OPEN UNIVERSITY GEOLOGICAL SOCIETY

WEST OF BRANCH

Northwest Highlands Field Trip

11th May to 18th May 2013

LED BY

DR IAIN ALLISON, UNIVERSITY OF GLASGOW

Achmelvich Beach M Donnelly

WRITTEN BY MAGGIE DONNELLY, ANNE MORTON, CHRISTINE HODGSON, LEXY CRUICKSHANK, JIM MARTIN, JOYCE RAMMELL

Compiled by M Donnelly

Tight folds in “Iain’s surprise quarry’” Cul Mor

I would like to thank our Leader, Dr Iain Allison, very much, for sharing with us his knowledge and expertise during the course of this week. Without him this trip would not have been possible. I would also like to thank him for the use of his Excursion Notes in the compilation of this booklet. Maggie Donnelly November 2013 .

The field trips were based approximately on the new ‘A Geological Excursion Guide to the ’, 2011, eds. Kathryn Goodenough, Maarten Krabbendam, published by Geological Society.

Day 1 – Excursion 1 Day 2 – Excursion 2 Day 3 – Excursion 10 Day 4 – Excursions 16 & 7 Day 5 – Excursion 6 Day 6 – Excursion 9

Looking towards , snow on the hills…………

Laxford Bridge

Loch Glencoul

4 Achmelvich

Loch Assynt

2 1

Traligill

Cam Loch 3 6 5 6 Oykel Bridge

Ullapool

UK North Sheet, BGS

Day 1 and the Achmore Duplex

Day 2 Shear Zone at Achmelvich

Day 3 Cam Loch, the Loch Borralan Pluton and the Loch Urigill carbonatite.

Day 4 Glencoul Thrust, Bridge and Traligill Valley

Day 5 Knockan Crag

Day 6 Glen Oykel & Oykel Bridge

Some Key Events…………..

Period Ma

After Dr Iain Allison

Achmelvich Canisp shear zone

Northwest Highlands Field Trip

11th May to 18th May 2013

Our trip was planned as an opportunitiy to visit localities, both familiar and unfamiliar, that are described in the new ‘Excursion Guide to the Northwest Highlands’. Our party of sixteen was accommodated in the very comfortable Inchnadamph Lodge, refurbished since our last stay, near the shores of Loch Assynt. Most of the group were from Scotland, but happily we were joined by four ‘further travelled’ from ‘South of the Border’ and abroad. The rocks we would see comprised Late Archaean Lewisian gneisses, reworked during the Laxfordian event ca. 1.7 Ga, the Group ( Formation) ca. 1000 Ma, the Cambrian/Ordovician of the Caledonian Foreland and Moine Thrust Zone, and Moine metasediments. Our weather was mixed, but fortunately improved as the week went on. So……….on Sat 11th May we gathered in late afternoon, and after dinner our leader gave us an introduction to the geology for the week. We then retired for a ‘sociable refreshment’.

Sun 12th May 2013

Loch Assynt and the Achmore Duplex. Margaret Donnelly

On a bright but rather cloudy morning we headed northwest for the shores of Loch Assynt beyond Skiag Bridge. Parking in a layby (NC 212 251), we set up a car shuttle, and were told to ‘go and look at the rocks’; so we made our way down the heathery slope towards the loch and came upon cross-bedded crimson sandstones, very fine to medium grained. Unfortunately this was not what our leader wanted us to find…………..he returned and led us down to the shore where there were Lewisian orthogneisses, intruded by a large black undeformed ultramafic Scourie dyke, in places 9 m wide, and trending ESE. The gneiss was composed mainly of pyroxene, quartz and feldspar, with westward dipping foliation, and the dyke was an olivine-rich metaclinopyroxene-norite. We climbed back up to the road, and examined the (crimson) sandstone rocks in the road cut. They were composed of (a) a poorly sorted, tabular-bedded very coarse material with clasts of gneiss and vein quartz supported in a muddy matrix, and (b) a better sorted coarse sand to granule material with matrix-supported pebbles, some with a desert varnish. A number of the beds had erosive bases, there was some parallel lamination, the entire matrix was arkosic, and there were millet seed sand grains and dreikanters. Our leader explained that these deposits had been formed by alluvial fans descending into lacustrine The ‘Double Unconformity’ M Donnelly environments, and that this was the Formation, lowest of the , laid down on top of a Lewisian palaeolandscape about 1000 Ma. At this particular location, the unconformity was just out of sight beneath the road but as we walked eastwards it rose up before us (NC 213 251) – reddish conglomerates and pebbly sandstones of the Diabaig Formation lying on top of the highly weathered cream to pale green Lewisian gneisses. Looking across Loch Assynt to the slope of Beinn Gharbh, we could see Torridon rocks on top of Lewisian, and Cambrian Quartz Arenite on top of both, overlapping the first unconformity. This is the ‘double unconformity’ – a line in space perpendicular to the surface of the hill. Further along the road (NC 218 250), the considerable topography of the unconformity was revealed as the Diabaig sandstones infilled huge troughs in the Lewisian palaeolandscape of ‘cnoc and lochan’. We walked on to locality NC 219 247, where massive trough cross-bedded sandstones of the Applecross Formation lay conformably over thinly bedded, laminated granule stones of Diabaig. A little further on, at NC 223 249, well developed glacial striae scored the Applecross which was smoothed and shaped by ice, forming roche moutonnees. After a small ‘debate’, it was decided that the ice had come from the WSW. At locality NC 225 248, the Applecross Formation was set back from the road, and composed of very coarse sand to Massive cross-bedding M Donnelly granule clasts, angular to sub-angular with little silt or clay. It had an abundance of terracotta-coloured feldspar, lithic clasts and vein quartz – an arkosic sandstone – and trough cross-bedding throughout. We climbed a little way uphill to find the top surface of some of the cross-beds and in this 3D view the direction of the palaeocurrent could be observed as towards 100⁰. These rocks were created by deep perennial braided rivers, forming a very large-scale braidplain, and their source was the WNW. The slope of the hill, which was now all Applecross Formation, was broken up into ‘levels’ and ‘rises’ – the ‘rises’ formed the channels of the rivers and the ‘levels’ were eroded mudstones (old channels) where the rivers had changed directions and conditions had become quiet. We walked on towards Skiag Bridge and came to the Cambrian Quartz Arenite (NC 229 247) – this is not quartzite, which is metamorphic, but a sedimentary rock of arkosic sandstone or quartz arenite. The initial outcrops are of light grey weathering, very coarse sandstones and granulestones, full of clasts of quartz grains, pink feldspars, and small scale cross-bedding – these are sub-arkoses. We had a clear view of the Basal Cambrian unconformity (which is not actually seen on the road) extending all the way up the hill, , above – white well-bedded quartz arenites of the Eriboll Formation, dipping east at 12-15⁰, lay over sub-horizontal Applecross Formation with an angular unconformity. As we progressed along the road the pale sandstone generally became finer, with less feldspar, and it was now a true quartz arenite, with planar tabular cross-bedding, bipolar current directions and herring bone cross-bedding (NC 231 245). There were numerous joint faces, and some slickensides indicating a fault. Then pipes and pimples started to appear in the rock………. a cold wind sprang up and clouds started to gather. We continued to Skiag Bridge and then 20 m up the road (NC 235 244) where there are classic outcrops of Pipe Rock. This has large scale planar tabular cross- bedding and is stained red by diagenetic iron oxide, but the pipes stand out as white tubes about 1 cm wide and tens of cms long. They are known as Skolithus and are thought to be the trace fossil of a filter feeder organism. The actual animal has never been found even though similar pipes also occur in Mesozoic rocks. Altogether there are three types of this trace fossil – Skolithus (a single pipe), Monocriterion (a pipe with a funnel), Diplocriterion (a U-shaped tube). Returning to the lochside road, we walked east to where we found the Pipe Rock conformably overlain by orange-brown Fucoid Beds of the An t-Sron Formation (NC 236 242). These have numerous black burrows on the bedding surfaces, and were originally thought to contain fossil seaweeds ………. hence their name. They contain both dolomitic wavy bedded siltstones with the trace fossils Skolithus, Planolites, Cruziana and Rusophycus, and cross- bedded dolomitic grainstones with echinoderm fragments, the latter interpreted as storm events. This is the earliest unit in the Cambrian to contain body fossils – trilobites of the Bonnia-Olenellus Biozone which is of late Early to Middle Cambrian. As we progressed we came to the Salterella Grit, conformably overlying the Fucoid Beds. These are cross-bedded quartz arenites and contain Skolithus as well as the body fossil Salterella, a cone-shaped shell only a few mms long. The organism is unknown but may have been a primitive mollusc. One notable ‘fossil-finder’ in our company found lots of good examples. Finally we came to the grey dolostones of the Ghrudaigh Formation conformably overlying the Salterella Grit, before making our way back to the cars in the layby on the Kylesku road for lunch……..it started to rain heavily! The rain eased……….we walked up the road and through a gate, to a ford over a small stream, and climbed the grassy slope looking for Fucoid Beds, Salterella Grit and the Ghrudaigh Member, in the streambed. On a number of occasions we found two or more of these in stratigraphical order, and then repeated, indicating a thrust, and in fact these are the imbricate slices of the Achmore Duplex – a large scale structure with about 150m between floor thrust and roof thrust, but with very thin individual ‘horses’. The Sole Thrust lies below the ford and the Duplex is developed beneath the Glencoul Thrust sheet. The hill flattened out on a level of rich green grass – a sure sign of the Ghrudaigh dolostone – before rising again, and the stream was seen to emerge from the ground. On the hillside above, the streambed is dry because of the underlying karst drainage, and the horses of the higher slopes are formed entirely of carbonates of the Group. The weather had improved, and we walked back down and then across the heathery moor to large exposed planes of quartz arenite and pipe rock – one area had abundant cross-beds but adjacent and above was a huge section with no cross-bedding. There was a suggestion that this bed may have been shaken up by an earthquake in the rifting basin, and so had lost its structure And then the rain came on heavily again…………we returned quickly to the cars, soaked, and to the hostel.

Monday 13th May Canisp Shear Zone at Achmelvich Anne Morton

The purpose of this day’s excursion was to look at the Canisp Shear Zone, starting in the south at Achmelvich Beach. The day before had ended with hail and wintry showers, and we woke to find a good dusting of snow on the hill tops. On our way to Achmelvich, we stopped on the road to Stoer, where we had a panoramic view over the exhumed Lewisian landscape that had been inundated by sand from the north-west. These Torridonian sediments were subsequently deeply eroded as a dissected peneplain, leaving the spectacular inselbergs of , Canisp, Cul Mor, Cul Beg of the Applecross Formation. Canisp and Cul Mor are also topped by the Cambrian quartzite (properly called quartz arenite). The resulting cnoc-and- lochan topography represents a Lewisian palaeolandscape. After taking group photos against this glorious backdrop (unfortunately into the sun), we got back in our cars and made our way to a small quarry. From here we scrambled across the Scourian gneisses towards Achmelvich Beach, stopping en route to examine the dolerite dyke intrusion, containing iron and titanium oxides, plagioclase and amphibole. This dyke is a time marker dividing the Scourian complex (relatively undeformed, 2400 Ma old, with a cross- cutting relationship to the country rock) and the Laxfordian (deformed) in which structures and intrusions appear parallel to the grain of the country rock. On Achmelvich Beach we found beautiful examples of Badcallian gneiss showing preservation of schlieren textures (differing from the main mass of the rock) containing lenses of elongated amphibolite with flat-lying foliation, up to 1 m across. This lies within the central part of the Monocline, and steepens up into the Canisp shear zone, which deformed Scourian gneisses during the Inverian event. The Scourian gneisses seen at Achmelvich are coarse- grained feldspathic gneisses containing sheared, elongated ‘augen’ of pure amphibole rock

Group photo on the road to Stoer A Morton

originally 2,900 Ma old – migmatitic gneisses of granulite facies metamorphism. Heading north into the shear zone we clambered up the rocks above Achmelvich Beach to find one of the Scourie dykes, a medium-grained metamorphosed dolerite in which the pyroxenes were altered to amphiboles and the feldspars, originally Labradorite ((Ca, Na)(Al, Si)4O8), an intermediate to calcic member of the plagioclase series), had been altered to predominantly sodic plagioclases. As a result of later Laxfordian shearing, parts of the dyke margins had retrogressed, producing greenschist facies biotite-muscovite schists and hornblende-biotite schists. Our next stop followed a precipitous scramble down steeply dipping rocks presenting knife edges with a deep sea inlet on one side and a sheer drop on the other, followed by a game of dodge with the incoming tide. Here we could see clearly how the foliation (almost perpendicular here) had steepened into the Inverian portion of the shear zone, with SE- Achmelvich Beach A Morton plunging lineation and isoclinal folds. We were looking for signs of the displacement direction, such as the lineation on foliation surfaces, and mineral alignment within the plane of foliation. On our steep climb out of this bay, heading into the main shear zone, the fabric became much more intense, with strong schistosity visible from the ends of rock slabs, and stretching lineation visible along the sides. Iain Allison remarked that in thin section we would be looking for indicators of the sense of shear – somebody suggested flow around phenocrysts or clasts. We were looking at 2,500 Ma old shearing with later Laxfordian (1700 Ma) faults. Further scrambling led us to a cleft in the cliffs, which marked the faulted southern margin of the Laxfordian Canisp Shear Zone. As we negotiated the narrow and precipitous cliff path, involving a bit of scrambling, we had a bird’s eye view of almost perpendicular knife- edges of rock boiling with surf below us. Looking back southwards we could see the Lewisian cnoc-and-lochan terrain with the Torridonian peaks of Suilven and Cul Mor in the distance. After a short stop where some people peeled off back to the cars, we followed the footpath around to a small headland to look at the unfaulted margin of the Laxfordian Canisp shear zone. Here the textures differed between the outer and inner parts of the headland, the outer part being composed of little-deformed felsic gneisses, while the inner part was mainly schistose mylonite with shallowly plunging mineral stretching lineations derived from felsic gneiss with concordant quartz veins. Here, on close examination, the amphiboles were quite large and there was some evidence of cross-cutting foliation. Battling with the wind, with some difficulty we skirted the cliffs and dropped down steeply to cross a wide gully before climbing onto a promontory where we were looking out for extreme examples of folding – refolded folds and sheath folds – apparently with nests of complex Laxfordian folds at centimetre and metre scales. Because of the difficulty of negotiating the slippery rocks in the teeth of a buffeting wind, we were not entirely successful in this, but we did find some indications before retreating to seek a late lunch, reaching the cars just ahead of a heavy shower of rain. Unfortunately the rain soon settled in, driving us into our cars and putting an end to the afternoon’s activities for a large number of the group.

Tuesday 14th May Cam Loch, Loch Borralan Pluton and the Loch Urigill Carbonatite. Christine Hodgson

We were well prepared for the hail and sleet as we crossed the boggy ground around the side of the Cam Loch. The weather cleared a little, we could see the white horses on the water and the quartzite exposure on the easternmost shore. Iain told us that this was one of the Lower Cambrian Ardvreck Group rocks for us to identify. We looked for clues in this quartz arenite which contained a few rounded clasts and small pink pegmatite veins. The absence of cross bedding and thin siltstone banding ruled out the Basal Quartzite Member and the Salterella Grit Member respectively, whilst the Fucoid Beds Member contains dolomitic shales. This left the Pipe Rock as shown on the map, although in our short search we had not found the distinctive surfaces as seen at Skiag Bridge. However burrows are not present throughout the entire Member. On the shore we crunched on syenite clasts. The Lewisian Iain and Cam Loch C Hodgson gneisses were exposed on the higher shoreline path and by a kilometre north-east we were above the Cam Loch Thrust which, as Iain explained, dips eastward fairly steeply here. We had walked over the unexposed Cam Loch Thrust which carries the gneiss over the quartzite and climbed towards the cliffs forming the eastern side of Cnoc an Leathaid Bhig. The Cam Loch Thrust runs at the base of these cliffs, carrying Lewisian gneisses over Durness Group dolostones in the Sole Thrust Sheet, but the thrust itself is not exposed here. Scrambling on up to a rowan tree we reached the rocks above the thrust; these belong to the Basal Quartzite Member and dip beneath the Lewisian gneisses. The explanation is that the erosion of a thrust sheet lying above a shallowly dipping fault can leave an isolated cliff-bounded allochthonous region – a klippe. In this case the rocks of the thrust sheet here are inverted (folded) by a major anticline giving the observed juxtaposition of beds. Further mountaineering reached the steeply dipping unconformity, as shown in the guide. We were now daunted by the weather and the terrain, so retraced our steps back to the cars. A smaller party, optimistic for less rain or just nerdy, set out after lunch to investigate outcrops of the Loch Borralan Pluton. This is the only alkaline pluton in the that includes nepheline syenites, and it is associated with a small body of carbonatite (how could anyone bear to miss that?) We parked by the Ledmore river and about a kilometre into the forestry plantation entered a small quarry, much of it cut in drift. Large angular fragments of various types of nepheline-syenite, probably broken material from road or quarry construction and not erratic blocks, were examined. Most blocks were grey, medium grained, meso- to leuco- cratic nepheline-syenite (alkali feldspars 1-5mm) with pink variants as veins cutting through. Mafic inclusions with sharp margins, and also some foliated or layered, were present. Other rock at the back of the quarry was jointed and fine-grained; this is similar to ‘ledmorites’ which make up a major part of the early unsaturated suite of the Loch Borralan Pluton. Leaving the forestry track we walked down the bank of the stream and looked at various well weathered and somewhat overgrown outcrops. The Nature Conservancy Council opened up a series of excavations here in the 1980s to investigate the relationships of the igneous rocks and the Cam Loch Klippe. We saw pyroxenite cut by syenite veins and further west a sheet of zoned pegmatite with striking euhedral, dark grey feldspars in the host rock of fine grained, more mafic, pink nepheline- syenite. It had even stopped raining.

Ledmore exposures

C Hodgson

Pegmatitic syenite in the Bad na h’Achlaise excavation - grey feldspars C Hodgson

Continuing west an important exposure shows unequivocally that igneous rocks of the Loch Borralan Pluton were intruded into the Basal Quartzite Member, considered to be part of the Cam Loch Klippe. Iain mentioned that local Ledmore Marble Quarry exposures show that the pluton also cuts across the Cam Loch Thrust. The pyroxenites are intrusive rocks, not metasomatic skarn rocks at the contact of intrusion with dolomite. The pyroxenite seen at the extreme west of the exposure is very soft, but has clearly been injected into quartzite. The quartzite is fenitised, with rosettes of pale blue amphibole. Some pink syenite veins formed a network in the quartzite, within which is a 10 cm vug lined with euhedral quartz.

NCC long excavated terrace C Hodgson

Returning to the forest road we went to a small bay on the shores of Loch Urigill. In an excavated hollow about 30m across there were several carbonatite outcrops – dark grey on weathered surfaces but the fresh surfaces were white, crystalline and course grained.

Blue rosettes and pink syenite veins C Hodgson

These are sovites (calcite-carbonatites), with xenoliths of nepheline-syenite and pyroxenite from the Loch Borralan intrusion.

Vug lined with euhedral quartz C Hodgson

Carbonatite exposures C Hodgson

Also seen in the area were rocks – phlogopite sovites which have an orange weathered colour due to the presence of plates of phlogopite within the calcite matrix of the rock. The last photos may show layering, brought about by the mineral chondrodite. Brecciated carbonatites are also present. The carbonatite was actually intruded into dolostones of the Durness Group, and is not an integral part of the Loch Borralan Pluton, but is considered to be associated with it. However the Loch Urigill carbonatite has not been fully reinvestigated since the area was described in 1994 and some geologists have other views on this only known example of this rock-type in the British Isles. The Allt a’Mhuillin Quarry which complements work on the early suite of the Loch Borralan Pluton is visited later in the week. It was another great Assynt day with Iain – thanks - well worth braving the wet!

Geological map for Excursion 10 available – showing the carbonatite site well etc

Wednesday 15th May Glencoul Thrust, Laxford Bridge and Traligill Valley Lexy Cruickshank

The morning started wet, so Iain decided to change the plan and do the Traligill valley in the afternoon and give us a “surprise” in the morning. Thus we spent the morning visiting two roadside geology sites. The first was Loch Glencoul on the A894, [NC 236 321]. We had a fantastic view of the Glencoul Thrust (the sun came out!) from the car park, which had an excellent North West Highlands Geopark information board explaining what we were seeing. This is an easterly-dipping thrust which emplaces ~3000 Ma Lewisian Gneiss Complex above Cambrian quartz arenite of the Eriboll Formation, which in turn sits unconformably on Lewisian Gneiss - a quartz arenite sandwich as it were. Heading north again, we drove 2.5 km past Laxford Bridge, into the Rhiconich Terrane to Loch na Fiacaill, [NC 232 486]. The road cutting here exposes a colourful candy- stripped combination of grey quartzo-feldspathic Lewisian Gneiss, black amphibolite and pink granitic pegmatite. The protolith is dated at 2840-2680 Ma, with the oldest mineral in UK coming from this site. The Lewisian Gneiss was cross cut by mafic and ultramafic Scourie dykes spanning 2400-2000 Ma. The granitic sheets intruded c.1855 Ma. The Laxford deformation c.1750 Ma reworked the dykes to amphibolite facies and aligned them with the Lewisian Gneiss. Loch na Fiacaill road cutting with Lewisian Gneiss, amphibolite and granitic pegmatite L Cruickshank

Back to the hostel for a quick lunch, after which we headed up the River Traligill to the east of the hostel to have a closer look at dolostones, [NC2671 2189 to NC 2671 2140]. The light grey Eilean Dubh Formation shows marked karstic weathering with characteristic clints (ridges) and grykes (clefts). Climbing further to the Glenbain Cottages we see the older darker grey Ghrudaidh Formation with the beds dipping steeply NW-SE. More climbing past the fir plantation when the rain starts again (no surprise there) [NC2673 2112], brought us back to the Eilean Dubh Formation – a series of imbricated thrusts, with Eilean Dubh Formation over Ghrudaidh Formation, which can also be seen on the cliffs on the south side of fast flowing swollen River Traligill (which is usually dry!). Just before a small footbridge, we turn right to the Lower Traligill Cave [NC2706 2089]. The cave is formed in the Ghrudaidh Formation (hanging wall), while the floor of the cave is Eilean Dubh Formation, again older over younger so another thrust. The cave is a major sink but today is resurgence. The bed of the River Traligill is the Traligill Thrust. Further up the hill we find another cave – this one dry. At the back is a small hole and the majority of the group are able to scramble inside and stand up. Like the previous cave this has developed in the Ghrudaidh Formation above the thrust plain. In this area reddish peralkaline rhyolite intrusions can be found.

Dry cave on the way to cave at base Cnoc nan Uamh

L Cruickshank

We headed up Cnoc nan Uamh, which is a klippe, floored by the Cnoc nan Uamh Thrust. Not far from the top of the hill we find an outcrop of Salterella Grit Member, above which is Fucoid Bed Member with Pipe Rock Member above that – another set of imbricated thrusts above the imbricated thrusts of the dolostones. The complicated thrust system of central Assynt could be seen from the top of Cnoc nan Uamh [NC2778 2048]. After a quick descent, the majority of the group opted for an early finish but a few resilient types decided to see the Bone Caves. We drove south and parked [NC 253 179]. The caves were ~ 2 km up the steep-sided valley of Allt nan Uamh. We passed numerous sources with water upwelling along the thrust planes. We noticed some of the steps on the way up were Pipe Rock Member, which would have been brought down the valley from Beinn nan Cnaimhseag above. These dolostone caves have yielded a mass of animal and human bones. The animals include bear, northern lynx (1770 yrs old), polar bear (18,855 yrs old) and reindeer (47000 yrs old). More recent remains from about 4500 yrs ago include red and roe deer, rabbits, badgers, grouse, pine martens suggesting a well-wooded habitat, very different from the bleak eroded valley we see today. Human bones were radiocarbon dated to 4515-4720 years old. Reindeer artefacts were also found (dated 850 yrs). There are four caves in the shadow of Creag nan Uamh, but we only visited two. Both were dry and well above the valley floor now, but when formed over 200,000 years ago would have been level with Allt nan Uamh. Glaciers have eroded the glen to leave the caves high and dry. There was a small tunnel leading down and back into the cave. Two brave members went down to explore. After a good look around they returned and we had a quick look at another cave then back down the valley and home for tea.

Looking from the Bone Caves across the valley of Allt nan Uamh L Cruickshank

Thursday 16th May. Knockan Crag Jim Martin

The weather sunny, best day of the trip so far and with the prospect of the forthcoming festivities at Knockan Crag and the evening lecture in to look forward to we set off in anticipation of an interesting and entertaining day ahead. We were not to be disappointed. Our first stop was just one kilometre south of Inchnadamph where a large lay-by provides good views of the Sronchrubie Cliffs. The lower layers of Pipe Rock, unexposed Fucoid Beds and thin crag of Salterella Grit appeared horizontal. However, at the top of the cliffs the dolostone beds were tilted at various angles immediately above the Sole Thrust. In the distance we could see the Achmore Duplex which we walked over the previous Sunday. Driving south, approximately along the Sole Thrust, the next stop was at a small roadside quarry opposite Loch Awe. The Loch Awe quarry was opened to produce crushed rock, high in potash, from the top of the Fucoid Bed horizon, and was intended for use as a slow release improver for agricultural land. Unfortunately the venture was not commercially successful. As a result of much searching during our visit to the quarry, two trilobite fossils were found. The dip of the quarry beds was the opposite to that across the glen, the regional eastern dip of about 15º, the explanation being the presence of a blind thrust a short distance below our feet. Our next stop was in Elphin, at a large lay-by beside a low roadside cliff, where we were asked to suggest a likely scenario for what we were seeing. The explanation was that the exposure was most likely a combination of thrust on the left side and a low-angle normal fault on the right, as viewed from the car park opposite. Leaving the hamlet of Elphin we set off on foot over a combination of footpath and tufted grass with the intention of arriving at Knockan Crag in time for the ceremony to celebrate the refurbishment of the visitor centre. En route we travelled over the Durness Limestone landscape typified by sink holes, caves (such as that at Uamh an Tartair) and disappearing and re-emerging streams. With so much rain earlier in the week the “Roaring Cave” was indeed roaring. From the cave we walked almost due west along the line of the Moine Thrust mainly over the Durness carbonates. To our right was Cnoc a’ Chollich Mhor, a Klippe formed by quartz arenites of the Eriboll formation. The carbonates themselves reflect variations in dip due to imbrications by the Elphin Lunch break on the Moine thrust plane on the way to Imbricate Stack. As we progressed Knockan Crag Jim Martin from east to west along the stream south of the Druim Poll Eoghainn we were walking along the Moine Thrust and stopped for lunch on the dip slope of the carbonates with a small cliff in Moine mylonites on the far side of the stream. As planned (some of us making it in time to get the pre-presentation nibbles) we arrived at the Knockan Crag just prior to the official re-opening of the refurbished visitor centre. There were speeches by officials of Scottish Natural Heritage and Angus Miller, chair of the Scottish Geodiversity Forum as well as the guest of honour, Professor Iain Stewart. The theme of the speeches was the importance of Scotland’s natural heritage in terms of attracting tourists and in particular the needs to both maintain and promote Scotland’s two remaining Geoparks. There then followed a demonstration of Cadell’s mountain building experiment by Graham Leslie, a member of the British Geological Survey with one of our number, Peter, lending a hand. To round off the afternoon Iain led us round the Crag Top Trail which gave us the opportunity to come “face to face” with the Moine thrust as well as examine the exhibits which had been recently refurbished and very well presented. The highlights of this circuit were: the Moine Thrust contact between the Durness Limestone and the Moine Rocks; Mylonite at Eagle Rock; the “hands on” demonstration of thrusting and of course the views. The programme continued with attendance at “An Evening with Iain Stewart” held in the Macphail Theatre at Ullapool High School which was very interesting and well attended. Iain gave us a preview of the new series he has been working on supplemented by amusing anecdotes. Our drive back to Inchnadamph was enhanced by a marvellous west sunset.

Knockan Crag exhibit showing, in simplified form, the stratigraphic layers sequence involved in the Moine Thrust: i.e. Lewisian Gneiss Complex, Torridonian Group, Basal Quartzite, Pipe Rock, Fucoid Beds, Salterella Grit, Ghrudaidh Formation Eilean Dubh Formation and the Moine schist.

Jim Martin

Friday 17th May Glen Oykel & Oykel Bridge Joyce Rammell

This was the postponed Excursion 9 to Glen Oykel, possible as the river level had now subsided. The weather was clear and the sun promised to shine and in fact proved to be the best weather of the week. We drove to Ben More Lodge to park and walked along the track beside the , stopping to look at the odd outcrop. Our first stop was at a small waterfall in the river. We all scrambled onto the exposed rocks to examine some good exposures of what appeared to be very mixed types of rock. These turned out to be coarse- grained pyroxene-rich syenites of the S3 phase of the Loch Ailsh pluton with veins of pinkish leucosyenites. The syenites contained angular xenoliths of dark green pyroxenite while the veins contained inclusions of the syenites. There was also evidence of deformation all over the exposure making for a number of different hypotheses. The height of the river meant we had to miss part of the planned excursion but followed the geological map upriver to find the fine-grained red rhyolite dyke which cuts across the coarse- grained syenites in the river bed. We searched for finer grains at the edge of the pluton, marking a margin but were Fold in the ‘surprise’ quarry Joyce Rammell not convinced. However, we did note the fracturing of the syenite against the dyke. It was suggested that as the pluton dates from around 430 Ma the dykes and sills intruded in the area are obviously younger but may be of a similar age. We lunched here in almost spring like weather. After returning to the cars, we drove towards the main road but stopped off in a quarry which Iain had found during a recce visit. In the quarry which had obviously been used for road stone we could see a huge fold in mylonites. We supposed it might have been folded during the emplacement of the pluton and proceeded to examine the broken debris, finding superb examples of small folds in the finely laminated layers. The company could probably have spent longer poring over the debris pile, especially when Hammy claimed the first fossil. Moving on, we drove to Oykel Bridge to have an overview of the Moine psammites in the river bed. Before metamorphism, these were sandstones and we could see some planar laminations on the downstream side and what appeared to be cross bedding on the upstream side of the bridge. The rocks formed reclined tight to open folds with amazing mullion structures – this is one of the ‘classic’ sites where this type of lineation can be seen. On our last night before heading off home, we enjoyed a Amazing mullion structures M Donnelly communal meal prepared by a few and cooked superbly by Pete Hodgkinson. Iain was given the usual vote of sincere thanks for leading the trip, his wonderful hand-drawn handout with riddles and passing on a little of his consummate knowledge of the North West Highlands.

Sunset on the road from Ullapool to Inchnadamph M Donnelly

Photos not credited – taken by M Donnelly