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of this sediment must have been transported from source areas some distance away
from the valleys themselves. Figure 159 presents an example of this situation, where,
at the western end of the cross-section, around We stbury, the Carboniferous Limestone
has been tilted and fractured, and then eroded to produce a distinct valley almost 1 km
across. This valley was then plugged by Triassic sediments consisting of angular coarse
blocks (breccias) derived from the nearby valley walls and sandstones derived from
nearer the valley centre. Local hollows in the hills are preserved in the same section
on the Mendip upland east of Priddy, and on the northern Mendip flank near Chewton
Mendip.
FIG
161.
Cheddar Gorge seen from the Heights of Abraham (Fig. 156,
c4
). (Copyright
Landform Slides - Ken Gardner)
Landscape D: The Bristol and Avon Valleys and Ridges
This Landscape lies north of the Mendips and west of the Cotswolds, and includes
the city of Bristol and the coastline from Weston-super-Mare to Avonmouth. North of
Avonmouth the landscape merges into that of the Severn Vale. The region is hilly but
does not reach the heights of the Mendips. The main river is the Avon, which runs
northwest towards the Severn, passing through Bath and Bristol on the way.
Like the Mendip Hills to the south, the discrete hill features of this scenery are the
result of Variscan fold structures in the bedrock. However, the folds are less regular and
continuous in their arrangement than in the Mendips, because the Bristol Landscape
appears to have developed across the transitional zone of the Variscan Front. Here the
east-west trends of the main Variscan mountain belt give way northwards to a more
open arrangement with a general north-south trend.
In spite of this important difference in the regularity and trend of the folds, the ba-
sic bedrock components are very similar to those of the Mendips. Carboniferous Lime-
