SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 21 
length reached the level at which the backs of the Bristol Downs 
were just laid bare. As the concurrent action of the two denudation 
processes continued through long ages, the effects where the ancient 
ridges of old rocks slowly emerged were different from those where 
pre-Mesozoic sculpturing had already formed broad and open valleys 
in which the more yielding Triassic and Liassic strata still awaited 
removal, as for example in the valley between Durdham Down and 
Kings Weston Down (see fig. 4). 
The action of the river-files was restricted to the rate at which they 
could cut through the harder rocks in the lower part of their course, 
for the erosion of the softer beds above was limited to the depth to 
which the notches in the harder strata were cut. And since the super- 
ficial waste, due to rain and weather, continued with unbating vigour 
over the more yielding area, the valleys here remained broad and open. 
But over the old ridges, whose existence was primarily due to their 
powers of resistance to an earlier superficial erosion, rain and weather 
had little power. The action of the file outran that of the sand-paper, 
and the gorges of the Avon ^ and the Trym resulted. They impress 
the eye from their narrow cleft-like form and their steep sides. But, 
gauged by the amount of material removed, they are characterised 
rather by defect than by excess of denudation. The quantity of 
sedimentary deposits swept away by the Avon during the formation of 
its striking gorge at Clifton is far inferior to that removed in an equal 
length af the river’s course above Keynsham, where the valley widens 
out and has a much less impressive appearance. 
Another example of the way in which the newer rivers cut defiles 
through the older ridges is seen near the northern boundary of the 
Gloucestershire coal field. A range of Carboniferous Limestone upland 
here forms the containing rim of the basin in which the lower- 
lying Coal Measures lie. From Almondsbuty through Tytherington, 
Cromhall, WTckwar, and so round to Chipping Sodbury, runs a tract 
of Carboniferous Limestone forming a horse-shoe ridge. Most of the 
drainage of the Coal Measure area is into the Frome which joins the 
Avon at Bristol. And one might naturally expect that the horse-shoe 
ridge would form the watershed of the whole of this drainage area. 
But this is not so ; for within the limestone rim, south of Cromhall, 
there rises a small stream which, instead of joining the Frome drainage, 
makes northward for the ridge, breaches it in the well-wooded and 
^The recognition of the existence of the old glacier lakes in the Vale of 
Pickering by Mr. C. Fox Strangways (‘ Mem. Geol. Survey, the Jurassic Rocks 
of Britain I, 1892, p. 423), and in the Cleveland Hills by Prof. P. F. 
Kendall ( ‘ Q. Joiirn. Geol. Soc.’, LVIII, 1902, p. 471, and ‘ Proc. Yorks. 
Geol. and Pol. Soc.’, XV., pt. i., 1903, p. 1) has suggested a new explanation of 
the origin of certain gorge-like valleys, viz., that they are the overflow channels 
cut by the escaping water from such ice-dammed lakes. Mr. F. W. Harmer 
has recently ( ‘ Q. Journ. Geol. Soc.’, LXIII., 1907, p. 483) suggested that 
similar ice-dammed lakes occupied the country in the neighbourhood of 
Trowbridge, and between Bath and Bristol, and that the gorges of the Avon 
between Bradford and Bath, and at Clifton, are the overflow channels from 
these lakes. 
