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   Half-day fieldtrips

Fieldtrip 1

Excursion to Ballachulish and Glen Coe

Godfrey Fitton & Ian Dalziel

Monday, 29th August

Fort William sits on the Great Glen Fault, which separates Moine metasedimentary rocks (deposited ~1000-900 Ma) to the north-west from younger Dalradian metasediments (~670-520 Ma) to the south-east. The objective of the first half-day excursion is to examine the Dalradian metamorphic rocks and younger intrusive and extrusive igneous rocks in the Ballachulish and Glen Coe areas. Moine rocks will be seen in the second half-day excursion. Dalradian sedimentary rocks were subjected to regional Caledonian metamorphism (ranging from chlorite- to garnet grade in the Ballachulish area) between 520 and 490 Ma. Intrusion of the Ballachulish igneous complex at 427 Ma produced a contact aureole 400 to 1700 m wide. The Glencoe volcano was active at about 410 Ma during the final stages of closure of the Iapetus Ocean and erupted a calc-alkaline basalt-andesite-rhyolite suite. The volcano is of historic significance as the original type example of volcanic caldera collapse.

View looking north-west from Bidean nam Bian, in the Devonian Glen Coe volcanic complex, toward Loch Leven and Ballachulish. The Ballachulish intrusive complex forms the high ground in the left middle distance (south of the Ballachulish Bridge); the lower ground to the north of the bridge is composed of Dalradian metamorphic rocks. Loch Linnhe and the Great Glen Fault run across the middle of the photograph (SW-NE). Moine metamorphic rocks form the mountains in the distance. Fieldtrip 2, the “Road to the Isles” half-day excursion, will comprise a transect of this part of the Caledonian orogenic belt.


Fieldtrip 2

Excursion along the “Road to the Isles”

Ian Dalziel & Godfrey Fitton

Wednesday, 31st August

The road due west from Fort William to the west coast port of Mallaig is the culmination of the traditional “Road to the Isles” through the Scottish Highlands, and leads to magnificent views (on a clear day!) of the Hebridean isles. In a geological context, our route traverses the Caledonian orogen northwest of the Great Glen fault, and provides a distant introduction to the rocks of the North Atlantic Tertiary igneous province, which form the islands. The objective of the excursion is to examine the Neoproterozoic Moine metasedimentary rocks forming the Northern Highlands in a transect from the west coast to the Great Glen.

Though intensely studied for over a century, this regionally metamorphosed and polyphase-deformed sequence is still highly enigmatic, and potentially of considerable significance in understanding global paleogeography. It is presently believed to record the effects of three major orogenic events at ~830 Ma (Knoydartian, of uncertain tectonic significance), 470 Ma (Grampian, arc-continent collision), and 425 Ma (Scandian, Baltica-Laurentia collision). We will also examine the pre-tectonic West Highland granite gneiss intruded at ~870 Ma, perhaps during an early aborted rifting of the Rodinian supercontinent.

Photo: Outcrop of the West Highland granitic gneiss, a ~870 Ma pre-tectonic intrusion in Moine metasediments, on the slopes of Beinn an Tuim above Loch Eil on the Road to the Isles. The view is eastward across the Great Glen to the Grampian Highlands with Ben Nevis, rising above Fort William, dominating the skyline. Ben Nevis is part of the Glencoe and Ballachulish igneous province to be visited in the first excursion.
last updated 28th July, 2005