The Borrowdale Volcanic Group and related geology on 1:10 000 sheets NY31NE and NY41NW : part of the 1:50,000 sheets 29 (Keswick) and 30 (Appleby)

This report describes the Borrowdale Volcanic Group, the Skiddaw Group, intrusive rocks and Quaternary deposits that crop out on 1:10,000 Sheet NY31NE (Glenridding) and part of Sheet NY41NW (Martindale). In addition, the structure of the area and mineralisation are also discussed.
An inlier of Skiddaw Group rocks crops out around Glencoyne Farm and Glencoyne Park. The Skiddaw Group exposures comprise dark grey, parallel-laminated mudstones, siltstones and fine-grained sandstones. They are crystal-rich with abundant muscovite and altered ferromagnesian minerals which may represent a volcanogenic component. The high degree of sorting and thin laminae have been interpreted as indicative of reworking and redeposition by turbidity currents.
The Birker Fell Formation is in contact with the Skiddaw Group in the northern part of the area, but the contact is not exposed. Though previously considered to be a thrust contact, more recent evidence suggests that the contact is interpreted better as a set of subvertical normal faults.
The Birker Fell Formation crops out in the northern part of the area and is also exposed to the east of Ullswater. It reaches a maximum thickness of about 1200m in the central part of the sheet (Glencoyne Head/Sheffield Pike) and also to the east on Patterdale Common. The northern area comprises a substantial thickness of autobrecciated basaltic andesite, andesite and dacite sheets with thin interbedded volcaniclastic sandstones or tuffs sparsely present. Most of these sheets are thought to be lavas but it is quite possible that some are intrusive and have gone unrecognised. In the eastern part of the area a succession of basaltic andesite, andesite and dacite sheets are overlain by a considerable thickness of volcaniclastic sandstone and andesitic to dacitic tuff and lapilli-tuff.
The Lincomb Tarns Formation comprises eutaxitic to parataxitic dacitic tuff with local accumulations of garnet. This formation is steeply faulted against the Birker Fell Formation along the major, roughly E–W trending Birkhouse Moor Fault. To the north of the fault there are small fault-bound blocks of Lincomb Tarns Formation within the Birker Fell Formation.
A successsion of volcaniclastic sedimentary rocks including bedded sandstones and siltstones, massive coarse sandstones and massive conglomeratic sandstones overlies the Lincomb Tarns Formation. These are assigned to the Esk Pike Formation.
A thick and laterally extensive flow-banded dacite lava, thought to be coeval with the Middle Dodd Dacite, overlies the Esk Pike Formation. The lava varies slightly in thickness and is locally autobrecciated. It contains sparse feldspar crystals and is locally overlain by a feldspar crystal-rich dacitic lava.
A substantial thickness of andesitic lapilli-tuff and lithic lapilli-tuff known as the Helvellyn Formation overlies the dacite. This formation is dominated by thin units of lapilli-rich tuff and lithic-rich tuff. Locally, especially in the lower part of the formation, large feldspar-phyric an-desitic fiamme are present.
The Deepdale Formation is a further succession of volcaniclastic sedimentary rocks which overlies the Helvellyn Formation and has a similar appearance to the Esk Pike and Seathwaite Fell formations. Bedded volcaniclastic sandstone, massive sandstone and conglomeratic sand-stone are intercalated with andesitic/dacitic tuff and lapilli-tuff. Penecontemporaneous shallow-level intrusive andesite sheets typically follow the fine-grained dacitic tuff units, although they are locally transgressive.
A distinctive quartz-phyric microgranitic dyke thought to belong to the Armboth-Helvellyn Suite crops out in the northern part of the area. The dyke has a dog-leg shape following both E–W and N–S faults. The presence of this dyke was critical to the development of past economic ore shoots exploited and developed at the Greenside Mine.
The area is dominated by major E–W normal and/or strike-slip faults. The Birkhouse Moor Fault downthrows the upper parts of the Borrowdale Volcanic Group succession against the Birker Fell Formation to the north. This major fault is the northern margin of a substantial fault-bound depositional basin. South of the Birkhouse Moor Fault, the basin succession is displaced by major NNE-trending faults. Thickness and facies variations across these faults implies that they were syn-depositional. A syn-volcanic fault on Catstye Cam marks the abrupt western limit of the dacite lava. The Birker Fell Formation in the northern part of the area is transected by a major E-W trending fault zone which truncates several N–S trending faults. The Greenside lead vein occurs along one of these N–S structures and ore shoots developed mainly along the steeper parts of the vein. The vein contains an assemblage of galena, sphalerite and minor chalcopyrite and silver and is considered to belong to the Early Carboniferous Suite of the Lake District.
This area is extensively glaciated with abundant glacial and alluvial deposits. Hummocky deposits of the Loch Lomond interstadial are ubiquitous in the upper corries. Peat cover is also extensive on some higher slopes. The Greenside Mine has generated large volumes of mine waste that are piled up in a series of platforms in Glenridding valley.