MacDonald, J. G. and Heriott, A. (Eds.) 1983. Macgregor’s Guide to the Geology of Arran. Geological Society of Glasgow. 2024 note: Most grid references have been recently estimated. They do not occur in the original publication.

Macgregor's excursion guide to the geology of Arran

Third Edition

Revised and edited by J. G. Macdonald and A. Herriot with contributions By B. C. King

Published by the Geological Society of Glasgow, 1983

The University, Glasgow G12 8QQ

Printed by Bell & Bain Ltd Glasgow

First edition 1965; Reprinted 1968; Second edition 1972; Third edition (Revised) 1983; Reprinted 1998

ISBN 0 902892 07 X

(Front cover)

(Rear cover)

Readers are reminded that the routes described in this Guide are not necessarily rights of way. The interests of the local landowners and residents should be respected at all times. Much of the moorland in the northern half of the island is used for game shooting. Recorded telephone information about the stalking programme for any day is available by calling the Hill Phone on (01770) 302363.

Further information on this and other matters regarding accommodation, transport or facilities may be obtained from the Tourist Information Office, The Pier, Brodick, KA27, Tel. (01770) 302140.

Transport information is also available from: Western Buses, Brodick Ferry Terminal, Isle of Arran, KA27 Tel. (01770) 302000.

Caledonian Macbrayne, Ferry Terminal, Gourock, PA19 1QP Tel. (01475) 650100.

Postbus Service, Brodick Sorting Office, Tel. (01770) 302507.

The Geological Society of Glasgow March 1998

Cover photograph by J. G. MacDonald

(Front cover) View towards Dougrie from Tormore Shore. Boulder of pitchstone on left foreground, outcrops of Permian sandstone on right. The mountain in the distance on the left is Beinn Bharrain (2345 feet), on the western side of the Northern Granite.

Foreword

Arran, that lovely island which dominates the broad estuary of the Firth of Clyde, has long been a favourite resort of the holidaymaker, drawn to it by its diversity of scenery and the grandeur of its northern peaks. To the geologist it has been for many years a place of pilgrimage, offering as it does within a comparatively small area a wide variety of rock formations and many striking, and indeed classic, illustrations of geological succession and structure.

The geology of the island has gathered round it over the years a considerable literature. This excursion guide has been designed to serve as an introduction to the subject, and it is hoped that it will prove of use to students of geology as well as of interest to those who find a perennial attraction in the scenic charm of the island.

The death of Dr. Murray Macgregor occurred in January 1966, shortly after the publication of the first edition of this guide. A short biography and list of other publications of this eminent geologist can be found in the "Proceedings of the Geological Society of Glasgow", session 108 (1967), pp. 17–24.

Foreword to the 3rd Edition

Arran continues to hold a fascination for geologists, both amateur and professional, but over the years since this guide was first published several important developments have taken place in basic understanding of geological principals and in our knowledge of the Arran rocks. Also the activities of the Forestry Commission have altered the physical access in some areas.

In this first major revision of Macgregor's Guide the opportunity has been taken to incorporate a number of changes which reflect these developments; at the same time measurements have been modified to take account of metrication and many of the figures have been re-drawn. Throughout, however, an effort has been made to preserve as much as possible of the original text so that the third edition can still be substantially thought of as the work of Dr. Macgregor.

1 Introduction: Scope of the Guide: Summary of Physical Features: Memoirs and other Publications: Transport and accommodation: Acknowledgements

2 Summary of the geology

1. Dalradian

2. Ordovician

3. Old Red Sandstone

3a. Lower Old Red Sandstone

3b Upper Old Red Sandstone

4. Carboniferous

5. New Red Sandstone

5a. Permian

5b. Trias

6. Mesozoic Fragments in the Central Ring Complex

7. Tertiary Period

7a. Early Tertiary denudation

7b. Tertiary Igneous episode

7c. Later Tertiary Denudation

8. Quaternary Period

8a. Time of widespread Ice-sheets

8b. Late-Glacial Interval

8c. Post-Glacial times

3 Archaeological notes

4 Descriptive itinerary

Excursion 1: Glen Rosa and Glen Shurig

Excursion 2: Brodick to Lamlash by the coast

Excursion 3: Corrygills and the Clauchland Hills

Excursion 4: Corrie Shore

Excursion 5: Locherim Burn and Coire nan Larach

Excursion 6: Glen Sannox and North Glen Sannox

Excursion 7: Loch Ranza, North Newton and the Cock of Arran

Excursion 8: Catacol, Loch Ranza, and Creagan nan Caorach

Excursion 9: Catacol, Loch Tanna and Pirnmill

Excursion 10: Pirnmill to Catacol

Excursion 11: (a) Kingscross and Glen Ashdale (b) Holy Island

Excursion 12: Largybeg to Bennan

Excursion 13: Drumadoon and the Tormore Dykes

Excursion 14: Central Ring Complex

5 Road Logs

6 Glossary of Place Names

7 References

8 Index

Illustrations

(Figure 1) Simplified physiographic map of Arran

(Figure 2) Simplified geological map of Arran

(Figure 3) The Northern Granite mountains

(Figure 4) Diagrammatic cross sections of the Dalradian

(Figure 5) Index to ornaments used on the geological sketch-maps, figs. 6, 7, 9, 11 and 13 to 16

(Figure 6) Geological sketch-map of the Brodick district

(Figure 7) Geological sketch-map of the Brodick—Lamlash district

(Figure 8) Plan of Dunan Beag cairn

(Figure 9) Geological sketch-map of the Corrie district

(Figure 10) Diagrammatic cross-section of the Carboniferous rocks on the Corrie shore

(Figure 11) Geological sketch-map of the Glen Sannox and North Glen Sannox district

(Figure 12) Plan of fort at Torr an t-Sean Chaisteil

(Figure 13) Geological sketch-map of the Loch Ranza—Cock of Arran district

(Figure 14) Geological sketch-map of the Catacol—Pirnmill district

(Figure 15) Geological sketch-map of the Kingscross—Whiting Bay district

(Figure 16) Geological sketch-map of the Largybeg—Kildonan district

(Figure 17) Geological sketch-map of the Drumadoon—Tormore district

(Figure 18) Geological sketch-map of the area around Ard Bheinn, Central Ring Complex

Plates

(Plate 1) The Northern Granite mountains. FrontispieceThe Northern Granite mountains as seen from near the Stone Circle at the summit of the Brodick—Lamlash road (113m). In the centre, Upper Glen Rosa is seen between Beinn a'Chliabhain on the left and Glenshant Hill on the right, with Cir Mhòr at its head.

The limit of the high ground coincides closely with the edge of the granite, and the mountains show abundant evidence of severe glaciation. The foothills are composed of Dalradian (Glenshant Hill) and Lower Old Red Sandstone rocks, while the foreground is mainly of Permian sandstones and breccias, on which portions of the dissected 300m (thousand-foot) platform can clearly be seen. Photo, A. Herriot

(Plate 2) Drawings of micro-sections of some typical Arran rocks

Front cover

Rear cover

(Figure 1) Simplified physiographic map of Arran showing areas covered by the excursion itineraries. Road log localities indicated by numbers. Contour interval 100 metres.

(Figure 2) Simplified geological map of Arran; based on the Geological Survey one-inch map.

(Figure 3) The granite mountains of North Arran. Contour interval 150m. A. The Saddle. 431 m; B. The Archer's Pass, a little over 610m; C. unnamed pass, about 610m; D. A'Chir ridge, about 713m.

(Figure 4) A, Cross section of Dalradian of North Arran from Newton to Corloch. B. Structural model of the Tay Nappe according to Shackleton (1958). Note the position of the slate at the core of the Aberfoyle Anticline corresponding with the Loch Ranza Slate Group in section A. Length of section B is about 30km. C. Major structures in the Clyde Area. Note how the Aberfoyle Anticline has been distorted by the intrusion of the granite (G).

(Figure 5) Index to the geological sketch-maps. (Figure 6), (Figure 7), (Figure 9), (Figure 11), (Figure 13), (Figure 14), (Figure 16).

(Figure 6) Geological sketch-map of the Brodick area to illustrate Excursion 1 and part of Excursion 2.

(Figure 7) Geological sketch-map of the Brodick–Lamlash area to illustrate Excursions 2 and 3. For key see Figure 5.

(Figure 8) Plan of Dunan Beag Cairn, Lamlash: from The Book of Arran, 1910. p. 76.

(Figure 9) Geological sketch-map of the Corrie area to illustrate Excursions 4 and 5. For key see Figure 5.

(Figure 10) Diagrammatic cross-section of the rock succession on the Carrie Shore.

(Figure 11) Geological sketch-map of the Glen Sannox and North Glen Sannox area. For key see Figure 5.

(Figure 12) Plan of the fort at Torr an t-Sean Chaisteil, Sannox; from The Book of Arran, 1910. p. 176.

(Figure 13) Geological sketch-map of the Loch Ranza–Cock of Arran area to illustrate Excursions 8 and 9. For key see Figure 5.

(Figure 14) Geological sketch-map of the Catacol–Pirnmill area to illustrate Excursions 9 and 10. For key to map, see p. 78.

(Figure 15) Geological sketch-map of the Kingscross—Whiting Bay area to illustrate Excursions 11 and 11a. Note that the Triassic sediments have been left blank. Where indicated the identity of the dykes is as follows: K^C crinanite, K^T tholieiite ^OK^T olivine-tholeiite K^A 'andesitic' quartz-dolerite, F^I pitchstone

(Figure 16) Geological sketch-map of the Largybeg–Kildonan area to illustrate Excursion 12. For key to map see Figure 5, p. 78. Note that the area occupied by Triassic sediments has been left blank.

(Figure 17) Geological sketch-map of the Drumadoon–Tormore area to illustrate Excursion 13.

(Figure 18) Geological sketch-map of the area around Ard Bheinn in the Central Ring Complex, to illustrate Excursion 14.

(Plate 1) The Northern Granite Mountains. (For explanation, see page 8)

(Plate 2) Micro-sections of some typical Arran rocks. Fig. 1. Permian sandstone, old quarries at Corrie. x12. Grains of quartz, felspar and quartzite, the surfaces of which have been rounded by wind abrasion ("millet seed" grains), are coated and cemented loosely by limonite. Fig. 2. Lower Carboniferous Sandstone, south side of String Road, 915m SW of Brodick Church. x13. Angular grains of quartz (dusty with fluid inclusions) and of decomposed felspar and siliceous rock are loosely cemented by clay. Fig. 3. Lower Old Red Sandstone, shore cliff, 800m S of Dougrie. x 11.5. Angular and unsorted grains of quartz (clear), decomposed felspar (grey) fragments of mudstone, igneous rocks and oxidized iron ore are compactly cemented by the fine-grained waste of similar material. Fig. 4. Pebbly grit, Dalradian, near SE end of Creag Ghlas Laggen, North Arran. x 10.5. Fig. 5. Cleaved grit, Dalradian, shore 69m ESE of Loch Ranza pier. x9–5. The rock has been sheared. Quartz and quartzite pebbles have been deformed and ground away until their long axes lie parallel to the schistosity which is strongly developed by parallel orientation of the chlorite and muscovite flakes of the matrix. Fig. 6. Slate, Dalradian, old quarries on hillside 2.4km E of head of Loch Ranza. x 13. The paler and darker bands represent more silty and more clayey alternations of the original strata. Within the paler bands the effect of shearing can be seen in the development of a lenticular schistosity on a microscopic scale. Fig. 7. Pitchstone dyke, Schoolhouse, Brodick. x12. Small euhedral crystals of quartz lie in a matrix of rock-glass from which numerous crystallites of pyroxene have grown. In other parts the rock contains euhedral prisms of zoned plagioclase and pyroxene which are not shown in this figure. Fig. 8. Granite, Glen Rosa. x12. The rock is composed of oligoclase (showing straight cleavage lines), orthoclase (turbid), quartz (clear, with tiny inclusions and cracks), and biotite (dark, with straight close cleavage). A small prism of zircon lies on the left side of the topmost biotite). Fig. 9. Olivine basalt lava, Lower Carboniferous, Corrie shore opposite schoolhouse. x10.5. Euhedral phenocrysts of purplish augite (bottom right) and olivine serpentinized along cracks (centre) lie in a matrix composed mainly of plagioclase laths, augite grains, and iron ore granules.

Macgregor's excursion guide to the geology of Arran

Foreword

Foreword to the 3rd Edition

Contents

Illustrations

Plates