54 PROCEEDINGS COTTESWOLD CLUB I90I 
an open ocean, undermining cliffs and eating away the land ; 
but they do not appear to have cut far into the Mendip 
area. ‘The invasion of the Rheetic sea introduced a more 
potent agent of denudation. The breakers of this exten- 
sive ocean were undoubtedly larger and stronger than 
the waves of the land-locked seas of the Permian and 
Trias, and it can hardly be doubted that to their agency is 
to be attributed the planing off of some of the asperities of 
the Paleozoic land. The level plateaux of Durdham and 
Clifton Downs are probably amongst the results of this 
period of denudation. 
Submergence went steadily on, and the sea began to- 
swarm with the reptiles and ammonites of the dawn of the 
Jurassic period. The Lower Lias deposits overlapped the 
Rheetic strata, and enveloped all but the loftiest islands of 
the Mendip archipelago. The high ground surrounding 
Bristol was submerged, for fossiliferous Lias limestone is 
found filling in fissures of the Carboniferous Limestone 
on Observatory Hill; and a patch of Lias still remains high 
up on Broadfield Down, to the south of the city. To the 
Lias succeeded the Oolites, and the whole, or nearly the 
whole area sank beneath the waves. 
There remain two points to be briefly discussed. These 
are the origin of the Avon Gorge and the beginnings of the 
Bristol Channel. 
It may be clearly shown that the gorge of the Avon had 
no existence in Pre-Mesozoic times. During the deposi- 
tion of the “ Dolomitic Conglomerate ” of the Valley Road 
section, the creek in which it was formed extended to the 
west, right across the line of the gorge to Leigh Down, 
and it sloped upward in that direction. This is proved by 
the existence of Keuper strata’ capping a Carboniferous 
Limestone precipice on the western side of the river just 
opposite the section. These beds are at a rather higher 
1 Pointed out to me by Prof. Lloyd Morgan. 
