Geological Excursions

Take with you the following if  required. Ordnance Survey map of the area. Bag for collecting rock specimens. Newspaper for wrapping specimens. Labels to identify specimens, noting where collected. Handlens. Geological hammer. Geological chisel if collecting fossil specimens. Geological Survey map of the area. Field notebook and pencil to note down your own observations. Camera.  Excursions: Iona; The Ross of Mull: Gribun; Carsaig and Glen More.
 

Excursion One - Iona:

Park in the official carpark at Fionnphort and take the ferry to Iona. At the jetty on Iona turn left and walk along the road by Martyrs Bay. At G.R.284237 you can see a glacial erratic boulder sitting on top of Torridonian sandstone. It was transported by ice across the Sound of Iona. (Many more can be seen on the cliffs along Iona’s south east coastline.) The Torridonian sandstone beds can be seen to be almost vertical at this locality. Walk further along the road and then turn off onto the sandy beach at Traigh Mhor. Follow the rocky shoreline until you find a vertically sided  gully in the low cliff at G.R.283231. This is where Torridonian sandstone is cut by a Tertiary dyke from the Mull Dyke swarm. The chilled margins of the dyke can be clearly seen against the gully sides. The coarser grained centre of the dyke is less resisitant to marine erosion. Islets just offshore consist of much younger Ross of Mull granite which means that an important fault must separate these from the Torridonian and the Lewisian. Return to the road and continue on until you come to the cross roads. Turn left up the rough track and continue to the end and on to the moorland (be sure to shut the gate). Walk in a SSW direction for about one mile towards a long sloping valley at the end of which lies Iona’s disused Marble quarry G.R.268218. (This can be confusing to find as several sloping valleys look very similar.) Here a narrow and largely worked out vein of white marble forms part of the Lewisian. The marble has been metasomatised and is streaked pale green with serpentine. Talc schist can be seen at the edges. Large weathered blocks of marble at the foreshore show good fold structures. Usually pieces of marble can be found here but for prettier pieces of marble and colourful banded Lewisian gneiss the wave washed pebbles at the beach in the bay just to the north-east at G.R.271219 offer the best specimens.

(This excursion takes about half a day, leaving you time to visit Iona Abbey and other places of interest or to explore the geology of the south and west of Iona.)
 

Excursion two - The Ross of Mull:


Park in the official carpark at Fionnphort and walk down to the jetty. From here note the view across the Sound to Iona. On Iona the Lewisian outcrops form higher relief whilst the Torridonian outcrops form lower relief. Remember that the Lewisian gniesses are about 2000 MY old, polycyclic, and part of an ancient archean craton which used to lie at high latitudes in the southern hemisphere, whilst the Torridonian sandstones represent sedimentary rocks laid down under deltaic conditions from sand and conglomerate sized material produced from the weathering and breakdown of an ancient super-continent (of which North America and Greenland were also a part).  Bear in mind that the Sound of Iona may mark the position of the Moine Thrust Fault which elsewhere in Western Scotland separates Moine age rocks from the much older Lewisian. Walk the short distance south of the ferry jetty to examine the granite. The Ross of Mull granite, which is Devonian in age and was intruded during the Caledonian orogeny about 414 MY ago, shows four stages in its evolution:- Intermediate igneous (tonalite) stage, pink granite facies, red granite facies, pegmatite and aplite late stage veining. Here close to the ferry jetty are rafts of angular tonalite (dark in colour with large pink feldspar phenocrysts) incorporated within the pink facies of granite. The angular chunks of tonalite are thought to be parts of a pre-magma fraction. From here also note the ‘castellations’ created by marine erosion of the closely jointed granite, giving castellated coastal scenery. Walk northwards across Fionnphort Bay and walk over the ice smoothed outcrops of granite towards Tor Mor Quarry. In this area you can see small granite tors created by weathering processes. Where closely spaced joints within the granite have weathered faster the relief is lower and grassy or boggy. Where the granite has widely spaced joints huge blocks of resistant granite form higher relief. Take a close look at the weathered granite surfaces and see how the constituent minerals of  dark brown or black biotite weather to green chlorite, and pink and white feldspar weather by hydrolysis to kaolin, whilst quartz resists weathering but loses cohesion. The quarry which was extensively worked until 1910 was reopened in 1986. It is still worked from time to time. Entry requires written permission from Scottish Natural Stones Limited, Edinburgh Road, Springhill, Shotts, ML7 5DT. A huge xenolith can be seen in an east facing vertical face in the top part of the quarry. This was almost certainly a roof pendant of Moine schist incorporated into the granite. The Ross of Mull granite is one of the world’s best examples of ‘stoping’ - inclusion of roof material within the granite. (If you do not have permission to enter the quarry then xenoliths can also be seen in granite outcrops at Knockvolagen.) Return to your vehicle and make your way towards Bunessan. Park off the mainroad near Bendoran Cottage at G.R.365227. Here the contact between the 1000 MY old Moine series and the 414 MY old Ross of Mull granite can be seen in the rocky outcrops behind the cottage. Contact schists are well developed with Cordierite, Sillimanite and Andalusite minerals found within the 1/3rd of a mile metamorphic aureole, said to be one of Britain’s best. Moine schist which was regionally metamorphosed during the Caledonian orogeny by deep burial has been heated and hardened further by the granite at the contact where it intruded. Small tight folds called ptygmatic folds, xenoliths of Moine schist and porphyroblasts within the schists can be seen. The change in landscape with the change in rock type should be noted. Continue on towards Bunessan and take the right turn towards Uisken and Ardalanish Bay. Here Moine schists and psammites are well exposed in coastal sections at both Uisken and Ardalanish. Uisken at G.R.396187 shows good Caledonian fold structures and also garnet grade facies in the predominantly biotote mica schist. As there is a small carpark at Ardalanish it is best to explore the geology here. (Any large group should first contact the owners of Ardalanish Farm for permission.) Here the originally clay sedimentary sequence contains sandy beds which have been metamorphosed to hard psammites (quartz rich meta-sediments or quartzites). Folding of Caledonian age and the structural trend which has produced the coastal scenery of narrow headlands and bays (better seen at Uisken) should be noted. Return to your vehicle and take the road into Bunessan and from here take the first road on the LHS outside the village towards Ardtun taking the first left towards Eorabus. Park on the grass at the end of the road and follow the signposted track to the Ardtun leaf beds at G.R.378248. Descend the steep gully which has fallen blocks at its base. (There are several gullies in the Ardtun headland and some are dangerously steep and deep.) In the Leaf Bed gully sedimentary beds deposited by rivers and lacustrine deposits can be seen outcropping between Tertiary lavas of the Staffa Suite. The interbedded leaf beds were deposited in times when Mull’s volcanoes were dormant and vegetation grew on the weathered lava flows. Carbonised fossil tree leaves of Platanus - Plane, Corylus - Hazel, Quercus - Oak, and Ginkgo - Maidenhair, have been found. It is only practical today to look for specimens by splitting open already quarried stone found at the base of the gully. Interbedded lacustrine deposits containing silicified chalk fragments from the Cretaceous, can be seen between the upper and lower lava flows. Further down the gully at sea level the Staffa Suite columnar basalt can be examined at close quarters. A basalt filled lava tube can be seen in the cliff on the eastern side whilst hexagonal/polygonal basalt columns are well shown in these cliffs and on the sea stack. N.B. Great care should be taken in this locality, especially in rough weather conditions.
 

Excursion Three - Gribun:
 

Park your vehicle off the road at Gribun and take the small track towards the island of Inch Kenneth. Here you will be at the Gribun foreshore G.R.446531. To the west of the track the Triassic sandstones, cornstones and conglomerates which form the lowest part of the Mesozoic portion of the upper basement rocks underlying Mull are well exposed. The Triassic rocks are about 200 MY old and represent a time when this portion of the earth’s crust lay in tropical latitudes similar to the Persian Gulf today. The polygonal, infilled mudcracks found in cornstones on part of today’s marine abrasion surface were once calcareous sediments forming under lagoonal conditions which periodically dried out. The conglomerates represent a shallow water phase at a time when Triassic seas were spreading. Walk on around the shore for about one mile to the cliffs below Balameanach Farm. Here at G.R.443333 a classic angular unconformity can be seen between the overlying Triassic conglomerate and the underlying Moine psammites which represent a buried landscape. The angular unconformity can be well seen by the small raised sea cave/arch which formed at that point due to a fault plane whose weakness was exploited (see photo P15). The dip of the underlying Moine psammites is seen to be at a much steeper angle and in a different direction from the overlying Triassic. Weathered bedding planes of the Triassic show large angular clasts. Cross bedding can be seen within the Moines. Joint sets on the Moines psammites are clearly seen. Return to your vehicle and drive to Balameanach Farm where you can park off the road. It is necessary to obtain permission from the owner of Balameanach Farm if you would like to see the next stage, the gorge section. If however you just intend visiting MacKinnon’s cave then the walk is signposted and no special permission is needed. Make your way from G.R.448330 towards the small gorge G.R.455327 at the entrance to Glen Seilisdeir. In the stream section closest to Balameanach the top of the Trias, the Rhaetic can be seen as a reddish brown limestone. No other clear exposures of the Mesozoic rocks can be seen until the gorge entrance as much is overlain by a landslip morainic feature thought to have been formed late in the 2nd glaciation to affect Mull. At the entrance to the gorge is a poor exposure of Cretaceous Greensand, followed further on and above by silicified chalk. The Cretaceous is represented by a thin sequence in Mull, but it is interesting to note that millet seed sand grains have been found in the silicified chalk suggesting possible proximity to an arid land mass at that time. Above the chalk exposure lies the first bed of the Tertiary, the Basal Red Mudstone. Rootlets have been discovered in the mudstone. Above this lies the first lava flow of the Tertiary volcanic series. A horizontal sill can be seen in the face of the gorge opposite. Return the way you came and follow signs to MacKinnon’s cave. In the high cliffs of the Ardmeanach peninsula transgressive (discordant) sills can be seen in these exposures at Gribun. Landslips, a postglacial feature of this area, can be seen below the steep cliffs. The cave lies at sea level some distance along the shore which can present difficulties as there are many huge boulders and a short cliff scramble to be negotiated. Arrange to reach the cave at low tide for although MacKinnon’s cave G.R.441323 is largely a raised sea cave with much of its great extent above sea level its entrance is tidal. The cave formed in the plane of weakness created by a fault. Small stalactites can be seen inside the aphotic zone. The outer part of the cave entrance has collapsed suggesting that the cave once much deeper. Block fall of sections of the cave roof along unsupported, well jointed beds, are possible hazards. It is vital to take a powerful torch if you intend exploring. A staff is handy too.
 

Excursion Four - Carsaig and Glen More:
 

Drive to Carsaig and park near the old jetty. Follow the track which leads west to Carsaig Bay. At G.R.534212 the Lower Jurassic (Lower, Middle and Upper Lias) are well exposed at Carsaig on the marine abrasion surface beyond the sands. Here, within the mudstones (with calcareous concretions) a limited and not very well preserved Liassic fauna can be found composed of Bivalves such as Ostrea and Gryphaea, Brachiopods, Belemnites, and Ammonites eg Uptonia. These sediments represent a time when Mull’s crust at this point was a region of sea some distance from land in tropical warm latitudes. Sandstones of the Middle Lias represent closer to land conditions. These exhibit unusually large calcareous concretions called doggers. Some sandstones contain poorly preserved fossil bivalves. Intruded into the faulted Jurassic sequence (and the overlying Cretaceous) are sills, cone sheets, and dykes. Beyond the stone wall a large transgressive sill overlies a shallow raised sea cave formed in sandstone which contains huge ovoid concretions called ‘doggers’. Below on the pebbly beach a large acid cone sheet with basic, veined margins and a ‘canoe’ shaped structure, possibly a xenolith, can be seen. If you continue further west a large stone run, recently augmented by further falls, contains good specimens of Mesozoic sediments and Tertiary  red bole, tachylite, pepperite, basalt, vesicular basalt, amygdaloidal basalt, and chalcedonic quartz filled vugs within some basaltic specimens. Your walk can be extended in a westerly direction to visit the Nun’s Cave and much further on, Carsaig Arches (see photo P31).  From Carsaig drive to Kinloch and park near the Kinloch Hotel G.R.521267. From here across Loch Beg a huge fold structure in the Tertiary lavas can be seen. Originally horizontally bedded lavas were bodily shifted aside by intrusions of the  Central Igneous Complex. From Kinloch drive to Craig Quarry G.R.582295 in Glen More. (This quarry is disused.) Exposures of gabbro cut by dolerite represents part of the once vast magma chamber containing basic magma which supplied many of the areas gabbroic intrusions. Megacrysts (huge crystals) which sank to the bottom of the magma chamber forming coarse grained pegmatites can be found in the lowest level of the quarry, whilst coarse grained gabbro can be found above. Late stage injections of basic magma into veins etc produced dolerites. (All these rocks have fundamentally similar chemistries, though different cooling histories.) From Craig quarry continue on through Glen More noting glacial morainic features of hummocky moraine and lateral moraine, dropped and partly moulded by stagnating ice during the Loch Lomond retreat 10,000 years ago.  Various features of glacial erosion can also be seen including the three lochs which can be viewed from the passing place at G.R.620304. Drive on to the road bridge over the Allt Mollach at Ischriff G.R.630312. Here is a Composite dyke which is very well exposed in a stream section. An original basic dyke about two feet wide which cut the gabbroic country rock was later intruded by acid magma producing an acid felsitic centre of about 18 feet. Good jointing at right angles to the cooling surfaces clearly seen.  Further up the stream section three dykes can be seen cutting the country rock before the old bridge. Further on upstream there are three more. The composite dyke can be seen as a distinct feature in the hill far behind. Continue along the A849 towards Loch Spelve where at G.R.707311 a small dolerite plug is made from flow aligned ‘big feldspar’ dolerite.