Arkley, S. L. B., Browne, M. A. E., Albornoz-Parra, L. J. and Barron, H. F. 2011. East Dumbartonshire Geodiversity Audit. British Geological Survey Open Report, OR/09/19. 265pp.
EDC 1: Twechar Quarry, Twechar
Grid reference:
Site type: Artficial quarry works
Site ownership: Not known
Current use: Disused
Field surveyor: Sarah Arkley & Luis Albornoz-Parra
Current geological designations: None
Date visited: 5th March 2009
Site map
Summary description
Disused quartz-microgabbro quarry, formerly used for roadstone. Recorded as an 'old quarry' in (Robertson, 1937).
This quartz-microgabbro intrusion is part of the Midland Valley Sill Complex intruded here into the lower part of the Upper Limestone Formation sedimentary rocks (between the Lyoncross Limestone and Index Limestone). The radiometric age of the sill complex is thought to be between 290–295 Ma (de Souza, 1979) and to reach a maximum thickness of 90 m around Cumbernauld (Forsyth et al., 1996)
Approximately 15 m high faces remain. At the north end of the quarry the top of the sill is exposed ⅔ rds of the way up the face; the conformable nature of the contact between the fine-grained chilled margin of the sill is and the bedded baked sandstone above can be clearly identified. Close inspection of the contact and overlying sediments is possible at the side of the quarry but the ground is quite steep, clearance of vegetation and a few simple steps would greatly improve access to this key section.
Other features visible at the site include quartz and calcite veining within the top part of the sill, coarser and more 'patchy' looking microgabbro within the main body of the sill. Columnar jointing, perpendicular to the cooling surface, is fairly well developed and slickenslides can be observed on some fault planes indicating the direction of movement.
Generally microgabbro from near the top or base of a sill is finer grained and more brittle than material from the middle of thick sills. This material was often worked into setts.
EDC 1: Stratigraphy and rock types
Age: Upper Carboniferous Formation: Midland Valley Sill-Complex
Rock type: Quartz-microgabbro
Age: Upper Carboniferous Formation: Upper Limestone Formation
Rock type: Sedimentary Rock Cycles of the Clackmannan Group Type
Assessment of site value
Access and safety
Aspect/Description
Road access and parking Site is 50 m from a road, car parking at the local tavern adjacent to the site. Safety of access Rough uneven ground on floor of quarry, boggy in places.
Safety of exposure Some quarry faces are high and potentially loose material should be removed.
Permission to visit Permission given by the tavern owner.
Current condition Good; moss and ivy growing in places on the quarry faces but generally they are well exposed. A few young trees within the quarry itself may in time obscure views. Minor amounts of household/garden rubbish in places.
Current conflicting activities None
Restricting conditions None known
Nature of exposure Vertical quarry faces
Culture, heritage & economic
Historic, archaeological & literary associations None known. Rating: 0
Aesthetic landscape Old quarry at the edge of the village of Twechar, revealing the underlying geology. Rating:2
History of earth sciences None known. Rating: 0
Economic geology Former road-metal quarry. Rating: 3
EDC 1: Geoscientific merit
Total Geoscientific merit score 41
Current site value
Community Pub adjacent to the quarry is visited on a regular basis. Rating: 10
Education A rare locality where the features typical of the top part of a sill and the contact with the deposits it has intruded can be observed. Rating: 6
Fragility and potential use of the site
Fragility Geohazard, Natural Overgrowth, Development. Pub has recently closed and land is potentially subject to redevelopment.
Potential use: School, on-site interpretation
Geodiversity value
Although there are numerous disused quarries in East Dunbartonshire within the Midland Valley quartz-dolerite sills and dykes, Twechar quarry is one of the few to expose the top or base of an intrusion. The nature of the contact between the igneous intrusion and the baked sedimentary rocks it intruded can be examined in the quarry and typical field characteristics observed. Rating: 6.