Cumbria has a rich and varied geology which contributes immensely to its variety of landforms and landscapes.
The High Fells and uplands of the Lake District dominate the county and comprise Ordovician marine sediments and outpourings of lava and ash from volcanoes. Large masses of igneous rock were intruded into these rocks during the early Devonian.
Surrounding the central core of the Lake District is a ring of rocks spanning the Carboniferous Period. These include the distinctive Carboniferous Limestone, which forms the limestone scenery of the Kent Estuary area and the Cumbrian part of the Yorkshire Dales area around Kirby Lonsdale.
The overlying Upper Carboniferous sandstones and shales of the Millstone Grit and the mudstones, shales and sandstones of the Coal Measures form a broad band of outcrop from the coast north of Whitehaven to the north of the Lake District. The lower land around the Solway Firth in north Cumbria, the Carlisle Plain and Vale of Eden are floored by mudstones and sandstones of Permian and Triassic age. These outcrop on the coast in the St Bees area, where they form the well-known red sandstone cliffs.
The landscape of Cumbria, and, in particular, the much loved mountains and valleys of the Lake District, were shaped by the enormous erosive forces of the glaciers and ice sheets of the Ice Ages of the past 500,000 years. Huge amounts of material were deposited by the ice sheets over the landscape leading to the formation of the undulating and hummocky lower-lying ground around the Lakes and the Pennines and resulting in very limited outcrop of the underlying rocks in these areas.
Rocks of Ordovician age (495-443 million years ago) form the characteristic mountains, radiating ridges, steep scarps and glaciated valleys of the High Fells of the Lake District. Two main groupings of Ordovician rocks are present. In the northern part of the Lakes, the rounded bulk of the hills of the Skiddaw area are formed by the mudstones and siltstones of the Skiddaw Slates. These were deposited in a deep water basin and yield the fossils of a group of extinct, planktonic, colonial animals known as graptolites, which are very useful for dating the rocks.
The second, and overlying group, of Ordovician age is the Borrowdale Volcanic Group, which form the central core to the Lake District and the rugged mountains of Helvellyn and the Scafell range. The Borrowdale Volcanic Group comprises a thick sequence of lavas and tuffs (ash falls), which were the product of an extended period of violent volcanic eruptions. The volcanoes would have rapidly built up their cones well above sea-level, so that most of the debris, which included ash falls and lavas, fell on land whilst the fine-grained tuffs were probably deposited in water.
Following the extrusion of the rocks of the Borrowdale Volcanic Group, there was a period of considerable earth movement and erosion, before the return of marine conditions and the deposition of the limestones and associated sandstones, tuffs and lavas of the Coniston Limestone Group. The shallow water limestones of the Coniston Group yield fossil brachiopods and trilobites and outcrop in a narrow band at the southern end of the Borrowdale Volcanic Group outcrop and as isolated areas at Cross Fell in the and to the west of Ulverston.
Marine conditions continued into the Silurian (443-417 million years ago) and deposition carried on without a break from the Ordovician. A series of mudstones, sandstones and silts were deposited, which contain fossil graptolites, brachiopods and trilobites. These rocks now form the gentler landscapes of the southern Lakes, outcropping from Ambleside southwards to a line running approximately between Ulverston and Kirby Lonsdale.
The sequence of Silurian rocks exposed in the Windermere area and across the Southern Lakes show that the marine basin in which they were deposited gradually filled with sediment over time, so there is a change from deeper water muds in the Lower Silurian through to shallow water sandstones and the onset of more continental conditions in the Upper Silurian and the beginning of the Devonian Period.
The Devonian (417-354 million years ago) was a period when much of Britain, apart from south-west England, formed part of a large continental landmass. In Cumbria there are no continental deposits of this age, however, the Devonian was marked by a great phase of mountain-building (the Variscan Orogeny). This compression of the crust resulted in uplift of the land and buckling of the earlier rocks, which is still discernible, despite later periods of folding.
Associated with these earth movements was the intrusion of large amounts of igneous rock. We now see, after repeated phases of deposition and erosion, the portions of what were then deeply buried igneous masses including the granite masses of Shap, Skiddaw and Eskdale (the largest granite body in the Lake District), as well as igneous masses around Ennerdale, Buttermere, Wastwater and Carrock Fell.
The end of the Variscan mountain-building phase resulted in the formation of a new land surface, on which the deposition of sediments began again in the late Devonian or early Carboniferous (354-290 million years ago) following the encroachment of the early Carboniferous sea. In Northern England and southern Scotland, the land over which the sea transgressed comprised a series of blocks and troughs or basins. Greater thicknesses of sediment were deposited in the troughs than over the blocks which tended to remain as relatively stable, shallower, areas throughout the Carboniferous. Much of Cumbria lay over what was then one of these block areas and this influenced deposition during this period.
Apart from the south-west Cumbrian coast, rocks of Carboniferous age form a broad swathe of outcrop around the older core of the Lake District. The oldest rocks of the area belong to the Carboniferous Limestone Series. These rocks are composed of limestones, sandstones and shales deposited in a shallow marine-estuarine environment, and outcrop in south Cumbria in the Ulverston-Furness area, in a broad lobe between the Kent Estuary, Kirby Lonsdale and up to Kendal and then in a broad band following the eastern and northern rim of the Lake District from Ravenstonedale, north-westwards to Penrith and then around the north of the Lakes to Cockermouth.
The Carboniferous Limestone is overlain by the Millstone Grit Series. In this area of England, the differences between the Millstone Grit and the older Carboniferous Limestone are less marked than further south, with sandstone common in the Carboniferous Limestone and limestone present in the Millstone Grit. In Cumbria, the Millstone Grit series consists of a series of limestones, marine shales and sandstones, and the ‘Millstone Grit’ itself, thick coarse-grained sandstones, siltstones and mudstones. These rocks were deposited in the late Carboniferous (approximately 300 million years ago) in a coastal environment where large river deltas were building out into the shallow marine waters. Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy land area in which the succeeding Coal Measures were deposited.
The Lower and Middle Coal Measures outcrop in north-west Cumbria from the coast at Whitehaven to Maryport and then easterly to south-east of Wigton. The rocks of the Coal Measures show a repeated coal, sandstone and mudstone cycle which reflects relative changes in land and sea level. The coals represent the fossilised remains of swamp vegetation which grew as luxuriant forests on the deltas, while the mudstones were deposited under shallow marine conditions. The Lower and Middle Coal Measures contain the workable coals of the West Cumbria Coalfield.
The Permian (290-248 million years ago) and Triassic (248-205 million years old) Periods in Cumbria are represented by red mudstones and sandstones that outcrop in a broken ring around the Lake District, from the south-west coast (south of Whitehaven) to the Solway Firth, Carlisle and the Vale of Eden. However, although these rocks have a relatively extensive area of outcrop, apart from at the coast, the solid rock geology rarely emerges from beneath a thick covering of glacial and post glacial deposits.
The Permian-Triassic rocks of Britain were deposited under arid conditions over a large desert plain with numerous basins and mountain ranges. In South Cumbria breccias and overlying marine or brackish water mudstones and limestones are probably of Permian age and represent deposits on the edge of the so-called Bakevellia Sea, a shallow sea that filled much of the present day Irish Sea basin. Thick sandstones formed by desert dunes are also present, particularly in the Vale of Eden.
Lower Triassic sandstones, such as those exposed at St. Bees on the west coast represent the deposits of large braided rivers that crossed the desert plain. These are overlain by the Mercia Mudstone Group, which comprise mudstones, sandstones and thin horizons of gypsum and rock salt. These rocks form the solid geology to north Cumbria along the shore of the Solway Firth to the River Eden estuary.
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
Apart from the peaks and fells of the Lake District, much of the bedrock geology of Cumbria is mantled by sediments deposited during the Ice Age by ice sheets moving over the area or from glaciers originating in the area. Perhaps, nowhere else in England is evidence for the tremendous landscape-shaping forces of the Ice Age more evident than in the glacier-scoured U-shaped valleys of the Lake District and the upland corries and craggy outcrops.
These valleys were carved out during the last main glacial period, the Devensian, some 12,000-50,000 years ago and are now occupied by the famous lakes such as Windermere and Coniston Water. There is a vast array of glacial deposits, mainly of till (boulder clay) lying over the surface of the County, deposited from beneath ice sheets and glaciers. The composition of the till varies depending on where the main ice flow was from, so that till over parts of northern Cumbria contains material derived from Scotland, while the Carlisle Plain received ice and its debris from the north, south and probably east.
During the melting of the last ice sheets and glaciers, over some 15,000 years, large amounts of material were released and transported by the meltwaters to form large spreads of fluvio-glacial sand and gravel deposits. Arctic, tundra-like conditions would have prevailed for the first few thousands years and during this time earlier glacial deposits would have been subject to repeated freezing and thawing leading to the formation of deposits of shattered bedrock (known as head) and large scree deposits.
The presence of large amounts of glacial material also led to modifications in the course of streams and rivers. Such an example is seen at Lake Windermere, where glacial deposits held back the rising waters of the lake during the melting of the glaciers, so that the lake now drains out through an overflow channel to the west rather than directly to the south.
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.