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March 9, 2016 at 7:57 pm #8818Anonymous
The Geology Group met at Merlin's Bridge Village Hall at 10.30 am on Wednesday 9 March. Here are the notes for the topic.
THE GEOLOGY OF THE LAKE DISTRICT
The Lake District National park in Cumbria was created in 1951 in recognition of its outstanding natural landscape sculptured from Palaeozoic rocks by the forces of weathering and erosion. Geologically the region can be divided into an older Lower Palaeozoic inlier surrounded by a younger Upper Palaeozoic margin.
Evolution of the Central Lake District; the Lower Palaeozoic inlier.
.The oldest strata in the central Lake District belong to the Skiddaw Group which are of Lower Ordovician age (Arenig 485-470 Ma). These were deposited on the floor of the Iapetus Ocean which separated what is now Scotland, Greenland and Labrador from NW Europe. This wide subsiding geosyncline received vast amounts of sand and mud throughout Arenig times which eventually produced the Skiddaw slates, a sequence of turbidites that have been deformed, folded and cleaved during the Caledonian orogeny. The graptolitic fauna of these rocks also clearly indicates the deep water marine conditions which existed at this time. However, as the Iapetus Ocean began to close, subduction was taking place below the Solway trench and magmas were generated in the subducted oceanic crust. These magmas fed the granitic intrusions which in turn produced the Eycott and Borrowdale volcanoes that built up some 4000 metres of andesitic lavas, tuffs and agglomerates belonging to the Borrowdale Volcanic Group. No marine fossils have been found in these rocks, confirming their terrestrial origin. Such volcanic rocks are resistant to erosion and form the highest and most rugged mountain scenery in the Central Lake District. By the end of Ordovician times (443 Ma) volcanic activity had almost ceased and a marine transgression occurred depositing thin calcareous mudstones which are known as the Coniston Limestone Group. The narrow outcrop of these rocks forms an important marker horizon separating the rugged Borrowdale Volcanic country from the more subdued topography of the overlying Silurian rocks to the south. These sediments of Silurian age (443-417 Ma) consist of some 3500 metres of mudstones, sandstones and greywacke that were deposited by turbidity currents in a vast subsiding trough.
At the end of this long period of Silurian sedimentation came the Caledonian Orogeny when continental collision and mountain building occurred. The Laurentian continental plate moving SE collided with the NW European landmass. Ordovician and Silurian sediments and volcanic tuffs were strongly folded and deformed with the predominant Caledonian structural trend aligned NE-SW. Platy minerals were reoriented to produce slates with strong cleavage often at a high angle to the original bedding. By the early Devonian (395 Ma) a huge batholith was emplaced below the Lake District and the Northern Pennines. This batholith has been exposed by erosion in a few localities as in the Shap; Skiddaw and Eskdale granites and the Carrock Fell complex. However, its overall presence has been proved by geophysical surveys (positive gravity anomalies) and a borehole in Weardale. The heat generated by these plutonic intrusions has resulted in the thermal metamorphism of the surrounding rocks and also in the formation of mineral veins from rising hydrothermal fluids.
The Upper Palaeozoic Margins of the Lake District
During the Devonian period (417-354 Ma), erosion reduced the Caledonian mountains to a peneplain and some of the waste products of this erosional process known as molasse deposits can be seen in the Mell Fell conglomerates on the north side of Ullswater. Then by the beginning of the Carboniferous marine transgression deposited a sequence of limestones and shales. These Carboniferous strata lie unconformably on the eroded Lower Palaeozoic rocks. There is a time gap of nearly 50 Ma between the end of the Silurian and the deposition of the first Carbonifeous strata. This unconformity is well displayed in a stream section near the Shap Wells hotel where steeply dipping Silurian slates are overlain by horizontal basal Carboniferous sediments which consist of a pinkish conglomerate containing distinctive orthoclase crystals of Shap granite. Since the Carboniferous beds could only have been formed after the granite had been intruded, exposed and eroded; the granite itself must be of Devonian age. This is confirmed by isotopic dating which puts the granite intrusion at 393 Ma and the basal Carboniferous at 350 Ma.
Upper Carboniferous coal measures are preserved in NW Cumbria around Whitehaven but these would have been more extensive before the Variscan earth movements further uplifted the Lake District, downfaulted the Vale of Eden and activated the Pennine fault system. Continental desert conditions were then established over the region in Permian times (290-252 Ma) as shown by the dune bedded sandstones exposed around Penrith. On the west coast around St Bees Head the Permo-Trias contains evaporates deposited in shallow water desert basins.
One of the most outstanding aspects of the scenery of the Lake District is the radial drainage pattern. The lakes and rivers all flow outwards from the central mountain hub like the spokes of a wheel. This drainage pattern is thought to have originated on the domed surface of post Devonian rocks that covered the region. Eventually, as erosion removed most of the cover rocks (except for the margins) the radial drainage pattern was superimposed upon the underlying Lower Palaeozoic inlier. This explains why the present superimposed drainage system bears little relation to the geological outcrops.
Finally, the Pleistocene glaciation (2 Ma – 10,000 BP) resulted in the sculpturing of the mountain core as the Lake District ice cap expanded and ice moved outwards down the radial valleys. Hence we see corries where glaciers were nourished, U shaped valleys eroded by ice and glacial till (boulder clay) deposited in the surrounding lowlands where ice sheets melted. Today the processes of weathering and erosion continue to mould the landscape.
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