structure of the Mendips
|At the end
of the Carboniferous and beginning of the Permian, continental drift
caused the final closure of an ocean to the south of Britain. Rocks
were pushed up into a vast mountain range that stretched thousands
of kilometres to the east and west. This phase of mountain building,
known as the Variscan Orogeny, is represented by the major time gap
(an unconformity) that occurs between rocks of Carboniferous and
Triassic age. It is during this time that the Mendip Hills were folded
into their characteristic 'whale back' form.
|The Mendips were close to the northern limit of
mountain building, and probably reached an original altitude of around
1500 m. Although complex in its detail, there are two key features
to the geological structure of the Mendips: the folding of the rocks
into four approximately east–west orientated upfolds (Blackdown,
North Hill, Pen Hill and Beacon Hill) and the development of gently
dipping fractures (known as thrust faults) along which major parts
of the rock succession have slid across each other.
Simplified geological map of the Mendip region showing the concentric
outcrop pattern produced by the four main periclinal folds. The Devonian
Portishead Formation is shown as dark brown, the Triassic rocks are
shown as salmon pink. Major thrust faults shown in red. (Click to
Cartoon to show the relationship between periclinal folds and
|| A close relationship
probably exists between the thrust faults and the folded rocks of
the Mendips. Intense pressures to the south of the British Isles
in the Late Carboniferous and Permian caused rocks within the highest
few kilometres of the Earth's crust to be pushed northwards over
more deeply buried strata. This movement occurred along large, gently
dipping thrust faults. The thrusts propagated through the rock from
south to north, with rocks at shallow depths sometimes becoming folded
ahead of the advancing thrust fault. In this way, the characteristically
narrow elongate folds of the Mendip Hills are thought to have formed.
All of the folds have steeply dipping or overturned beds on their
northern side and more gently dipping beds on their southern flank.
|The Mendips are an example of anticlinal folds,
recognised on geological maps by a concentric outcrop pattern with
the oldest rocks at the core. The distinctively shaped variety of
anticline that forms each of the Mendip Hills is known as a pericline.
Both anticlines and periclines are prone to rapid erosion after their
formation because strata are stretched and weakened when they are
buckled upwards. This explains why old, deeply buried rocks at the
cores of anticlines rapidly become exposed. On Blackdown, erosion
probably reached Devonian strata at the core of the anticline by
the Late Permian or Early Triassic.
Block diagram to illustrate the concentric pattern of older rocks
surrounded by younger rocks that has been produced by erosion of
the Mendip periclines.
Cartoon to show how thrust faults can move older rocks on top
of younger rocks.
Thrust faults can produce some unexpected geological relationships.
They allow older strata to be moved up from depth and be emplaced
on top of younger rocks. In some cases, far-travelled slices of
thrusted rock can be almost entirely eroded away, leaving small
isolated surface outcrops of older rocks on top of younger rocks.
This is the explanation for the occurrence of Carboniferous Limestone
on top of Coal Measures at Upper Vobster, near Frome. Older thrust
faults can also be folded by deformation associated with the development
of younger thrust faults.