Geoscience Reference
In-Depth Information
Landscape C: The Mendip Hills
The Mendip Hills extend across the Somerset-Gloucestershire border, from Weston-
super-Mare in the west to Frome, some 40 km to the east. They rise to over 300 m
above sea level, and their landscape of limestone cliffs, plateaus and gorges provides a
striking contrast with the surrounding lowlands.
The Mendips are defined by a number of anticlines (upfolds) that have brought
Devonian Old Red Sandstone and Carboniferous Limestone to the surface. These older
rocks have resisted erosion to form uplands distinctly higher than the surrounding
softer and younger bedrocks. The pattern of anticlines and small faults was created dur-
ing the Variscan mountain-building episode some 300 million years ago.
A cross-section through the North Hill upfold (Fig. 159), just north of Wells, illus-
trates some of the features found in many of the Mendip anticlines: a core of Devonian
sandstone is flanked by layers of Carboniferous Limestone that are steeply tilted (to
about 40 degrees) and cut by small faults. The Devonian and Carboniferous bedrocks
have resisted erosion and form rolling upland plateaus, possibly relicts of some earlier
erosion event. Perhaps they are wave-cut platforms, recording a time in the past when
sea level was very high and/or the land was lower, similar to the wave-cut platforms
identified in the Southwest granites. The highest elevation of the cross-section is in the
Devonian bedrock, at 305 m.
Another cross-section (Fig. 160) reconstructs the wider Variscan fold and fault
pattern in a north-to-south direction across much of Area 8. We can picture a geometry
of this sort forming by using an analogy: imagine a weak paper tablecloth violently
pushed over the table-top. The paper will fold and tear as it is bunched up. Similarly,
the upper layers of bedrock have folded and faulted as they slid northwards over a
flat-lying surface (the table-top) and over each other. The flat-lying surface must have
formed by fracturing along some weak layer deep in the ancient (Silurian or older) bed-
rock.
