22 
SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
picturesque gorge of Tortworth, and so joins the waters of the Little 
Avon which falls into the Severn near Berkeley. Etheridge believed 
that the gorge was a faulted line, but careful examination shows that 
there are no dislbcations of any importance, and that the defile is a 
denudation feature. When the river, whose drainage area has probably 
been materially decreased by the encroachment of the Frome basin, 
began to flow, the horse-shoe ridge was still buried beneath the 
Secondary strata over which the stream took its northward way in 
accordance with the slope of the surface as it then existed. And the 
gradual emergence of the old limestone ridge could do nothing to 
divert its well-established course. 
But the same horse-shoe ridge is not only breached from within by 
an outward-flowing stream, but it is also cleft from the opposite direc- 
tion by a tributary of the Frome near Chipping Sodbury. On either 
side of the old British encampment of Old Sodbury two streamlets have 
their source within a mile of each other ; one flows to the east, the 
other to the west. The former is a parent stream of the Bristol Avon ; 
the latter, of its tributar^q the Gloucestershire Frome. The easterly 
rill, mingling its waters with those of the other streams, runs pa.st 
Malmesbury, sweeps round by Chippenham, and flows in a bold curve 
through Trowbridge, Bradford, and Bath to Bristol, where it is joined 
by the waters of the other rill which have flowed through Chipping 
Sodbury, Yate, and Stapleton. Not only, therefore, does the lesser 
stream cut through the Carboniferous Limestone ridge, but the longer 
river threads its way through the Oolitic upland round Bath, here 
cutting boldly through the range of hills on the easterly dip-slopes of 
which it had its birth not many miles further north. 
When we remember the length of time that has elapsed since these 
streams began to flow, and since the later denudation of the district 
began to carve the surface into sculptured relief, we shall cease to 
wonder at the depth of its gorges and the broad sweep of its more open 
valleys. And when we realize how diverse are the rocks which have 
been exposed to these long-continued erosive influences, we shall under- 
stand how it is that the Bristol Avon and its tributaries have been 
instrumental in producing such varied and beautiful scenery. Nor 
must we forget that during the vicissitudes of a changing climate, and 
in consequence of variations of relative level in sea and land, denudation 
may well have been often far more intense than it now is. The cliffs 
of the Avon Gorge did not always look down on a tidal mud-laden 
stream. There were once some hundreds of feet of descent between the 
present site of Bristol and the sea many miles distant. When the 
streams began to flow the climate was tropical, as the fossils of the 
Tertiary deposits of the London Basin show ; a warm ocean current 
may have coursed through Asia to the Mediterranean, and through the 
heart of Europe to the London Basin ; and a tropical rain-fall may 
have swept the valley slopes and swelled our streams. As Tertiary 
time elapsed this current was diverted, and the climate passed through 
successive phases of lowering mean temperature until the increasing 
cold culminated in the glacial epoch. Although there is no evidence 
