An Orogeny (derived from the Greek Oros for mountain and Genesis for
creation) is a long-lived deformational episode of mountain formation,
involving powerful compressive folding and faulting of the Earth’s crust.
Laurussia
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O
eic
Rh
Gondwana
The geological history of the British Isles has been affected by three
orogenies over the last 600 million years. The Caledonian Orogeny
(between 490 - 390 million years ago (mya)), the Variscan Orogeny (about
290 mya) and the Alpine Orogeny (about 25mya).
= Location of Great Britain
A globe showing the layout of the continents 370 million
years ago, prior to the Variscan Mountain building period.
Image copyright to Ron Blakey, Colorado Plateau
Geosystems, Inc
Variscan
Mountain
Belt
The Variscan Orogeny saw the ancient southern hemisphere continent of
Gondwana collide with the northern hemisphere continent of Laurussia,
closing the Rheic Ocean. This created a massive mountain belt extending
across what is now modern-day Russia, Western Europe and on to North
America, welding together Gondwana and Laurussia to form the massive
supercontinent of Pangaea.
Pangaea
= Location of Great Britain
A globe showing the supercontinent of Pangaea 300
million years ago. Image copyright to Ron Blakey,
Colorado Plateau Geosystems, Inc
North of the Variscan Front the effects of orogenesis were still felt, though
the intensity of deformation greatly decreased. The Pennines, Mendips,
Malvern and Abberley Hills all owe their existence to this period of powerful
earth movements.
For the Malvern and Abberley Hills the Variscan Orogeny re-activated a
line of weakness in the Earth's Crust, known as the East Malvern Fault.
This north-south orientated fault can be traced from Bristol through
Malvern and Martley and on to the north of Kidderminster. Locally the crust
was strongly compressed in an east-west direction; deep seated layers of
rocks were thrust up along the East Malvern Fault, crumpling the
previously horizontal layers of rock as it did so, creating great folds along
the axis. These folds now manifest themselves in the line of Abberley Hills,
including Penny Hill.
Variscan folds in sandstone, St Anne’s Head,
Pembrokeshire
2. Compressive forces exerted upon the layers
bbof rocks cause them to buckle and fold.
Syncline
3. When the rocks are stressed beyond their
bbstrength they tear apart along fault planes.
Syncline
b
m
i
L
Li
Anticline
m
b
Compression
Compression
Fault
Diagrammatic representation of folding and faulting
North-east section of the Canyon. The hatched area depicts the
surface of a layer (bed) of limestone
Layers of nodular
limestone with thin
siltstone partings
In the northern face of the Canyon you can follow some individual layers of
rock from the base to the top of the cutting in a near enough straight line,
others layers are truncated and some follow sinuous paths. These sinuous
layers are folded - a phenomenon called 'Cascade' folding. The long-lived
Variscan orogeny consisted of many phases, with deformation intensity
varying both in magnitude and location along the East Malvern Fault. Some
deformation was very localised, subjecting layers of rock to such stresses that
after being tilted they lost their form, causing the rocks to ‘cascade’ under
gravity.
Bentonite
South-west Britain lay on the northern margin of the Variscan Front, the
deformation zone. Here, no mountains were formed but the rocks were
severely folded. Variscan fold structures can be seen in the sea cliffs of
Pembrokeshire.
Pangaea
1. Rocks are deposited in horizontal layers.
The three-dimensional section of the rock face seen at the north-east section
of the Canyon shows the steep angle at which these once horizontal layers of
rock are now tilted.
Thick, grey
siltstone layer
The northern face of the canyon. The course of five layers of
nodular limestone are shown in different colours, highlighting that
the intensity of folding within the rock layers increases towards the
bentonite layer, which lies on a fault plane. The thickness of the
bentonite layer is exaggerated at this location due it filling the void
created by folding/faulting. Bentonite layers within the Much
Wenlock Limestone found in the woods around Penny Hill and
further afield across the Midlands are significantly thinner.
During phases of earth movement large quantities of superheated water are
expelled from the rock. These hot watery fluids contain minerals in solution.
When the conditions are suitable, these minerals crystallise out of solution
forming veins or thin coatings along
joints in the surrounding rocks. As
such, veins of calcite are commonly
found in Penny Hill, the superheated
waters having dissolved calcium
carbonate from the limestone before
precipitating it back as a calcite vein.
A calcite vein in a nearby quarry. The vein
fills a void created along a fault plane
(red). The beds either side of the fault
have been displaced by 10cm (black).
Inset - close up of calcite vein
At the north-west end of the canyon among the layers of grey nodular limestone is a
layer, 30cm thick at its maximum, of a light coloured, soft clayey material. It is believed
this is a layer of volcanic ash, called bentonite. During the Silurian Period, when the
limestones of the Canyon were forming on the floor of a tropical sea, there were
Ash billows from Mount St Helens, 1980 © USGS; Photo
sporadic volcanic eruptions producing a great deal of ash. The volume of ash
by Jim Vallance
expelled is potentially on a par with that produced during the eruption of Mount St
Helens, 1980. The ash rained down and settled to the sea floor, probably killing most of the sea creatures
during each fall. The location of the volcano, or volcanoes, which produced this ash is still contentious,
though the current thought is that the volcano was located in what is now south-west Dingle, Ireland.
Interpretation by Herefordshire and Worcestershire Earth Heritage Trust
www.EarthHeritageTrust.org.uk
01905 855184 / [email protected]
Registered Charity No 1144354