Caol Ila, Islay
Geological Conservation Review site | GCR #3303 | 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
The Caol Isla GCR site is of international importance for determining the environment of deposition of the rocks immediately overlying a late-Precambrian glacial deposit, the Port Askaig Tillite.
The sequence is crucial in this respect for it forms a transition between the tillite and its possible cap dolomite- the Bonahaven Dolomite Formation.
In addition, the well-exposed coastal section provides a representative section for the lower part of the Bonahaven Dolomite Formation of the Argyll Group, and enables a bed-by-bed study to be made of the sedimentary structures in this tide-dominated, shallow water sequence. Features of particular interest include sand-filled cracks, which form polygonal patterns on the bedding surfaces. Their 3-dimensional morphology suggests that they originated by loss of water from the sediments after burial, possibly triggered by movements on a fault that was active during development of the sedimentary basin. The site is made more valuable by the fact that the rocks are almost unaffected by tectonic strain, an unusual situation in the Dalradian.
David Webster
Dec. 4, 2014
?Syneresis cracks in siltstones (base of bedding plane) in Bonhaven Fm Member 1.
David Webster
April 12, 2014
ripples on bedding plane in Bonahaven Fm Member 1 fine-grained sandstones.
David Webster
April 12, 2014
Metamudstones at the base of the Bonahaven Formation. Contains metamorphic biotite and several (?3) cleavages.
David Webster
May 11, 2021
Bedded metasandstones at the top of the Port Askaig Formation (Member 5)
David Webster
(PDF) pdf file of Excursion 7 from 'A Guide to the Geology of Islay' 2025 Edition. The Caol Isla section is Excursion 7b.