The geology of England's Lake District is dominated by sedimentary and volcanic rocks of mainly Ordovician age underpinned by large granitic intrusions. Younger sedimentary sequences outcrop on the edges of the Lake District area, with Silurian to the south, Carboniferous to the north, east, and west and Permo-Triassic to the west and east. The entire area was covered by a Mesozoic sequence that was eroded off during the Paleogene uplift related to the opening of the North Atlantic. During the Quaternary the area was affected by repeated glaciations, which sculpted the current mountainous landscape.
Early Paleozoic
In early Paleozoic times the area was on the northern margin of the microcontinent of Avalonia, which was moving northwards as the Iapetus Ocean began to close. Initial deepwater sedimentation recorded by the Skiddaw Group was followed by the development of a continental arc caused by the subduction of Iapetus oceanic crust beneath the Avalonian margin. This magmatism produced both the Borrowdale Volcanic Group sequence and the older granitic intrusions, which are the preserved magma chambers to the volcanics. Continental collision began near the end of the Ordovician as southward subduction ceased and northward subduction beneath the margin of Laurentia began, forming the Southern Uplands accretionary wedge. As the collision progressed the Lake District area became part of a foreland basin in which the Windermere Supergroup sediments were deposited, initially interfingering with the volcanics and then overlaying them. The collision reached its end during the Early Devonian, part of the Acadian Orogeny with the final closure of Iapetus and the intrusion of late orogenic granites.
Skiddaw Group
The Skiddaw Group are the oldest rocks known from the Lake District. They are mainly Ordovician in age, from possibly Cambrian up to Llanvirn (upper middle Ordovician). The sequence, which is up to about 5 km in thickness, consist mainly of mudstones and siltstones, with lesser amounts of sandstone. Following deformation and low-grade metamorphism during the Acadian Orogeny, they now have a well-developed slaty cleavage, giving rise to their common name, the Skiddaw Slates. The group is divided into two main areas, the northern and central fells, to either side of the WSW-ENE trending Causey Pike Fault, with distinct successions developed.
Eycott Volcanic Group
The Eycott Volcanic Group (EVG) forms the youngest part of the preserved Ordovician sequence in the northernmost part of the Lake District. This set of mainly andesitic lavas and tuffs, with related minor intrusions is of Caradocian (Sandbian to Early Katian) age. In terms of their chemistry the EVG are transitional between moderately potassic tholeiitic and calc-alkaline in type.
Borrowdale Volcanic Group
The Borrowdale Volcanic Group (BVG) lies unconformably above the Skiddaw Group, and is of Caradocian age. It is similar to, but distinct from, the Eycott Volcanic Group. It consists of a thick sequence of basaltic, andesitic, dacitic and rhyolitic lavas and pyroclastic rocks with interbedded volcaniclastic sediments. It is subdivided informally into two parts known as the Lower Borrowdale Volcanic Group and Upper Borrowdale Volcanic groups. It is further subdivided into many formations, of which only a few have regional extent.
In terms of chemistry, the BVG are moderate to highly potassic calk-alkaline in type.
Lower Borrowdale Volcanic Group
The lower part of the succession is dominantly andesitic. Locally in the western part, the lowest formation is the non-volcanic sandstones of the Latterbarrow Formation. In the Furness Inlier the Greenscoe Tuff Formation is the lowermost unit. In the southwest at Millom Park the Whinny Bank Tuff Formation is the lowermost unit, overlain by the Po House Tuff Formation. The Birker Fell Andesite Formation makes up most of the lower BVG, varying in thickness from 980 m to 2700 m.