So, another couple of weeks have gone by since the last field trip, so what time is it? Time for Another One.
Last Thursday the class (the entire honours class this time, not just Limnology Students [the few, the brave, the mighty]) headed back up to Loch Lomond to the hamlet of Balmaha. This tiny little village sits practically on top of the Highland Border Complex / Boundary Fault, making it our third visit to this Terrane Boundary (the previous being Arran and Stonehaven).
The purpose of the trip was to get us thinking about relationships between Terranes, the evidence for there actually being different Terranes on either side of the fault, and to give us something to think about for our Stratigraphy essay (due in on Monday – yes, I’m procrastinating).
The trip was pretty interesting, if fairly brief. We only really looked at 3 localities; part of the Complex itself, some Upper Devonian ORS Conglomerate and an earlier Lower Devonian ORS Conglomerate. We didn’t go far enough North to cross over onto the Dalradian metasediments.
The Outcrop of the Highland Border Complex consisted of a Serpentinite outcrop in the forest, and some poorly exposed Breccia on the beach, containing clasts of Serpentinite and Gabbro (Cherts, Dolomite, Pillow Lava and Trondjhemite clasts can supposedly be found if you hunt hard enough, but the exposure is poor). If you fancy trying to track this spot down yourself I highly recommend this leaflet [pdf] (location 5), produced by the Strathclyde RIGS group (Part of the Geological Society of Glasgow).
Serpentinite Clast in Breccia
Unfortunately this was the point in the day where the Scottish Winter began to make its presence known. Remember those nice sunny photos last time? It wasn’t like that:
Loch Lomond in the Rain
The first of the two Conglomerate outcrops we looked at on our way south (location 4 in the RIGS leaflet) was the Late Devonian one. It’s a fairly typical ORS Conglomerate, as previously seen on Arran, of mostly metamorphic clasts in a red sandstone matrix. It probably represents an alluvial fan deposit coming off Dalradian Highlands.
ORS Conglomerate outcropping on the Shore
The Second Conglomerate outcrop (1 and 2 on the leaflet) is a younger outcrop. On first glance it appears roughly similar, being part of the ORS unit, however upon closer examination the clasts are of an entirely different origin. We were seeing Quartzites and various igneous clasts rather than the vein quartz and psammites from the Dalradian. A textural difference is noticeable too, these are much rounder than the sub-angular clasts of the first outcrop. This would seem to indicate that in the Lower Devonian the area to the South of the HBF was not in proximity to the Dalradian rocks of the North.
Foreground: Lower Dev. Conglomerate. Background: Lomond North Basin
View Down towards the South Basin
Well that’s it for the Balmaha trip overview! It was a fun and informative trip, made all the more-so by the fact that it was taken by experts on the subject – including Tim Dempster and Brian Bluck.
For further reading on the subject try the (oft-recommended by me) field guide below:
Lawson, J.D. & Weedon, D.S. 1992. Geological Excursions around Glasgow & Girvan. Geological Society of Glasgow.
Edit: Also, the number of google search hits this is getting from what I assume are my classmates working on their essays is brilliantly high.
Tags: Devonian, Geology, Loch Lomond