The report provides an overview of the main findings from the first field season in the Assynt
District of the Moine Thrust Project. Detailed mapping in the eastern part of the Assynt halfwindow has resulted in a new interpretation of the geometry and behaviour of the Ben More
Thrust. This reinterpretation of the thrust satisfactorily resolves the conflicts between the various
previous models. The remapping confirmed that the Ben More Thrust can be traced, as shown on
the published 1923 Assynt District geological map, along the western flank of Na Tuadhan to
Bealach a’ Mhadhaidh. The Ben More Thrust is then traced to [NC 30026 24416] where it is
displaced across a steep reverse fault to [NC 30514 23953]. It then continues NNW as a readily
traceable feature placing gneisses of the Lewisian Gneiss Complex over quartzite along Leathaid
Riabhach [NC 298 252]. Here the Ben More Thrust progressively steepens into a sub-vertical
structure that has gneiss to the NE and quartzite to the SW. The thrust follows a prominent gully
along Leathaid Riabhach to A’ Chailleach. From here the Ben More Thrust more or less follows
the top of a monoclinally folded quartzite that forms the summit of Beinn Uidhe and is exposed
in the valley floor NW of A’ Chailleach. It retains thrust geometry with hangingwall gneisses
and footwall quartzites and becomes a steep feature that approximately follows ‘Glen Beag’ (the
un-named glen south of the Stack of Glencoul). The Ben More Thrust meets, but does not
displace the Glencoul Thrust at the head of Loch Glencoul. Therefore it is proposed that there is
a branch line here where the two thrusts meet so that all the rocks NE of Loch Glencoul and east
of Loch Beag are part of the Ben More Thrust Sheet. Figure 2.7 in the report provides a clear
pictorial description of the geometry of the Ben More Thrust in the northern part of the Assynt
half-window.
A significant new ductile structure has been identified within the Ben More Thrust Sheet, termed
the Coire a’ Mhadhaidh Detachment, that mostly follows the Lewisian gneisses/quartzite contact.
It has been traced from the northern limits of the Loch Ailsh intrusion across Ben More Assynt,
along the eastern slopes of Na Tuadhan, across Cailleach an t-Sniomha to the west of Gorm Loch
Mòr and immediately west of the Stack of Glencoul into Glen Coul (Figure 2.1 in the report).
The sense of shearing in the detachment is almost always top-to-west. Similar smaller shears
have also been recognised within the Lewisian gneisses in the thrust sheet. However, no ductile
shearing was noted at the gneiss/quartzite contact below the Ben More Thrust.
Several of the complex imbricate structures mapped by previous workers were revisited. The
imbricates in the Loch an Eircill–Loch nan Caorach area appear to be simpler than shown on the
published Assynt District map. An alternative solution is provided for the southern termination
of the Glencoul Thrust south of Inchnadamph although it is noted that more detailed work needs
to be done, notably south of Conival.
Brief descriptions are given of Moine rocks above the Moine Thrust in the north-eastern part of
the Assynt District map. There appears to be a lateral facies change with semipelitic schists
dominant in upper Glen Cassley and psammites becoming dominant to the north. Fabrics
associated with several deformation phases have largely obliterated sedimentary structures
although transposed bedding traces can be seen between a spaced foliation that controls the
flaggy character of the psammites.
Widely spaced traverses across the major Lewisian outcrop areas, within the Assynt half-window
as well as in the western foreland to the thrust belt, largely confirmed the work of the primary
surveyors. Thus all of the Lewisian comprises orthogneisses, mostly hornblende-gneisses but
with more felsic pyroxene-bearing gneisses in the north, that all contain ultramafic and mafic pods and layers. The traces of the various Scourie dykes are correctly shown on the published
Assynt District map. The Canisp Shear Zone has been traced eastwards, south of Canisp,
eventually disappears under Cambrian quartzites. A second parallel shear has also been
delineated north of Loch Assynt. The polyphase nature of ductile deformation in the Lewisian
gneisses elucidated by previous workers is confirmed. However, the deformation state of the
gneisses is extremely variable on all scales, with intense deformation confined to specific (shear)
zones that vary in thickness from several centimetres up to hundreds of metres.
Descriptions of the numerous minor intrusions and the Quaternary deposits studied during the
fieldwork are given in separate reports