The land mass that is now Arran has wandered across the face of the earth through varying latitudes and in different geological settings. The rock types found on Arran reflect the latitude of the land mass at the time of their deposition or formation and the geological conditions that prevailed at that time, as well as forces that may have acted on them after their deposition. These pages are based in part on information contained in McKerrow and Atkins, 1989 , MacDonald and Herriot, 1983 and Gunn 1903 and references there in, and from field observations by Arran Heritage Museum Geological Volunteers. Any mistakes are truly of the volunteers own making!
The main rock types found on Arran were formed during the Cambrian Period (590-505 million years ago or Ma), the Ordovician Period (505-438 Ma), the Devonian Period (408 -360 Ma) , the Carboniferous Period (360-286 Ma), the Permian Period (286-248 Ma), the Triassic Period (248-213 Ma) and Tertiary Period (65-2 Ma), see the simplified geological map below for their distribution. There are also some special outcrops of rocks formed during the Jurassic ( 213-144 Ma) and Cretaceous ( 144-65 Ma) periods. There are no rocks on Arran from the Pre-Cambrian (4600- 590 Ma) and the Silurian (438-408 Ma) periods. There are wonderful examples of moraines and glacial tills, as well as beautiful U-shaped and hanging valleys produced during the Quaternary Period (2 Ma to present).
The Rocks of ArranClick on the plus signs below to see some examples of the rock types formed during a specific Geological Period found on Arran, and for some overview of the geological processes occurring on Arran and the surface of the earth during that Period. Click on the Cambrian Period button below to begin moving through the examples in chronological order.
If we look at the simplified map of the solid geology of Arran above, we can see that the Dalradian deep marine sediments occur on Arran in the northern part of the Island. After their deposition as marine turbiditic rocks, the Dalradian Rocks found on Arran have undergone metamorphism to produce metamorphosed greywackes and schists. This metamorphism was due to major geological events that were occurring during the evolution of the earth, in particular the Caledonian Orogeny, and took the Dalradian marine sediments to Chlorite grade metamorphism (that is, that they were cooked and compressed up to ca. 400°C temperature and 500 kilobars pressure), the Dalradian rocks on Arran were also folded when the Tertiary granite that forms the northern granite mountians of the island was emplaced.
Although the Dalradian metasediments on Arran have undergone a complicated geological history of metamorphism and deformations, it is still possible to see graded bedding in them (Image 1.) , this fining upwards of the sediment is typical of deep marine turbiditic sequences. Image 1, is a photograph of the Dalradian metamorphosed sediments found in the North Sannox burn just beside the road bridge and car park, where you can still see the coarser grains at the base of each depositional unit fining upwards. In other locations such as Imachar the original Dalradian sediments were more muddy and when they were metamorphosed a schistose cleavage was produced Image 2. The cleavage plains contain micaceous type minerals of muscovite and chlorite aligned in the same direction, this can easily be seen, by studying the rock down a petrological microscope Image 3.
In Glen Rosa Dalradian rocks can be observed in contact with Devonian rocks (Gavrock member of the Lower Old Red Sandstone) Image 4. This contact is thought by some to represent a possible extension of the Highland Boundary Fault, others have described the contact as an unconformity. You may want to go and have a look for yourself! There are numerous outcrops of the contact between the Dalradian rocks and the Tertiary Granite that forms the north Arran mountains. An example of the contact between Tertiary Granite and Dalradian can be seen in the North Sannox Burn Image 5. In other outcrops on Arran the Dalradian rocks can be seen to have been locally metamorphosed by the intruded granite, this is in addition to the general metamorphism of the Dalradian rocks during the Ordovician.
During the early Ordovician (Tremadoc age, 505-488 Ma) The Grampian Orogeny took place and the Dalradian sediments found on Arran underwent folding and metamorphism Image 3. The Grampian Orogeny was related to large scale plate tectonic events and readers should refer to Mckerrow et. al. 2000.
During the Silurian the Arran land mass was at a latitude of about 15°N.
The Lower Old Red Sandstone of Arran outcrops in many places and is represented by conglomerates with rounded clasts of quartzite and vein quartz, red pebble rich grits and thin bedded sandstones and siltstones. During the interval between the deposition of the Lower Old Red Sandstone and the Upper Old Red Sandstone. The Lower Old Red Sandstone sediments were folded and underwent faulting due to major crustal movements, they were also denuded and eroded by the elements and in some locations completely removed. In Image 1 we can see an example of the Gavrock member of the Lower Old Red Sandstone, it is exposed in Glen Rosa, here it is green coloured as it has been altered. The iron in the coatings on the quartz grains has been reduced and has turned from red to green.
1. The Inverclyde Group (now including The Kinnesswood found at Hutton’s Unconformity), sands and mudstones with cornstones.
2. The Strathclyde Group, sands and shales above basalts lavas and mudstones.
3. The Lower Limestone Group, sediments laid down in deltas and limestones, some of the members of this group ar highly fossiliferous (Images 1 , 2 and 3).
4. Limestone Coal Group, sediments laid down in deltas and coals.
5. Upper Limestone Group, deltaic sequences with limestones.
6. Passage Group, fluvial (river) laid sandstones and volcanic ashes.
7. Coal Measures, river sediments and sediments formed in deltas with forests, these rocks contain abundant fossilised plant remains, including fossil rootlets and fossil tree roots (stigmaria) (Images 3 and 4).
1. Corrie Sandstone (sub unit of the Brodick Beds), aeolian sandstones (Image 1).
2. Brodick Breccia (sub unit of the Brodick Beds), fluvial conglomerates.
3. Brodick Beds, aeolian and fluvial sandstones and conglomerates (Image 2).
4. Lamlash beds, aeolian and fluvial sandstones and conglomerates.
The lower fine grained sandstone beds are displaying cross-bedding.
These rocks were thought to have been laid down in a lacustrine environment.
a.) During most of the Tertiary except the very last part, basaltic dykes (with a range in compositions) were intruded into country rocks Image 1. In some places on Arran in particular around the coast at Kildonan, there are so many dykes to see, that they are called dyke “swarms” Image 2.
b.) The next simplified sub-division of the Arran tertiary events/rocks, is thought to have occurred early in the Tertiary Period (ca. 63-60 million years ago), and involved the production of basalt plateau lavas, erupted through fissures. The only remnants of these lavas can be seen in the collapsed caldera of the Tertiary Central Ring Complex of Arran (see d below).
c.) The next Tertiary sub-division is the Northern Arran Granite unit/event (ca. 60.3-58.8 million years ago). A massive granite pluton forced it’s way up through the north of Arran, forming the northern Arran mountains (Image 3) and tilting and locally metamorphosing the Dalradian schists. The granite has a coarse grained inner portion and a finer grained outer portion. Associated with the northern granites are aplitic rocks and pitchstones intrusives.
d.) The fourth sub division of the Tertiary of Arran, is that of the Central Ring Complex, with ages at ca. 58.3 million years plus or minus 4.4 million years. During this time in the central area of Arran a volcano existed, the caldera of the volcano collapsed on itself and and there were other minor volcanoes within resurgent caldera. Many volcanic rock types were produced including granites, gabbros, felsites, basalts, andesites, pyroclastics and other volcanic rocks. During the collapse of the volcano of the Central Ring Complex, blocks of Jurassic and Cretaceous sediments fell into the collapsed caldera and are now the only remains of those sediments on Arran.
e.) The fifth general sub division of the Tertiary of Arran, can be said to have taken place at around 58.5 million years ago plus or minus 1.6 million years. during this time in the Tertiary the Ails Craig plug was formed, it is composed of a riebeckite granite, the Holy Isle Sill was formed Image 4 , the stunning Drumadoon Sill was formed Image 5, and the sill that the Isle of Pladda is partly composed of formed. numerous other acidic and basic sills and dykes were formed and a basic cone sheet formed as well.
The granite mountains of Arran have two distinct types of peak, the peaks to the east of Glen Iorsa have jagged, rugged, angular high peaks, some examples being Cir Mhor and Goat Fell (Image 2). By contrast the western granite hills are more rounded, this is thought to have been related to more ice-sheet activity on the lower western mountains. At later periods in the glacial activity and the ice sheets were at lower heights, corries were developed on the northern sides of some of the peaks, i.e. the Devil’s Punchbowl in Glen Sannox (Image 3.).
There are abundant depositional features on Arran related to the Quaternary glacial action. There is abundant boulder clay covering the southern part of Island and some of the western part of the Island. Between Machrie and Dougarie, drumlin features have bee observed in the boulder clay. In the glaciated valleys of the northern granite mountains, abundant moraines can be found. These terminal and lateral moraines were the last deposits of the retreating valley glaciers (Images 4 and 5).