Ashgill Quarry

Also within

Cumbria County LAKE DISTRICT National Park

Condition (Natural England)

Geology & landscape

Geological Feature: Rock Outcrop · Mining / Industrial: Quarry · Geological Period: Ordovician, Silurian · Rock Type: Limestone, Mudstone, Shale

Landscape processes: Volcanic / Igneous

Description

Area,154.9 hectares 382.8 ac Description The site is situated on the south eastern slopes of the Old Man of Coniston, three kilometres south west of Coniston village. It is centred around Torver Beck, extending from Ashgill Quarry in the south west to Timley Knott in the north east. The site is of considerable geological importance. Ashgill Quarry and the large area of land to the south east is of stratigraphical interest with rocks bearing important sequences of fossils and the area between Torver Beck and Timley Knott at the northern end of the site has rocks displaying unusual structural features related to past earth movements. Ashgill Quarry gives its name to the Ashgill Series of Marr, the uppermost series of the Ordovician. The strongly cleaved but fossiliferous rocks have enabled geologists to subdivide the succession into the Lower Ashgill 'Phillipsinella Beds', the Middle Ashgill 'Mucronatus Beds, Ash and White Limestone' and the Upper Ashgill 'Ashgill Shales'. A locality of great historical value, exhibiting the type developments of Ashgill stratigraphical units of great importance in correlation with other areas. The streams passing through the central part of the site rise in Ordovician rocks and flow downstream cutting successively through Llandovery, Wenlock and Ludlow sediments. The area has a complete Wenlock sequence including the Brathay Flags, the Lower Coldwell Beds and the Middle Coldwell Beds, containing a full suite of graptolite biozones from centrifigus to ludensis, with the possible exception of the ellesae zone, which is probably present in the continuous sequence from linnarssoni to lundgreni biozones in Ashgill Beck. The Lower Coldwell Beds are known only in the western part of the Lake District. They thin out to the east of Skelgill and are absent in the Howgill Fells. These proximal turbidites are the only coarse clastics in the Wenlock of the Lake District. An extremely useful and important site for the construction of a graptolite biostratigraphy for Wenlock strata in the Lake District. It enables long range correlation of Lake District strata with the type Welsh Borderland succession. At the northern end of the site on the fellside between Torver Beck and Timley Knott there are exposures of a sequence of strata spanning the Ordovician/ Silurian boundary, inclined quite uniformly southeastwards. Rocks of the Borrowdale Volcanic Group are overlain by the Coniston Limestone Formation of the Ashgill Series, overlain in turn by a Silurian sequence comprised of Skelgill Beds, Browgill Beds and Brathay Flags. The geological structure of this area is complex, however and of considerable interest in studies of the structure of this region. The rocks are cut by a series of fault dislocations, predominantly of wrench type, oriented north west to south east and mostly of dextral displacement. An unusual feature of the faults is the apparent termination of the great majority at or close to the Ordovician / Silurian boundary. However, detailed study has shown that these faults do not terminate but rather turn into parallelism with the bedding and change their nature from wrench faults to thrusts. The explanation of this unusual feature lies in the presence of a soft shale formation, the Skelgill Shales, which have acted as a focus for the distortion and displacement generated by stresses imposed during the Caledonian mountain building episode orogeny. This shale layer has thus absorbed many of the wrench faults which displace the volcanic rocks and the Coniston Limestone Group. Thrusts within the Skelgill Shales also truncate some of the upper mudstone formations within the Coniston Limestone Group and result in a variable width of outcrop for the Skelgill Shales. This structural style can be recognised within the Skelgill Shales for the whole 25 mile length of its outcrop across the Lake District. Furthermore, a number of major faults of regional significance, such as the Coniston and Brathay Faults, are terminated at this shale layer by conversion into bedding thrusts in the way described above. This area thus demonstrates very clearly the characteristics of one of the major structural features of Lake District geology.