The document summarizes the geology of the Isle of Raasay in Scotland. It describes the island's basement rocks as Lewisian gneiss formed over 3 billion years ago. Sediments from eroded mountain ranges were deposited as Torridonian Sandstone over 1 billion years ago. Younger Mesozoic sediments including Triassic, Jurassic and Cretaceous periods were deposited in various shallow marine to terrestrial environments. During the Cenozoic era, the region experienced volcanism and intrusions of basalt and granite. Glaciation during the Pleistocene sculpted the modern landscape and deposited till.

1. 1. Introduction
Raasay is a small island in the north-west of Scotland, measuring around 5km at its widest point, and having a total length of approximately
20km. To its west across the Sound of Raasay is Skye and the town of Portree, and to the island’s east across the Inner Sound is mainland Scot-
land. An unusually wide range of lithologies is seen in close proximity, juxtaposed by large-scale faults, which allow nearly 3 billion years of
Earth history to be condensed onto one small island. Work was carried out on the island over a six week period from May-July 2014, and the ge-
ology was recorded through use of outcrop mapping and break-in-slope data. More detailed mineralogical work was carried out on the gneisses
and some of the igneous units, while the sedimentary units were characterised through sedimentary logs and small-scale biodiversity studies of
fossil assemblages.
Geology of the Isle of RaasayMorton, J. D. and Baird, A. S.
School of Earth and Environment, University of Leeds
Raasay in relation to Skye and mainland Scotland
2. The Precambrian (c.3-1 Billion Years)
A basement of Lewisian Gneiss (1) was formed by the high temperature and pressure alteration of a predominantly igneous protolith at great
depths within the crust. At least four phases of deformation can be inferred: the first being the formation of a pervasive fabric, followed by at
least two periods of folding. They were then intruded by the Scourie Dykes, which have been subsequently metamorphosed to amphibolites. The
resulting mountain range was then eroded and deposited as the Torridonian Sandstone; firstly in lakes (2) and then later in braided rivers as the
lakes gradually became filled with sediment and the topography was levelled (3). A significant unconformity is then seen before deposition re-
sumed in the late Palaeozoic.
4. The Cenozoic (65-0 Million Years)
Eruption of basaltic lavas blanketed the eroded sedimentary sequence in the Palaeogene,
followed by the intrusions of mafic rocks in the form of the Skye Lava Group and Little
Minch Sill-Complex, and felsic rocks of the Raasay Granite. The enigmatic ‘explosion
breccias’ are also dated to this period of volcanism, and have been suggested to be col-
lapse structures related to volcanic vents. The region was later heavily glaciated during
the Pleistocene, where the present landscape was formed, along with vast deposits of till
that cover much of the island and a number of large faults. Peat was later deposited at
higher elevations during the Holocene.
© Crown copyright/database right 2014. An Ordnance Survey/EDINA supplied service.
Skye Lava
Group Basalts
Cullaidh Shale
Bearreraig Sandstone
Scalpay Sandstone
Ardnish/Breakish
Formation
Stornoway Formation
View facing north from Rubha na Leac on Raasay, with the Triassic Stornoway
Formation visible in the foreground, and progressively younger Jurassic sediments
in the distant cliffs.
Schematic model of the east coast of Raasay in
the present day.
References:
Blatt, H. & Tracy, R. J. 1996. Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed. New York,
Basingstoke: W. H. Freeman & Co.
British Geological Survey. 2006. Raasay. Scotland Sheet 81W. Bedrock and Superficial Deposits.
1:50000 Geology Series. (Keyworth, Nottingham: British Geological Survey).
Hesselbo, S.P. and Coe, A.C. 2000. Jurassic sequences of the Hebridean Basin, Isle of Skye, Scotland.
41-58 in Field Guide Book, International Association of Sedimentologists Meeting, Dublin 2000. Gra-
ham, J.R. and Ryan, A. (editors) (Dublin: University of Dublin.)
Park, R. G. 2002. The Lewisian Geology of Gairloch, NW Scotland. Geological Society Memoir. No.
26.
Page, K. N. 2003. The Lower Jurassic of Europe: its subdivision and correlation. Geological Survey of
Denmark and Greenland Bulletin. Vol. 1 pp. 23-59.
Selley, R. C. 1966. Tertiary Fumaroles of the Island of Raasay, Inverness-shire. Geol. Mag. Vol. 103
No. 6.
Turnbull, M. J. M. Whitehouse, M. J. & Moorbath, S. 1996. New isotopic age determinations for the
Torridonian, NW Scotland. Journal of the Geological Society. Vol. 153 pp. 955-964.
Rubha na Leac Section
Balachuirn Section
Northern Raasay Section
Geological Map of the Isle
of Raasay
4. 5. 6.
7.
8.
9.
10. 11. 12.
3. The Mesozoic (252-65 Million Years)
Subsidence on the Screapadal Fault allowed for accommodation of sediments on a half-graben structure, beginning in the late Permian. The ob-
served sequence began with the Stornoway Formation conglomerates (4) in the Permo-Triassic, followed by a rise in sea level to the shallow,
storm-agitated marine conditions of the Breakish and Ardnish Formations (5). The rise in sea level continued into the deep-marine Pabay Shale
(6), before shallowing again into the fossiliferous Scalpay Sandstone (7). This is followed by the deposition of two shale units sandwiching a thin
ironstone in the latest Lower Jurassic (8), before deposition of the Bearreraig Sandstone (9) in shallow, shelf seas. The environment changed to
one of brackish lagoons and prograding river deltas (10) in the Middle Jurassic, before the region was uplifted and tilted in the Upper Jurassic and
Cretaceous (11). Deposition resumed briefly in the Late Cretaceous in the form of the Morvern Greensands (12).
1. 2. 3.
Stratigraphic Column for the Isle of Raasay
5. Conclusions
The findings of this study are:
 Basement of Archaean gneisses.
 Neoproterozoic fluvial and lacustrine sediments .
 Westward-dipping succession of Mesozoic sedi-
ments (including terrestrial fans, shallow and deep
marine and lagoonal with associated deltas.
 Palaeogene mafic and felsic volcanics (including
‘explosion’ breccias).
 Extensive erosion and faulting during and after the
Pleistocene glaciation.
Key Fossils
Leptechioceras macdonelli
Simpsonibelus sp.
Spiriferina sp.
Pseudopecten equivalvis
Dactylioceras athleticum
Gryphaea arcuata
Cardinia sp.