| The Mendip
plateau is incised by numerous dry valleys which deepen into spectacular
gorges such as Cheddar, Ebbor,
and Burrington Combe,
as the steep flanks of the hills are approached. The largest of
these, Cheddar Gorge, is entrenched up to 120 m deep.
The origins of these dry valleys and gorges have been debated for
over a century. The earliest ideas involved earthquake rifting and
similar catastrophic phenomena. The first plausible hypothesis, that
the gorges were formed by cavern collapse was put forward in 1862,
a myth that is still often perpetuated in many modern geological
texts. This theory remained popular until 1927 when it was suggested
the gorges were cut by surface rivers.
Neither Cheddar or any of the other gorges and dry valleys are collapsed
caverns, except possibly the Wookey Hole ravine. The size of the
smallest gorge on Mendip is still far larger than even the largest
Mendip caves such as GB Cave, and Lamb Leer. Many dry valleys
have cut though existing cave passages. Furthermore, most stream
caves on Mendip descend rapidly at first before levelling out at
depth, whereas the steepest section of both Cheddar and Ebbor Gorge
is near the mouth.
The Mendip dry valleys and gorges were incised over the last million
years or so by meltwater rivers during the Ice Age, when the caves were
blocked by ice, gravel and frozen mud. Although the area was never
glaciated, the region was still deeply frozen much of the year. Torrents
of summer meltwater from the snow caps poured off the hills. The material eroded during these periods
was deposited as large alluvial fans (marked as 'head' on most geological
maps) extending out from the gorge mouth, an excellent example occurs
at Burrington Combe.
On western Mendip, many modern valleys follow earlier filled-in Triassic
valleys where erosion has picked out the softer Mercia Mudstone,
in preference to the harder, more resistant Carboniferous Limestone.
Apart from a few exceptions in the extreme east, all the valleys
cut into the limestone are now dry. This is due to the development
of underground drainage and the formation of extensive cave systems.