The Geology of Colonsay

The planned talk by Simon Cuthbert on Glenelg and Eclogites has been postponed due to illness and will be rescheduled

Instead David Webster will talk about the geology of Colonsay.

The oldest rocks on Colonsay belong to the Rhinns Complex – named after the Rhinns of Islay where they mainly occur. On Colonsay there is a small fault-bounded outcrop consisting of moderate-grade metamorphic gneiss that was originally an igneous rock formed some 1,800 million years ago in the Precambrian. The majority of the rocks outcropping on Colonsay are sandstones, siltstones and mudstones of the eponymous Colonsay Group that were deposited on top of the Rhinns Complex. However, they are much younger, having been deposited about 800 million years ago probably in a foreland basin setting before the break-up of the Rodinia supercontinent and are likely correlated with the Grampian Group of the Dalradian. The earliest rocks are deep-water turbidite deposits, whereas the later rocks were deposited in shallow marine, deltaic and fluviatile settings. They were then folded and underwent low-grade metamorphism about 470 million years ago in the first phase of the Caledonian Orogeny.
There are a number of faults that cross the island in a NE-SW direction; many of these are splays or offshoots of the Great Glen Fault that runs offshore to the north-west of Colonsay. Movement along them at the end of the Caledonian Orogeny, about 430 million years ago, generated new folds which modified some of the pre-existing structures. During their active phase these faults acted as conduits for magmas that fed volcanoes at the surface and include a spectacular explosion breccia which was used by Dorothy Reynolds (Arthur Holmes’ wife) in support of the then popular ‘granitisation’ hypothesis. A slightly younger and very unusual igneous intrusion can be found at Kilchattan where Holmes carried out some of the first attempts at radiometric dating.

David Webster graduated with a geology degree and worked in the oil industry and then in local government in Scotland before retiring and building a house on Islay, from where he undertook an MSc at Stockholm University on the rocks of western Islay and Colonsay. He has authored guide books to the geology of Islay, Jura and Colonsay and is an active member of the group researching the glacial rocks of the Port Askaig Formation on Islay and the Garvellachs.