24 SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
of glaciers in our district snow must have lain thickly on the hills in 
winter, and, when it melted, produced torrential floods. 
The small streams of to-day must often have been swollen to many 
times their present dimensions, and exerted a denuding influence far in 
excess of its present amount. Geological uniformity does not exclude 
wide differences in intensity of action, and when we look down from the 
slopes above Limpley Stoke on the Avon creeping sluggishly seawards, 
we must not suppose that such was the stream that carved the valley 
through the Oolitic hills ; not such was the stream that swept down 
the large rounded masses of grit and limestone which are found in the 
Twerton gravels, near Bath. No such stream as this could have 
produced the Avon Gorge at Clifton. 
Some further allusion may here be made to a geological feature, 
which is particularly well illustrated in our district, namely, to the 
revelation by the later denudation of the sculptured surface which was 
produced in pre-Mesozoic times before the area was invaded by the 
salt waters of the Keuper lake, or w^as smothered beneath later 
Secondary strata. It is, no doubt, impossible to say for certain to 
what extent the ridges and valleys in the older Palaeozoic rocks have 
been remodelled by the later erosion which has revealed their 
existence. 
The valley of the Trym between Durdham Down and King’s Weston 
Down (see fig. 4), which has been eroded in an anticlinal fold of 
Carboniferous and Old Red Sandstone rock, is clearly a pre-Mesozoic 
feature, as its lower portion is occupied by flat-lying Triassic beds, 
before whose deposition some 1500 feet of Old Red Sandstone, 
2000 feet of Carboniferous Limestone, and 6000 — 7000 feet of Coal 
Measures must have been removed by denudation. Similarly, the 
hoi'izontally-lying Triassic beds, occupying the valleys to the north 
and south of Broadfield Down, show that these hills are of 
pre-Mesozoic date. 
At various points in the Bristol area, and notably on Durdham 
Down, in the neighbourhood of Sea Walls, the highly inclined beds of 
limestone have been planed off to form an almost absolutely level 
surface. Such a surface could not have been produced by subaeriel 
erosion, and it is probable that the final planing of the ridges was 
effected by the waves of the Jurasssic sea. On these planed surfaces, 
by further subsidence of the land, were deposited the somewhat 
abnormal Liassic strata, which, as has been already mentioned, are 
found on Broadfield Down and elsewhere, resting directly on the 
Carboniferous Limestone. It is inconceivable that these Liassic beds 
could have been preserved in their present position if the planed 
surfaces in their neighbourhood were the result of a post-Liassic 
denudation ; and evidence of a similar kind is presented in such 
amount, and so well distributed throughout the district, as to justify 
the conclusion that the Carboniferous Limestone ridges, which form 
so marked a feature in our scenery to-day, are ancient denudation- 
features revealed by, and only to a small extent re-modelled by, the 
later forces of erosion. 
