Geology pages

Only the Southwest boundary of the Tollie gneiss lies within the mapping area. Two lithologies are observed to have a boundary with the Tollie gneiss: the Garbh amphibolite and the Druim breccia. The disposition of the Druim breccia and the Tollie gneiss show that the boundary clearly represents an unconformity. The boundary between the Garbh amphibolite and the Tollie gneiss was not observed (c.f. Criteria for placement of boundaries). The unit has a minimum thickness of 2250 metres measured from the core of the Antiform to the boundary of the Garbh amphibolite

The Tollie gneiss contains a major Antiform, the Tollie Antiform (Park 1970, Odling 1984), this is fully described in the structural section of the report.

Exposed surfaces of the Tollie gneiss show the rock to have a light overall colour, with preferential weathering of mafic bands observed. The individual bands vary in thickness between 40 and 5 millimetres with the average width lying towards the narrower end of this range (Plate 2). Fresh surfaces of the gneiss showed a grain size which varied between 1 and 205 millimetres with the mineralogy of the rock varying between the mafic and felsic layers.

The felsic layers had a mineralogy consisting of: quartz 50%, feldspar 30%, biotite 15%, and amphibole 5%.

The mafic layers in the gneiss had the same minerals comprising the rock but the proportions differed from those observed in the felsic layers the mode of the rock is biotite 60% quartz 30%, feldspar 5%, amphibole 5%.

A thin section was taken of this rock (Figure 3). The section was observed to be comprised of quartz 50%, plagioclase 20%, microcline 20% and chloritised biotite 10%, with accessory sphene and apatite. This modal composition suggesting the section was taken from a felsic band in the Tollie gneiss (no amphibole was observed in the thin section).

A poorly defined "L" fabric is believed to be defined by the biotite crystals (with the gneissose banding representing an "S" fabric).

No evidence of retrogression from granulite facies is observed (which considering the mineralogy, is not particularly diagnostic as to whether the rocks have been to granulite facies or not.

It was noted that the Tollie gneiss became finer grained towards the Southwest. The finer bands begin as intercalation’s within the wider bands immediately to the west of Altan Odhar (NGR 83597779) and become dominant to the west of Meall Airigh Mhic Criadh. The Tollie gneiss can almost be described as a coarse schist (i.e. the foliation of the rocks is 1-2 millimetres in width) in the valley of the Allt Eas Ghairbh Ghraid.

The structures observed in the Tollie gneiss are fully described in the structural section of the report.