Geoscience Reference
In-Depth Information
Drainage and agriculture created today's characteristic Somerset Levels landscape: flat
straight-sided fields of lush pasture, bounded by artificial streams (called
rhynes
), wil-
lows and occasional poplar trees. The rhynes augment the natural drainage provided by
the rivers Parrett and Brue.
The bedrock of this Landscape varies considerably. The hills and ridges of higher
ground are formed mainly of Triassic and Jurassic clays and limestones that have resis-
ted erosion by the ancestors of the present-day rivers and streams. Much younger sed-
iment has accumulated in the lower ground, particularly during the later stages of the
Flandrian transgression that followed the last cold episode of the Ice Age. In the inland
areas, the clay is buried under a surface blanket of peat, the youngest sediment in the
region, which formed between 7,000 and 2,000 years ago.
The low but often distinctive Mid-Somerset Hills are due to the resistance to
erosion offered by thin limestones in the lowermost part of the Blue Lias, which forms
the oldest layer of Jurassic bedrock. The sharpness of the toes of the slopes just above
the flat ground of the Levels suggests that the hills may have been eroded not only dur-
ing the Flandrian sea level rise, but also during the earlier (Ipswichian) high-stand of
the sea, about 130,000 years ago.
Glastonbury Tor (
b2
) and the Pennard Hills (
b3
) are very clearly defined features
of the Mid-Somerset Hills, both of which have been eroded from layers within the
Early Jurassic Lias group. The distinctively shaped Tor of Glastonbury is capped by
Bridport/Yeovil Sand and owes its well-defined form to the resistance of this material
to erosion (Fig. 102). The lower slopes of Glastonbury Tor and most of the Pennard
Hills are composed of earlier Jurassic silts that also appear to have resisted erosion
more than the surrounding mudstones. One possible explanation for this is that miner-
alisation from hot water migrating up faults has strengthened the surrounding bedrock:
the Pennard Hills, Glastonbury Tor and Brent Knoll (in Area 8) are all associated with
northeast-southwest trending faults.
FIG
102.
Glastonbury Tor (Fig. 99,
b2
) and some of the Somerset Levels. (Copyright Dae
Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
