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Geo-Holiday on KERRERA

This month saw the accidental combination of two of my favorite pastimes: geology and kayaking!

A kayaker's paradise

Located off the Highland town of Oban, the Isle of Kerrera creates a sheltered harbour for Oban and a sea kayakers paradise of secluded bays to explore! I had not even considered the geology of the Island before we first landed there, where we met a group of geology students mapping the coastline. It quickly became clear from exploring the exposed coastline that the were some significant features to be seen.

Geological map of Kerrera. From http://mapapps.bgs.ac.uk/geologyofbritain/home.html

Approaching the island from the east, the metamorphic shale bedrock formed beautiful steeply dipping beds. Softer layers have been eroded away by the action of the waves, resulting in small narrow covers along the coast ideal for exploring and testing your kayak paddling skills in!


However, reaching Rubha Seanach at the southern tip of the island and paddling into the Bay Pot a' Chroinn revealed much younger and more impressive Devonian Conglomerate Cliffs, one of which has made a good strategic location for Gylen Caslte, built

in 1581 by the Clan MacDougall.


Gylen Castle, Kerrera

There is a large time gap between the deposition and deformation of the Dalradian bed rock (deposited ca. 541-1000 million years ago) and the deposition of the overlying Devonian Conglomerates (deposited 393-419 million years ago). As a result, there is a spectacular angular unconformity seen across the island, where sub-vertical slate beds lie below horizontal conglomerate beds.


The Devonian conglomerates are part of a group of rocks names the Old Red Sandstones. During the Devonian, a huge mountain range called the Caledonian Mountains sat over north Scotland. At the time, the UK lay close to the equator in an arid dessert-like environment. These conglomerates were, therefore, deposited by destructive flash floods bringing eroded material down form the Caledonian mountain range. As a result, in addition to the unconformity being angular in nature, it is also heavily erosive, with large chunks of the underlying slate bedrock being ripped up and deposited as clasts withing the conglomerate. This flooding of the region that led to the deposition of this conglomerate bed would truly have been a catastrophic event.

The unconformity showing deformed Dalradian slates below Devonian conglomerates.

Another lovely feature of the Kerrera Geology is the much younger igneous intrusions that can be seen to cut through the older rocks of the island. There were intruded in to the Daradian slates and Devonian Conglomerates in the Palaeogene, around 55 million years ago. They are related to the extensive volcanic activity which resulted in the opening of the Atlantic Ocean (see blog post of Giant's Causeway, Ireland).


These light pink-brown microgabbroic dykes are easily spotted within the dark grey slates of the bedrock, and as can be seen on the map, they show a clear NW-SE orientation across the island. The can even be traced across the Sound of Kerrera to the mainland!


Overall, the Isle of Kerrera was an unexpected geological haven. Seeing it from the kayak gave a whole new perspective on the rocks and allowed us to access the outcrop where it would otherwise be impossible. It reminded me of how James Hutton must have felt when he explored the inaccessible East Lothian Coastline by boat and discovered his famous unconformity at Siccar Point. I hope this is the first of many sea kayak geo-trips - keep your eyes peeled for more on the blog soon!

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