Lake District

It has taken over 500 million years of Earth history to create the Lake District, the first 100 million years overshadowing everything that followed. This was the time that the major rock groups formed which were then uplifted into the mighty Caledonian mountain chain – the eroded remnants of which form the present-day Lake District. The Lake District can be broadly divided into 3 areas geologically: a Lower Palaeozoic core, an Upper Palaeozoic border and glacial features. The Lower Palaeozoic core

1. The Skiddaw Group

Skiddaw Group rocks are the oldest in the Lake District, forming almost one third of the mountain core, and outcropping in a wide belt in the northern and western fells. There are also inliers of Skiddaw Group rocks such as around Haweswater and Ullswater and, further east, they reappear at the base of the Pennine Fault escarpment below Cross Fell. These rocks are very difficult to interpret. They are mainly greywackes, sandstones, siltstones and mudstones (some now metamorphosed to slates). They were deposited by turbidity currents in fairly deep water on the continental slopes of the former Iapetus Ocean. They are more than 3,000 m thick. Their monotonous and uniform grey colour make them difficult to interpret as evidence such as sedimentary structures is difficult to find, and fossils are rare. Structurally they are also difficult to interpret as they are very complex. Besides having undergone many phases of folding, they are also highly cleaved (hence the old name ‘Skiddaw Slates’) and they have been altered by thermal metamorphism and mineralisation.

2. Ordovician volcanic rocks

Volcanic rocks succeed the Skiddaw Group, indicating new environmental conditions. This was a direct result of subduction of the Iapetus ocean crust. The Iapetus was gradually becoming narrower as the American and European tectonic plates drifted towards one another during the Ordovician period. During the early Ordovician, Britain – like many of the Earth’s continents – was situated in the southern hemisphere. Northern Britain was part of the North American continent which lay about 15oS and southern Britain was part of a small continent at about 60oS, much of which was covered by a shallow sea. In between these fragments there lay about 4,000 km of Iapetus ocean! Over the next 100 Ma the Iapetus gradually closed as the rate of subduction overtook that of seafloor spreading. By the Devonian (400 Ma ago) the ocean had closed completely. The igneous activity is a direct result of this subduction and it indicates that subduction took place beneath the southern fragment of the British Isles. Early igneous activity was extrusive and produced large volumes of andesites, ashes, ignimbrites and other volcanic material. Such volcanism is typical of volcanic island arcs, or convergent plate boundaries over subduction zones. (The marine fossils associated with nearby sediments also indicate an island arc setting.) Without a doubt this must have been the most exciting period of Lake District geological history as up to 6,000 m worth of volcanic eruptions were spewed out and violently erupted in the area. The oldest eruptions are recorded in the Eycott Volcanic Group. These were partly erupted under water, as some of the last Skiddaw Group sediments are interbedded with them. They differ chemically from the later groups – being mainly basalt and basaltic-andesite lavas. The rocks of the Borrowdale Volcanic Group (BVG) succeed the Eycott Volcanic Group. These represent the main volcanic episode. They are the real heart of Lakeland – extending from Wasdale and the Duddon Valley in the west through Scafell to Hevellyn, High Street and Haweswater in the east. Over 6,000 m of these rocks were erupted in only 10 million years during the mid-Ordovician. Today they cover an area of 800 km2. Detailed examination of the rocks has helped to build a fascinating picture of the type of volcanic activity responsible. It resembles that of Mount St Helen’s (of the Cascade Ranges) for instance that erupted so spectacularly in 1980. The evidence shows that the volcanoes erupted above water, on land, but some of the material collected in short-lived freshwater lakes. Streams then eroded fresh piles of volcanic material, transporting it and reworking the debris. Two major phases can be distinguished in the BVG: (i) The first phase in which 2,500 m of rock accumulated, was dominated by lavas flows with only occasional explosive eruptions. There were several vents or centres of activity, but these are notoriously difficult to locate. The lavas, usually andesitic, flowed across the landscape. These flows were often fairly localised, their extent and thickness depending on the precise composition of the magma and the local relief. There was occasional explosive eruptions forming air-fall tuffs. (ii) The second phase marked an abrupt change to explosive activity with ignimbrite sheets, pyroclastic surge and fall deposits. Enormous volumes of material were erupted very quickly, and with violent eruptions of andesite, dacite and rhyolite, much of which fell back to the surface as air-fall tuffs. The present Lake District is very different from the original Ordovician volcanic terrain. Much has been removed by erosion. Large parts of the volcano broke away, tilted and sank back into the molten volcanic pile – caldera collapse. The sheer variety of rock types, their differing responses to weathering and erosion, and the effects of many faults, have all led to the development of a remarkably intricate landscape.

3. The Windermere Supergroup

More than 5,000 m of very varied sedimentary material, the Windermere Supergroup, lie on top of the volcanic rocks. The succession ranges in age from the late Ordovician to the end Silurian. These rocks form the third major belt of the Palaeozoic core, extending across the southern Lake District (the Duddon Estuary to the Howgill Fells). Towards the end of the Ordovician the Iapetus had almost closed, the Skiddaw Group rocks had been folded and the Borrowdale Volcanic Group uplifted. This is indicated by the lowest unit of the Windermere Supergroup, the Dent Group (formerly called the ‘Coniston Limestone’) which lies above an unconformity – an erosion surface cutting down into the BVG and the Skiddaw rocks. These two groups must have been folded and uplifted prior to the deposition of the Dent Group. It outcrops between the Duddon Estuary and Shap. Besides the thin limestones which suggest shallow marine conditions, this group contains some volcanic materials from the last dying throes of the Ordovician volcanoes. During the Silurian there was continuous sedimentation and the seas deepened once more as subsidence occurred. Thick turbidite sequences of sandstones, greywackes, siltstones and mudstones were deposited.

4. Intrusions

The igneous intrusions are the fourth and very distinctive group of rocks in the Lower Palaeozoic core. They range in size from substantial plutonic intrusions such as the Eskdale Granite to small plugs, dykes and sills. The Shap Granite is a well-known outcrop on Shap Fell which produces a beautiful rock from a quarry into the fell. Granites are often a product of partial melts of a dioritic parent. The granites now seen at the surface of the Lakes today have been exposed due to considerable erosion. Most will have been emplaced at 5 km depth or greater. Devonian and Carboniferous rocks of the outer Lake District By the Devonian the new Euro-American continent created by the Caledonian orogeny was now in desert latitudes of around 20oS. Sediments of Devonian age are very rare in this area, mostly because it was a period of erosion. However, conglomerates can be found as the Mell Fell Conglomerate now forming the rounded hills of Great and Little Mell Fell near the lower end of Ullswater. This conglomerate was deposited as a series of alluvial fans and it is a coarse-grained and poorly sorted rock. A finer, but similar, deposit can be seen in the stream by the Shap Wells Hotel, Shap Fell. It contains fairly fresh crystals of pink orthoclase feldspar (from the Shap Granite nearby) which indicates that by this time (latest Devonian-earliest Carboniferous) the granite was exposed at the surface. The stream conglomerate is immediately overlain (at another locality downstream) by the Carboniferous Limestone (hence confining the date for the bed nicely – post granite 393 Ma (+/- 11 Ma) and pre-basal Carboniferous (base Dinantian). In fact, by the beginning of the Carboniferous erosion had removed a significant part of the uplifted mountains, allowing shallow seas to extend over the land. By the end of the Dinantian these tropical seas, rich in life, covered the whole of Lakeland, resulting in widespread limestone deposition. The succeeding Namurian and Westphalian deposited a series of deltaic sandstones and siltstones, with coals forming within the Westphalian. They were eroded from the Lake District massif, but they can still be found around the margins of the Lake District. The Carboniferous rocks almost completely encircle the Lower Palaeozoic core, forming distinctive terrains. The Permian and Triassic Permo-Triassic rocks outcrop in the Vale of Eden and along the Cumbrian coastline. These rocks were mainly deposited under desert conditions (within the northern hemisphere c. 15oN). At this time, the exposed dome of Carboniferous rocks was being eroded rapidly from the inner core area and thick sediments were being laid down in the surrounding basins. The arid environment is indicated by a series of desert dune sandstones (the Penrith Sandstone), wadis (representing flash floods and called the Brockram), evaporites, sabkha and other sediments relating to desert and peripheral environments. Quaternary What happened to the Lake District over the 200 million years from the Triassic to the Quaternary is largely guesswork since the region has left very little evidence! A very small patch of Jurassic rocks lie to the west of Carlisle, the nearest Cretaceous rocks are in Ireland, and there are no Tertiary rocks at all in the close vicinity. Material from all these areas was no doubt laid down, but erosion has removed the evidence. However, the effects of the Ice Age over the last 2.6 million years are fresh in the landscape of the Lakes. Although there were actually around 11 cold phases during the ‘Ice Age’ it is actually the evidence of the last one – the Devensian which ended only 10,000 years ago – which has the strongest signal on the landscape and deposits. Glacially-cut U-shaped valleys are evidence of the erosive forces of moving ice. Ice and freeze-thaw action which continues each winter to the present-day have all helped to sculpt a beautifully rugged landscape. Weathering and glacial erosion have shaped many Dinantian limestones into karst landscapes; limestone pavements are obvious features around Orton for instance. Erosion has given rise to the dip and scarp topography of the extreme north and the southeast of the Lakes – a topography akin to the Pennines. On the lowlands surrounding the Lakes the area is covered by unconsolidated deposits of till (diamicton), moraines, drumlins, sands and gravels and erratic blocks. The Lake District is in no way the classic glaciated landscape of, say, the Alps, but glaciation is still a formidable signature seen Information taken from
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