The geology of this area is difficult to summarize briefly, but three main groups of rocks were studied - Dalradian rocks north of the Highland Boundary Fault, the rocks of the Highland Border Complex, and younger rocks of the Midland Valley to the south.  The reader is referred to the Geological Society of Glasgow's excellent  guidebook "Geological Excursions around Glasgow and Girvan", excursion 10, and, in particular, locality 3 - Lime Craig Quarry.

Lime Craig quarry is situated about 1 kilometer south-east of the David Marshall Lodge, near Aberfoyle.  On the way up to that quarry,  we examined typical Highland rocks, the Dalradian slates and grits.  These are metamorphosed to a low grade only in this area.  Some of the grits show graded bedding, from which the way up, and hence the regional structure - a synformal anticline at the southern edge of the Tay Nappe -  has been worked out.

At Lime Craig quarry, we examined some of the rocks of the Highland Border Complex, Ordovician in age,  which are believed to represent a sliver of ophiolite (oceanic crust), which was obducted onto the continental crust, instead of being subducted down into the mantle as is normal, so that it is seen at the surface today.  Serpentinite, an ultrabasic rock typical of ophiolites is seen.  Elsewhere in the area, black shales and cherts are also to be found.

On the east face of the quarry, steeply dipping conglomerates, of Lower Old Red Sandstone age, form high ground but are geologically part of the Midland Valley.  They contain poorly sorted, but very well rounded clasts of andesite and quartzite.  These do not have an obvious source in adjacent metamorphic rocks of the Highlands, and this has led to the suggestion that there was a considerable strike-slip component to movement on the HBF, and that tracts of land to the north and south represent, in fact, different terranes.

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