
striatioii to be seen here or elsewhere in the Bristol district. It may, 
parenthetically, be remarked that the term Dolomitic Conglomerate 
^ See “The Mendips; a geological reverie ”, ‘ Proc. Bristol Nat. Soc.’, new 
series, V. (1885-8), p. 236. 
