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Thursday 20 April
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We left Aberfeldy heading south to the viewpoint where we could look over Strathtay to the hills beyond, including Farragon and the barite workings on Ben Eagach. Our journey took us past the “Famous Grouse Experience” near Crieff, but it was really a bit too early in the day for a visit - and we were here for the rocks not the scotch. Our aim was Comrie, the earthquake centre of Scotland, close to the Highland Boundary Fault. To the north of the town is the Comrie Igneous complex that we were about to investigate.
After a pause for breath we left West Lodge Caravan Park and headed eastward to the forest trail to investigate the emplacement of the Comrie granite c. 408 Ma, and the effects on the country rock that here was pelitic Aberfoyle Slate. At the first outcrop we found unaltered slate and looked for bedding and cleavage. The bedding was at c. 70° and the cleavage c.75°. There was little evidence of sedimentation. There was an interesting “Arboreal dyke” cutting the rock face at one point.
About 200m along the track we found a hint of spottiness in the slate but they were very small and overwhelmed by the much larger spots of rain that were now falling. After a further 100m we found that the slate had become more massive, having lost much of its fissility. When we had travelled another 50m or so we were presented with much harder hornfels that had lost any structure and although the notes suggested that it should contain cordierite if anyone found any they kept it very quiet.
We came across a dyke of feldspar porphyry running ENE-WSW with an adjacent minor fold that had been baked by it, but as it was thermal metamorphism only there was no distortion. We then began a treasure hunt in the woods looking for the intrusion itself. After what seemed forever we spotted some “pink stuff” that was declared to be the elusive microgranite or was it microdiorite? After a descent through the trees we emerged back at the caravan park and set off for Funtullich for the afternoon session.
After a 3 mile drive along Glen Lednock we arrived at our lunch stop but we were all keen to have a close look at the rocks that were outcropping at the road side. We found hornfels cut by pink microgranite (quartz, oligoclase and biotite) with some small xenoliths. There was also microdiorite that too was cut by granite veins. A climb up the hill to the east took us closer to the contact with the microgranite mass where we found xenoliths of diorite.
The area to the east looks equally interesting on the map with a profusion of dykes of felsite, lamprophyre, porphyrite and dolerite but distance and time constraints didn’t allow.
We then returned to Comrie before heading westwards along Loch Earn and then northwest along Glen Ogle . Some of the party stopped off at Killin to take photos of the Falls of Dochart that were impressive, but the flow had dropped off since we went by on Saturday and you could imagine what they would be like in full spate. (Oh, all right, I confess – some of us did indulge ourselves in the tearooms with superb cakes and pies.) There was then a long drive along the south bank of Loch Tay along a narrow potholed road until we reached Tomnadashan Copper Mine. After much scratching around we found some molybdenum, as well as the white rabbit, (see Monty Python and the Holy Grail for details). |
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