Loch Assynt Region
After leaving the Isle of Rum the party visited the far NW of
Scotland in the region of Loch Assynt. This is a classic region
for geological studies dating back to the 18th century. The
coastal region exposes some of the oldest rocks in Europe: the
2.8 billion year old Lewisian Gneisses, which were eroded and
overlain by a thick sequence of flat-lying red sandstones, the
Torridonian Group, which itself spans >300 million years of
geological time. At that time these rocks were deposited by
rivers on the edge of the North American continent, which broke
apart at 600 million years ago (Ma) to form the Iapetus Ocean.
As the margins of the continent subsided marine sediments were
laid down over the Torridonian, forming the succession of sandstones
and limestones now exposed along the shores of Loch Assynt (Pipe
Rock, Fucoid Beds, Salterella Grits and Durness Limestones).
Click here to read Rob Sohn’s poetic tribute to the Durness group.
At 470 Ma the margin of North America collided with a chain
of volcanic islands, similar to modern Taiwan, that caused folding
and faulting, and the formation of the Appalachian Mountais
in the US and Canada and the Grampian Highlands in Scotland.
As the final act in this mountain building the rocks of the
Assynt region were bulldozed and repeated by a thrust sheet
of metamorphosed rocks riding on top of the Moine Thrust. Click here
to watch a movie depicting the Moine Thrust at Knockan Crag (note that
some players may not display the video – Quicktime works best). This
region became a classic example of how rocks deform to compressional
deformation and in particular the debate as to whether crystalline
rocks were formed under the ocean, or were precipitated from
magmatic liquids in the way we understand today.
Click on the slide show to see images from the Loch Assynt field excursions.