Geological History of Arran | Isle of Arran Heritage Museum

Arran-Simple-Basex

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 Arran

Click 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.

1Cambrian

The oldest rocks on Arran are thought to have been laid down during the Lower Cambrian, they belong to the Southern Highland Group, in particular the Dalradian Subgroup. Similar Southern Highland Group rocks found at Callendar in Central Scotland, contained a fossil trilobite definitively age dating them to the early Cambrian. No such evidence has been found in the Dalradian rocks on Arran, but, it is likely that they were initially laid down during the Cambrian. The Dalradian rocks found on Arran were deposited on the bottom of a deep Ocean as sedimentary rocks such as greywackes , black muds and shales. They were laid down when the land mass that is now Arran was at a latitude of 27°N. These marine sediments have been classified as part of the Dalradian Subgroup which can be be seen outcropping in other parts of Scotland between the Highland Boundary Fault and the Great Glen Fault (Harris et. al., 1978).

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.

1. Graded bedding in a Dalradian greywacke.

2. Dalradian schists near Imachar, the original muddy deep marine sediments have been metamorphosed to produce shists with obvious cleavage planes.

3. A view of some Dalradian Schist, observed down a petrological microscope.

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.

4. The contact between the Devonian Lower Old Red Sandstone and Dalradian in Glen Rosa.

5. A contact between Tertiary Granite and Dalradian rocks, in the North Sannox burn.

2Ordovician

The Ordovician rocks on Arran were a subject of some controversy for quite some time. However, recently fossil, geological and geochemical evidence has been found to strongly suggest that a sequence of rocks are indeed of Ordovician age and belong to the Arenig sub unit of the Ordovician (488-478 Ma), and part of the Highland Boundary Complex. They can best be observed in outcrop in the North Sannox Burn, just below the road bridge. At this point Dalradian (Cambrian) deepwater marine sediments, progress into a suite of Ordovician rocks consisting of black phyllites, brecciated volcanic agglomerates, black slaty shists and pillow lavas. These rocks were formed in a deep marine environment with the pillow lavas issuing onto the sea floor and cooling in pillow shapes. In Image 1., we can see the contact between the Dalradian and the Ordovician rocks. In Image 2, we can see the beautiful pillow lavas that were formed as basaltic lava issued onto the sea bed and cooled rapidly to form pillows.

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.

1. A contact in the North Sannox Burn, between Dalradian rocks and Ordovician rocks. The hammer head rests on the Dalradian rocks and the shaft on the Ordovician black pyllites.

2. Ordovician (Arenig age) pillow lavas exposed in the North Sannox Burn.

3. Folding in Dalradian Rocks due to the Grampian Orogeny that occurred during the Ordovician Tremadoc period.

3Silurian

There are no rocks of Silurian age found on Arran. It is thought that the folding in the Ordovician rocks found in North Glen Sannox, occurred during the Silurian and was related to the Grampian Orogeny.
During the Silurian the Arran land mass was at a latitude of about 15°N.

4Devonian

During the Devonian Period the Arran land mass was at a latitude of 0°N and the predominant environments were fluviatile and lacustrine, i.e. rivers and lakes. The Devonian rocks of Arran belong to a sub-division of the Devonian called the Old Red Sandstone which itself can be sub-divided into the Lower Old Red sandstone and the Upper Old Red Sandstone. Numerous geologists and sedimentologists have studied the ORS rocks of Arran and the Midland Valley of Scotland and for more detail on further sub-divisions and contention in the classification of the sediments readers are referred to McKerrow & Atkins (1989) and references therein.

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. Lower Old Red Sandstone-Gavrock member, Glen Rosa

2. Cross-bedded ORS on Corrie Shore.

3. An Upper Old Red Sandstone Conglomerate exposed on Corrie Shore.

4. A mud cast in the centre of a reduction spot on the surface of an Upper Old Red Sandstone Unit exposed at Corrie Shore.

5Carboniferous

During the Carboniferous period, the Arran land mass was at a latitude of 8°N. The rocks of the carboniferous period found on Arran and predominantly sedimentary rocks. If we take the more recent sub-divisions as listed in McKerrow and Atkins 1989, the Carboniferous rocks of Arran can be subdivided into the following groups from oldest to youngest. The Carboniferous rocks of Arran can be studied in detail at the coastal sections at Corrie and at Laggan, where they are exposed along with some interesting structural features. Within the various Carboniferous units abundant fossils can be found, including corals, brachiopods, bivalves, crinoids and plant fossils including root and stems, to list but a few. Please do not hammer outcrops to collect fossils. Many fossils can be viewed in situ in outcrops and are best left there.

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. A limestone bed at Laggan, a member of the Lower Limestone Group, containing abundant shells of the Brachiopod Gigantoproductus.

2. A imestone outcrop at Laggan, a member of the Lower Limestone Group, containing abundant fossils including crinoids, spiriferids, fenestella and zaphrentids.

3. A limestone outcrop at Laggan, a member of the Lower Limestone Group, containing abundant fossils including crinoids, spiriferids, fenestella and zaphrentids. The unit is fractured and calcite and sphalerite line the fractures.

4. An outcrop of sandstone from the Carboniferous Coal Measures. It is exposed on the shore at Corrie and contains abundant fossilised rootlets.

5. An outcrop of sandstone from the Carboniferous Coal
Measures. It is exposed on the shore at Corrie and
hosts a fossil tree root or stigmaria (to the right of the pencil).

6Permian

When the Permian rocks of Arran were laid down. Arran was at a latitude of 13oN and aeolian and river conditions prevailed, resulting in sandstones deposited in river systems and wind blown systems. The Permian rocks of Arran have been subdivided into two major units the Brodick and Lamlash Beds. The Brodick beds have been further subdivided into another two units The Brodick Breccia and The Corrie Sandstone. Going from oldest to youngest the Permian sub units can be listed as follows:

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.

1. A junction between the Carboniferous river sands (on the left) and Permian wind blown sediments (on the right), on Corrie Shore.

2. Permian sandstones and conglomerates (Brodick Beds), exposed on Brodick shore.

The lower fine grained sandstone beds are displaying cross-bedding.

3. Permian sandstones (Lamlash Beds), exposed on
Clauchlands shore. The Holy Isle is in the background.

7Triassic

The Triassic rocks of Arran, consist of sediments laid down in marine and lacustrine scenarios (Auchenhew Beds), siltstones (Dereneneach Formation) and black mudstones (Westbury Formation). They were laid down when the Arran land mass was probably at a latitude of 32°N. The Auchenhew Beds can be seen in outcrop in many places on the south of the Island.

1. Auchenhew beds outcropping at Levencorroch, the rocks are fine grained, clay rich, parallel bedded and lacking marine fossils.

These rocks were thought to have been laid down in a lacustrine environment.

2. Fine grained parallel bedded lacustrine Auchenhew Beds, exposed in the burn at Levencorroch.

3. Lacustrine Auchenhew Beds, exposed in the burn at Levencorroch. Desiccation cracks, suggest that the lakes in which they were deposited were impersistent (i.e. dried up on a regular basis).

8Jurassic

During the Jurassic Period, Arran was at a latitude of 39°N. The Jurassic rocks found on Arran were deposited during the Liassic sub-division of the Jurassic . Unfortunately the Jurassic rocks found on Arran are not in situ, i.e. in the position of deposition. They are represented in blocks of rock that have fallen into a volcanic vent that was active during the Tertiary, so they are not in their original location, however, they were deposited on the Arran land mass. The Liassic rocks of the Jurassic, found on Arran are found in the Allt nan Dris burn near Balmichael Farm, close to Shiskine village. They consist of marine clays and sands and have yielded an interesting selection of Liassic fossils (Gunn 1903, re printed 1998, Warrington et. al., 1980)

9Cretaceous

Similar to the Jurassic rocks of Arran, the Cretaceous rocks found on Arran are found as blocks that are out of their original depositional stratigraphic situation. The Cretaceous rocks found on Arran are blocks of rocks that have fallen into the vent of a Tertiary volcano. They were deposited during the late Cretaceous when the Arran land mass was at a latitude of 43°N. They consist of marine deposited limestones, that host Cretaceous fossils and chert/flint nodules. The Cretaceous rocks can be found on Ard Bheinn and Creag na Feadh ( Image 1.), near Glenloig. Fossils found in the Cretaceous blocks, match that of the Cretaceous found in Antrim, Northern Ireland.

1. A cave at Creag na Feadh where a block of cherty Cretaceous limestone has been extracted over time for various uses, some of the limestone is still present.

10Tertiary

During the Tertiary Period Arran was at a latitude of 51-54°N. During this time a lot of plate activity took place and the opening of the North Atlantic occurred. Arran lay in an area where a lot of volcanic activity took place, this activity was in part related to major plate movements. It is during the tertiary that Arran began to take on a shape similar to it’s present form. There are some excellent texts on the Tertiary rocks and geology of Scotland and Arran and readers interested in a more detailed discussion of the tertiary Period should refer to McKerrow and Atkins 1989, MacDonald and Herriot 1983 and Sutherland 1982 and references there in. Our simplified approach to the Tertiary of Arran is to break it into several generalised events and related units (a-e), based on the more detailed proposed Tertiary Chronolgy of McKerrow and Atkins 1989. The division of the Tertiary volcanic rocks of Arran and the surrounding Isles is based on geochemical analyses of rocks for elemental and isotopic ratios coupled with field observation of the Tertiary Rocks in situ (McKerrow and Atkins, 1989, page 26, and references there in).

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.

1. A Tertiary igneous dyke, cross-cutting Permian wind blown sediments at Corrie shore.

2. A tertiary dyke on the shore at Kildonan with the Isle of Pladda in the background.

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.

View-of-Arran-Granite-Mountains-from-Goat-Fell

3. A view of some of the Granite Mountains of North Arran taken from Goat Fell.

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.

4. A view of the Holy Isle from Arran, the main rock forming the Holy Isle is a Tertiary sill, composed of a riebeckite trachyte (akin to granite).

5. A view of the Drumadoon Sill, which displays beautiful columnar jointing and is composed of quartz-feldspar porphyry. During the latter part of the Tertiary period on Arran, a lot of the Central Ring Complex was uplifted and eroded, erosion of the northern granite mountains also occurred.

11Quaternary

During the Quaternary period Arran would have looked quite similar to what it looks like today. During the Quaternary period, Arran moved through a latitude of 55-55.5&degN, where it is today. During this period the surface of the Island was acted on by numerous forces, in particular periods of Glacial Erosion. These multiple glaciation events eroded the surface of the island to look as it does today. For a detailed description of the events that took place in the quaternary, readers are referred to McKerrow and Atkins, 1989 and MacDonald and Herriot, 1983. During the Quaternary glacial activity, the northern Arran granitic mountains were thought to be a centre of ice sheet accumulation in their own right from that of the Scottish Highlands (Sissons, 1976). The movement of the ice away from this centre eroded the mountains and gouged valleys to greater depths, thus producing classic U-shape valleys and hanging valleys, that can be seen in Glen Rosa (Image 1), and Glen Iorsa.

1. The U-shaped valley of Glen Rosa, with hanging valleys
on either side and to the rear.

2. Beinn a Chliabhain-Cir Mhor-Goat Fell

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).

3. A Qauternary corrie on the north eastern side of Cioch na h-Oighe, locally it is called the Devil’s Punchbowl.

4. A glacial moraine deposit in Glen Rosa, the moraines are located on the floor of the U-shaped valley in a line of view with the tree in the foreground.