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A journey through the Ben Lawers Synform north of Loch Tay |
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Hilary Black |
The rocks in the Synform are Argyll Group, comprising in structural order: Loch Tay Limestone, Ben Lui Schist, Auchlyne Formation (Farragon Beds), Ben Lawers Schist, and, because the rocks lie on the inverted lower limb of the Tay Nappe, these are in reverse stratigraphical order so the older units are in the centre of the synform.
We drove along Loch Tay and turned north up to wards Ben Lawers. The first stop was a quarry in the Loch Tay Limestone, a dark grey crystalline limestone in a very persistent bed running from Campbeltown in the west to Deeside in the east. Its depositional conditions are not known and it is possibly a reworked limestone. There were bands of impurities with mica, calcite and quartz and some local folds to be seen.
We then drove on past the Ben Lawers Centre and walked westwards along the track towards the hydroelectric intake in the Allt Tir Artair. The track passes small outcrops of garnet-mica schist, Ben Lui Schists, but the track itself was made with Ben Lawers Schist, waste from the tunnels for the Lochan na Lairige hydroelectric scheme. Looking down to Loch Tay we saw the dogleg formed by the Loch Tay Fault.
As we went up the stream from the intake we left the Ben Lui Schist and passed into the Auchlyne Formation with its mixture of rocks, elements of both the Ben Lui and Ben Lawers schists and some rather flaggy psammitic rocks, evidence of high strain. Before we reached the boundary with the Ben Lawers Schist we found a nice sheltered spot for dinner, very welcome in the cold wind. The poor gentleman who was already there having his dinner rather had his peace disturbed by a large group of geologists! Afterwards we made our way up to the calcareous pelites of the Ben Lawers Schist.
Looking across to Creag na Caillich, we could see
the prominent appinite sill with its vertical cooling joints, unusually, this is
mainly ultrabasic but can also be basic and more acid especially in its lower
parts. The top forms a sharp contact and has baked the country rock to form
hornfels and there was a Stone Age axe factory utilizing the hard rock. The
rocks in the sill were also quarried for aggregate for the reservoir dam. A
discussion whether to go to the quarry followed with many opting to return to
the cars. A dedicated group carried on in a brief snow storm. As we began to
pass regular heaps of stones, Graham told us they were droppings from the aerial
ropeway that had carried the rock down from the quarry, so we looked at these
instead of continuing to the quarry! Here we found rocks with prominent
hornblende crystals.
Left: Snow on the tops as we move upstream (pic: Don Cameron)
Returning to the cars and driving up to Lochan na Lairige we stopped by a road cutting in the Ben Lawers Schist that displayed examples of three different generations of folding. The rocks were laid down in shallow waters and hydrothermal activity had deposited sulphides forming stratabound mineralisation, the weathering of which causes characteristic staining of the rocks. At the north end of the reservoir we went down to the beach to hunt for examples of rutile (titanium oxide) and garbenschiefer (randomly orientated dark green hornblende porphyroblasts). We failed to find the rutile but Graham did find a good example of the radiating hornblende crystals. The rest of us found other nice bits of rock!
Continuing towards Glen Lyon we stopped at a tributary of the Allt Bail a’Mhulinn (NN 593 414) where we saw the Loch Tay Limestone again. In the stream there was a very good example of ribbed weathering. We went along to find the junction with the Ben Lui Schist but only found the amphibolite associated with it.
We returned to Aberfeldy via Glen Lyon and visited Fortingal where there is an ancient yew tree said to be 5000 years old. The extent that it had covered was marked out but there is not a lot left now. A path was marked out indicating peoples that had been around since it started, from the likes of the Stone Age people and the Romans, and what the tree had been used for.