Allt Mhainisteir
Geological Conservation Review site | GCR #3288 | Structural and Metamorphic Geology | Dalradian
Scotland's geosites are chosen because of their local, national or international importance. Take only photos, leave only footprints: avoid causing any damage to this site. You can walk almost anywhere in Scotland without the need to ask permission or keep to paths, but you have a responsibility to care for your own safety, to respect people's privacy and peace of mind and to cause no damage.
The right of access does not extend to quarries, building sites or any land where public access is prohibited, or to the collection of geological samples.
Summary
A cross-section through Grampian and Appin Group rocks in the Kinlochlaggan Syncline, within the Geal-Charn - Ossian steep belt. Here, the Grampian and Appin groups are juxtaposed tectonically by the Allt Mhainisteir and Aonach Beag shear zones, and the rocks are highly strained.
The Allt Mhainisteir GCR site provides a coherent c. 4 km-long traverse across the most complete Kinlochlaggan succession in the Geal-charn-Ossian Steep Belt, thus providing the correlative framework for other sections preserved within the steep belt. Despite the presence of high ductile strain on both the north-west and the south-east sides of the Kinlochlaggan succession, Grampian Group psammites can still be seen to young consistently towards this limestone - quartzite - semipelite - amphibolite succession; hence the Kinlochlaggan succession lies stratigraphically above the Grampian Group and is readily assigned to the Appin Group. Stratigraphical relationships are sufficiently well-preserved to be able to determine local stratigraphical omissions in the Grampian and Appin Group succession, thus providing evidence for the punctuated nature of the depositional record in this sector of the Dalradian basin.
This GCR site also provides a well-exposed section through the Inverpattack Fault enabling examination of the complex history of ductile and brittle deformation associated with one of the number of sinistral strike-slip faults which have a major effect upon the distribution of lithological units in the Northern Grampian Highlands, and whose effects must be compensated for in any attempt to reconstruct a depositional framework for the metasedimentary rocks of the region.
Site information
Access
Parking is available in a lay-by on the A86 just at NN 555 899, just to the east of the junction to Gallovie. Alternatively, the Pattack Falls car park to the east could be used. The site is a pleasant walk along the River Pattack from here (approx. 5 km).
Further information
https://largeimages.bgs.ac.uk/iip/mapsportal.html?id=137358…
BGS map sheet NN58NW
Safety notes
In parts, the Allt Mhainisteir cuts through a steep-sided gorge, and care is required when traversing the sides. It is frequently necessary to cross the stream, and this is safer and more convenient to do when water levels are lower.
Related geosites
In collections:
Daniel Burgess
(PDF) 10k map and locality descriptions of the site.
Daniel Burgess
Nov. 4, 2023
New (2016) pipeline for the Pattack Hydro project. There is a good deal of disturbed / made ground where the pipeline has been buried, but this does not appear to affect the exposures of the Inverpattack Fault Zone, which lie in a deep gorge slightly to the east.
Daniel Burgess
Nov. 4, 2023
In the vicinity of the Allt Mhainisteir slide, looking approximately south. To the left, tightly folded and strained psammites of the Creag Meagaidh Psammite Formation. To the right, varied interfolded and tectonically attenuated Appin Group lithologies. The shear zone itself does not appear to be exposed.
Daniel Burgess
Nov. 4, 2023
Metacarbonate rock (Coire Cheap Formation), cross-cut by pods of boudinaged pegmatite.
Daniel Burgess
Nov. 4, 2023
Confluence of the Allt Liath nam Badan and the Allt Mhainisteir. Kinlochlaggan Quartzite Formation.
Daniel Burgess
Nov. 4, 2023
Thinly laminated metacarbonate rock of the Coire Cheap Formation.
Daniel Burgess
Nov. 4, 2023
Tightly folded Creag Meagaidh Psammite Formation, close to the Allt Mhainisteir Slide.
Daniel Burgess
Nov. 4, 2023
Overview of the site, looking SW to Geal Charn from approx NN 544 872.