Ben Lawers Synform & Ben Lui Fold (excursion)

Leaders: Dr John Mendum (BGS) and Dr Graham Leslie (BGS)

Please note that, due to unforeseen circumstances, the dates of this trip have had to be changed from the original dates (20th – 23rd September).

The trip will be to the Loch Tay area, where we will investigate the Ben Lawers Synform and the Ben Lui Fold. Accommodation will be B&B or dinner, B&B, in Killin or Lochearnhead.

We will examine the Dalradian and Caledonian geology of Breadalbane, mainly to the north of Killin. The aim is to identify key elements of the local Dalradian stratigraphy within a south to north structural traverse, from the inverted flat belt of the Tay ‘Nappe’ across the Ben Lawers Synform and then across the trace of the Ben Lui Synform into the right-way-up stratigraphy that lies structurally beneath the Tay ‘Nappe’. We will also take in some Quaternary glacial features that link to both the Main Late Devensian and Younger Dryas (Loch Lomond Readvance) phases. Glacial geomorphology in Glen Dochart developed on the eastern periphery of the Younger Dryas Loch Lomond Stadial ice sheet that was centred over Rannoch Moor; the glacial geomorphology gives some insight to the nature of ice sheet retreat from the region.

We intend to start at the top of Glen Ogle in the inverted ‘Flat Belt’ of the Tay Nappe where the Loch Tay Limestone Formation is exposed in a stream section. We may then look at glacial features in Glen Dochart. Subsequently we will travel northwards to visit the structurally higher but stratigraphically lower Ben Lawers Schist Formation around Lochan na Lairige where small scale structural complexities abound. Representatives of the Farragon Beds, partly volcanic units, are exposed at the northern end of the reservoir provided the water level is moderately low. We will check the succession on the way down to Bridge of Balgie confirming that we are still inverted, until we have crossed the trace of the Bridge of Balgie Fault whence the succession now youngs to the south in Grampian Group psammites. Here, we are on the right way up lower limb of the Ben Lui Syncline. The fault breccia is seen at Bridge of Balgie. The structural and stratigraphical picture and the role and nature of the Ben Lui Fold will be investigated in Glen Lyon and outcrop of breccias visited on the north shore of Loch an Daimh where much of the Appin and Argyll Group succession is absent. If time permits, other exposures in the Ledcharrie Burn section (south side of Glen Dochart) may be visited on the last day.

The localities will range from some burn sections to longer walks on tracks and some roadside views of the overall structure and stratigraphy. It will hopefully be not too difficult or strenuous. The aim is to give a broad picture of the Dalradian stratigraphy and Caledonian structure of the district – needless to say this is quite complex – and also to encompass some of the Quaternary history – particularly the features and deposits relating to the Loch Lomond Readvance and the Main Late Devensian glaciation. There will be some flexibility in the programme – and of course the weather can be important in this regard (views of clouds, not hillsides, etc.).

The price will be announced when the numbers are known and the accommodation arranged.