Major structures in Morar and Knoydart, Scotland 51

the eastern portion of the Knoydart Fold upwards and westwards with respect to the western portion. The whole structure is then folded by the Morar Antiform in the west and the complimentary Ben Sgriol Synform in the east.

ACKNOWLEDGEMENTS. The first author acknowledges the help of Professor R. St. J. Lambert and colleagues at Queen Mary College, the second the assistance and help of Professors J. Sutton and J. G. Ramsay during the tenure of a D.I.S.R. research studentship at Imperial College, London.

5" References

BROWN, R. L., DALZIEL, I. W. D. & JOHNSON, M. R. W. I97O. A review of the structure and stratigraphy of the Moinian of Ardgour, Moidart and Sunart--Argyll and Inverness-shire. Scott. J. Geol. 6, 309-335 . JOHNSTONE, G. S., SMITH, D. I. ~; HARRIS, A. L. 1969. Moinian assemblage of Scotland. Am. Ass. Petroleum Geologists, Mem. I2, 159-18o. KENNEDY, W. O. I955. The tectonics of the Morar Anticline and the problem of the north-west Caledonian front. Q. Yl. geol. Soc. Lond. IIO, 375-39 o. LAMBERT, R. ST. J. i958. A metamorphic boundary in the Moine schists of the Morar and Knoydart districts of Inverness-shire. Geol. Mag. 95, 177-194. & POOLE, A. B. 1964 . The relationship of the Moine schists and Lewisian gneisses near Mallaigmore, Inverness-shire. Proc. Geol. Ass. Lond. 75, I-I4. POOLE, A. B. 1966. The stratigraphy and structure of north-eastern Morar, Inverness-shire. Scot. J. Geol. 2, 38-53 . POWELL, D. 1964 . The stratigraphical succession of the Moine Schists around Lochailort (Inverness-shire) and its regional significance. Proc. Geol. Ass. Lond. 75, 223-25 °. 1966. The structure of the south-eastern part of the Morar Antiform, Inverness-shire. Proc. Geol. Ass. Lond. 77, 79-Ioo. RAMSAY, J. G. 1958. Moine--Lewisian relations at Glenelg, Inverness-shire. (2. J1. geol. Soc. Lond. Ix3, 487-523 • & SPRING, J. S. 1963. Moine stratigraphy in the Western Highlands of Scotland. Proc. Geol. Ass. Lond. 73, 295-32e. PAST, N. 1963. Structure and Metamorphism of the Dalradian rocks of Scotland. In The British Caledonides, Oliver& Boyd, I24-i42. RICHEY, J. E. & KENNEDY, W. Q. I939. The Moine and Sub-Moine series of Morar, Inverness- shire. Bull. Geol. Surv. Gt. Br. 2, 26-45. SUTTON, J. & WATSON,J. 1958. Structures in the Caledonides between Loch Duich and Glenelg, North-West Highlands. Q. Jl. geol. Soc. Lond. Tx4, 231-255 . TONES, L. H. & SIMPSON, J. B. 1934. Suture. Prog. Geol. Surv. Gt. Br. 65-66.

Received 13 April 1972 ; revised typescript received 8 February 1973; read 9 May 1973.

ALAN BRYAN POOLE, Department of Geology, Queen Mary College, London E I. JOHN STUART SPRING, 2/I 90xendon Street, London SWI.

DISCUSSION DR D. POWELL said he was intrigued that the Morar slide invoked by Kennedy in I955 to explain the structure of the Morar area had been, in part at least, reinstated. Both Lambert in I958 and the speaker in 1964 indicated that there is no evidence in south Morar for the Morar slide in the position postulated by Kennedy.

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Powell in 1966 had, however, given evidence viz., attenuation of stratigraphic units and rotation of local second phase fold structures, for the existence of a zone of tectonic discontinuity both within and immediately below the outcrop of the Morar Striped and Pelitic Group in SE Morar, in an analogous position to that which Drs Poole and Spring now, and Johnstone et al. in 1969, suggest for the Knoydart slide. In view of this could the authors give details of the behaviour of tectonic structures across the Ladhar Bheinn Pelitic Group-Barrisdale Psam- mitic Group contact ? Since 'slides' such as that in SE Morar may be zones or planes of strain dis- continuity resulting from differences in behaviour during deformation of thick psammite and adjacent pelitic units and are thus likely to follow contacts between such rock types, could the authors detail the evidence for the down- cutting of the Knoydart slide into the Lower Morar Psammite across Loch Nevis and elsewhere ?

DR SPRING pointed out in reply that the slide now proposed in an area which could broadly be called the NE quadrant of the Morar Antiform is at a different, higher level than the Morar Basal Slide of Kennedy (I955) , which in this area follows the top of the Lewisian and of the Basal Pelitic Group. But moving south, the present authors propose that the slide leaves the bound- ary at the base of the Striped and Pelitic Group and passes down, temporarily at least, into the underlying psammite. This must be so because at this point, between Loch Nevis and Loch Morar, the base of Group 5, the Striped and Pelitie Group is folded relatively tightly around the hinge of the Knoydart Fold, and it is an essential feature of the authors' proposal that the slide is contempory with, and not older than, the fold (see Fig. 4). The authors' present work does not allow them to make an extrapolation still further south, into the SE quadrant of the Morar Antiform, but they note with interest that here Dr Powell recognises a zone of discontinuity which is again at the base of the Striped and Pelitic Group~though it must be emphasised that this is a different outcrop of this Group, the Knoydart outcrop having terminated southwards at the Knoydart Fold closure near Tarbet. The authors would agree with Dr Powell that there are discrepancies in second age minor fold structures as they are followed across the Knoydart Slide, although it is difficult to be specific as the structures are developed in massive psammite and in pelitic mica schists respectively and are consequently of different types.

DR J. A. WINCHESTER remarked on the rapid increase in metamorphic grade that occurs east of a line approximately coincident with the proposed trace of the Knoydart Slide in Morar, and asked whether this might be additional evidence supporting the existence of a slide.

DR POOLE agreed that in Knoydart rocks to the east of the slide are at a higher metamorphic grade, whereas the psammites to the west are lower grade rocks showing massive current bedding over large areas. In this context it is noteworthy

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that the recently published Survey Map (sheet 6 i) for this area shows this bound- ary as the 'approximate western limit of regional migmatisation.' Additional metamorphic grade evidence for the existence of the Knoydart Slide is provided by Lambert's remapping (1958) of the Moine-Sub Moine boundary between rocks in two metamorphic states, and both on the NW side of the Morar Antiform and on the east side near Tarbet this boundary lies in the psammitic Group at essentially the same position as the authors propose (Fig. 4) for the Knoydart Slide.

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