MacDonald, J. G.
An illustrated PDF download is available from the Geological Society of Glasgow website.
MacDonald, J. G.
An illustrated PDF download is available from the Geological Society of Glasgow website.
J. G. MacDonald
An illustrated PDF download is available from the Geological Society of Glasgow websiteVersion 1.1
Geological Society of Glasgow Registered Scottish Charity No. SC007013
Geological Society of Glasgow Excursion Itineraries
© J.G. MacDonald and the Geological Society of Glasgow 2015
This is a major revision of Excursion 4: Corrie Shore, pp. 103–112, published originally in: J. G. MacDonald and A. Herriot (eds), 1983. Macgregor's Excursion Guide to the Geology of Arran. 3rd edition. Geological Society of Glasgow.
Access to unpublished data relating to the Corrie Limestone and the Coal Measures succession, held in the archives of the British Geological Survey, Edinburgh, is gratefully acknowledged.
The main purpose of this excursion is to examine the Carboniferous rocks of Arran in an area where the rock succession is clearly displayed at the roadside. The excursion also offers an opportunity to examine the junctions of the Carboniferous strata with the underlying Upper Old Red Sandstone and the overlying Permian.
The localities are readily accessible along a 2 km shore section. The only diversion involves a steep climb to locality 8 to examine the Corrie Limestone. The shore section is largely accessible at any state of the tide but it is best to avoid high spring tides. The going is rough over rocky outcrops and boulder beaches the latter often being slippery, especially after stormy weather when they may be strewn with seaweed, so stout footwear is recommended, especially with ankle protection.
There is limited car parking at localities 1 and 13 and a larger car park on the south side of the Corrie Hotel. Refreshments may be obtainable at a few small tea rooms or the hotel bar.
Corrie is situated to the east of the northern granite. The strata are generally inclined to the SE so the oldest rocks (Old Red Sandstone) occur in the north at locality 1 and they become progressively younger toward locality 12 (Permian). The outcrop pattern is broken up by a set of NNW trending normal faults. As a result of this the succession seen in the shore section is displaced and repeated inland where it crops out in stream sections, especially in the Leucheram Burn
At the roadside a huge erratic block of granite known locally as the Cat Stone (Clach a' Chait); is approximately 4.3 m in height, 5.5 m long and 2.75 m wide. About 50 m to the north a small parking area on the east side of the road provides access to the shore where red, cross-stratified and often coarse-grained sandstones crop out. These beds of Upper Old Red Sandstone, dip to the south-east at angles of between 24° and 30°. Sparsely pebbly horizons contain clasts mainly of vein quartz. Note that the dips steepen locally in the vicinity of small crush-lines or faults. Access to locality 2 is to the south along the shore.
A red conglomeratic horizon, 21 m or so thick, crops out here. It is poorly sorted and loosely packed, with a variety of clasts, notably of vein quartz (up to about 120 mm in length), quartzite, sandstone, and semi-pelitic chlorite-schist set in a sandy matrix. The proportion of clasts decreases upwards. It is followed southwards, in upward succession, by a series of red, cross-stratified sandstones, often showing on their upper surfaces irregular ridges which it has been suggested may represent in-filled mud-cracks.
This locality can been reached by walking along the shore from locality 2, or from the road through a gap in the vegetation opposite the wooden bungalow at the north end of Corrie Village. Cross-bedded Upper Old Red Sandstone is succeeded by a thick upstanding bed of conglomerate
thickness | |
(f) Pale-grey sandstone dipping southeast at 25° to 28° | ∼6 m |
(e) Cornstone, pale-coloured, sandy, nodular | ∼0.75 m |
Gap in succession | 0.9 m |
(d) Cornstone, nodules in red shaly matrix | ∼4.5 m |
(c) Mudstones, red, with beds of pale-grey sandstone | ∼1.2 m |
~~~~~~~~~~~~~~~ line of disturbance — possible fault ~~~~~~~~~~~~~~ | |
(b) Cornstone, pebbly, associated with calcareous sandstone | 1 m |
(a) Conglomerate, massive, reddish in colour, the top of the local Upper Old Red Sandstone succession | 15 m |
The lower part of the succession of the Clyde Plateau Volcanic Formation is encountered here. It consists of purplish porphyritic olivine basalt with phenocrysts of augite, olivine and occasionally plagioclase feldspar. The basalt is columnar and the flows have been tilted at about the same angle as the sedimentary rocks about and below. The columns have been spheroidally weathered
The upper part of the lava succession, also consisting of olivine basalt, is generally fresh, unweathered and provides excellent specimens for making thin sections for examination under the microscope. Look for the slaggy, decomposed, amygdaloidal tops of individual flows. Thin ramifying veins of calcite are common near the top of the lava formation. Above the lavas a succession of beds of reddish sandstones and shales, includes a 460 mm thick bed of red limestone. These strata dip southeast at about 25°. Some of the beds contain small fragments of volcanic ash. They are succeeded by a 3.0–3.6 m thick purplish basalt lava flow, much veined by carbonates.
The Corrie Limestone is the basal member of the Lower Limestone Formation in Arran. This important marker horizon within the Lower Carboniferous succession of the Midland Valley of Scotland is found at many localities with a variety of names (e.g. the Hurlet Limestone in the Glasgow area). On the shore it was excavated to form the Old Corrie Harbour, and mined extensively inland along its outcrop for over 360m.
A path from the bend in the road at the harbour leads uphill to the entrances to day-holes (formerly called ingaun e'es), that gave access to the old underground workings
Rocks belonging to the Lower Limestone Formation crop out along the shore to the south of Corrie Old Harbour. They include thick posts of white sandstone some of which are fine-grained and exceptionally pure. The hard compact sandstone found at this locality was quarried
A reddish 600 mm thick limestone here has been tentatively correlated with the Index Limestone. Like the Corrie Limestone the Index Limestone is widespread throughout the Midland Valley and is of particular importance as it marks the top of the Limestone Coal Formation, a source of commercially workable coals. The limestone at this locality has yielded shelly fossils including brachiopods such as Latiproductus latissimus, small gastropods and a few bivalves. The strata extending southwards to the little bay in front of the Corrie Hotel contain several other marine limestones, ranging in thickness between 300 and 900 mm, separated by shales and occasional sandstone beds which together have been tentatively placed in the Upper Limestone Group. Due to lack of continuous exposure and the occurrence of minor faults or crush lines the estimated thickness of the Group at about 42 m must be regarded as approximate.
This locality includes all the strata seen in the little Bay south of the Corrie Hotel as far as Locality 12. Results of research carried out during the revision of the stratigraphy of the Upper Carboniferous in Arran (Leitch 1942, pp. 141–154) place these rocks in the Upper Carboniferous, certainly in the Coal Measures although a 9 m thick, massive sandstone at the base may be equivalent to the upper part of the Passage Formation. The conglomerate at its base probably marks an unconformity of early Passage Formation age known on the mainland throughout much of Lanarkshire.
Some 100 m or so south of the Corrie Hotel, a sudden change in lithology and sedimentary structures of the rocks cropping out on the foreshore, marks a major unconformity between the Upper Carboniferous strata and those of the succeeding Permian, Corrie Sandstone. The plane of unconformity has been described (Bailey 1926) as being marked by scattered, minute pebbles of vein-quartz and iron pan mixed with rounded quartz grains. The pattern of sedimentation in the Corrie Coal Measures comes to an abrupt end with the sandstone at the top of the succession. Highly irregular contortion of the bedding
The abrupt change in lithology above the unconformity
Bailey, E.B. 1926. Subterranean penetration by a desert climate. Geological Magazine, 63, 276–280.
Clemmensen, L.B. and Hegner, J. 1991. Eolian sequence of erg dynamics: the Permian Corrie Sandstone, Scotland. Journal of Sedimentary Petrology, 61, 768–774.
Gunn, W. 1903. The Geology of North Arran, South Bute and the Cumbraes. Mem. Geol. Surv., GB.
Leitch, D. 1942. The Upper Carboniferous Rocks of Arran. Transactions of the Geological Society of Glasgow, 20, 141–154
MacDonald, J.G. and Herriot, A. (eds), 1983, Macgregor's excursion guide to the Geology of Arran. Geological Society of Glasgow.
Monaghan, A.A. and Parrish, R.R. 2006. Geochronology of Carboniferous–Permian magmatism in the Midland Valley of Scotland: implications for regional tectonomagmatic evolution and the numerical time scale. Journal of the Geological Society of London, 163, 15–28.
Trueman, A. and Weir, J. 1968. A monograph of Carboniferous non-marine Lamellibranchia. London, Palaeontolographical Society.
Young, G.M. and Caldwell, W.G.E. 2011. Early Carboniferous Stratigraphy in the Firth of Clyde area: new information from the Isle of Bute. Scottish Journal of geology, 47, 143–156.