Geological
Society
Excursions in 2002
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Week Excursion to the Isle of Arran
As a microcosm of Scottish geology, Arran proved to be an excellent choice for this week-long excursion (18 to 25 May 2002). From our base in self-catering houses near Machrie on the west side of the island we spent six days in the field looking at examples of almost all of Arran’s very varied geology. A group of leaders from the party each contributed their own expertise. Dr Henry Emeleus and Professor Brian Upton led on the igneous geology of the Arran’s granite bodies and associated dykes and sills. Dr John Roberts taught us the principles of examining sedimentary geology, with particular reference to the Dalradian metasediments and their structure. Dr Graham Smith took us over the baryte mineralisation associated with the contact of the Northern Granite body with the Lower Old Red Sandstone along the line of the supposed Highland Boundary Fault in Arran. And Dr Jason Hilton, a palaeobotanist, was expert at interpreting sedimentary sequences in the Old Red Sandstone, the Carboniferous and the Permian.
Starting on the Carboniferous sediments and lavas at Corrie on the east coast, we got our eye in for sedimentary structures and the deltaic and fluvial sedimentary environments in which alternating limestones, sandstones and shales were deposited. The Carboniferous succession on Arran correlates with that in the Midland Valley of Scotland, although the Arran sequence is very much thinner. From the north end of this section at Sannox Bay, a pleasant walk up Glen Sannox took us to the old baryte workings where in one or two places we saw the original mineral veins in the bedrock. The abandoned site is much over-grown, but old spoil heaps provided ample mineral specimens.
Our next destination was the south coast, for a day on the dykes and sills which intrude the Triassic and Permian of the southern part of Arran. At Kildonan, a swarm of Tertiary crinanite and dolerite dykes is exposed along the foreshore, and there is easy access to a felsite sill above the village. Working our way northwards up the east coast road, we looked at the crinanite Dippin Sill, with an underlying quartz dolerite sill, at dykes showing mixed dolerite and crinanite magmas at Kingscross Point, and at the crinanite Claughlands Sill. This was a good day for confirming one’s identification of different igneous rocks and for recognising multiple phases of intrusion in both dykes and sills. However, wet and windy weather dissuaded us from doing the coastal walk from Claughlands Point back to Brodick to complete our intended itinerary.
Later in the week, a bracing day’s walk northwards along the coast from Blackwaterfoot took us along the base of the great Drumadoon felsite sill on the way to the King’s Caves (figure 1), cut by postglacial seas into Permian sandstone cliffs. We were lucky to find the gate at the mouth of the main cave open, and were able to go in to see the early Christian wall-carvings. Working our way north along the shore we looked at the series of composite dykes known as Judd’s Dykes. Another “igneous” day took us to the contact of the Northern Granite with Dalradian schists above Catacol in the northwest of the island (figure 2), a rewarding scramble up several hundred feet of heather and rock to find the exposed contacts.
Our most strenuous day was the long tramp of about 11 km around the north coast from Lochranza (figure 3) to Laggan Cottage and back over the hill track to Lochranza. With a fine day we had stunning views across the Sound of Bute to the mouth of Loch Fyne. The coastal path leads to “Hutton’s unconformity” where Lower Carboniferous sandstone overlies Dalradian schists, then into Permian conglomerates and aeolian sands, and finally through a Carboniferous succession of calcareous sands with limestone horizons, and a coal seam evidenced by remnants of former coal workings. A magnificent boulder field of Permian conglomerate - huge blocks up to 5 metres across fallen from the cliff-top - makes the going very difficult along some of this route. It was a testing day for all of us.
Although rain and gales made this the wettest May week we have ever experienced,
we planned things to make use of shelter wherever possible (though hardened
geologists in the party poured scorn on the cowardice of the rest!).
As a prime tourist destination, Arran provides comfortable hotel bars
for lunch stops in many of its villages, while a small brewery, a distillery,
a folk museum, a cheese shop, a sweet factory and many craft outlets offer
dry havens for shopping and guided tours.
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Figure 1. A break for lunch at the King's Cave, Permian sandstones. |
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Figure 2. Examining the contact between the Northern Granite and the Dalradian schists above Catacol. |
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Figure 3. Examining folding in Dalradian schists at Lochranza. |
Weekend Excursion to the Cromarty Area
A party of 24 members, spouses and guests attended a successful weekend excursion to Cromarty on 28-30 June 2002, following a suggestion by Dr Mike Taylor (National Museums of Scotland) for an excursion to celebrate the bicentenary of the birth of Hugh Miller (the celebrated stonemason, geologist and newspaper editor). Members were accommodated in the Cromarty Centre - a field study and conference centre of the Robert Gordon University of Aberdeen. The weather, as throughout much of 2002, was mixed, but fortunately the heaviest rain and drizzle occurred en-route to, or between, localities and did not impede the programme.
The scientific leader for the excursion was Sinclair Ross (President of the Highland Geological Society and EGS member). Dr Lyall Anderson (National Museums of Scotland) contributed the palaeontological expertise and also narrated extracts from Hugh Miller’s writings at the appropriate locality (the “Siccar Point tradition”). Mike Taylor and Christine Thompson (both National Museums of Scotland) provided the historical background on Miller.
On Friday evening, three talks were delivered by the leaders in the well-equipped
lecture theatre of the Centre. On Saturday, the Moinian, Devonian
and Jurassic rocks of Eathie (fig. 1), adjacent to the Great Glen Fault,
were seen in the morning, followed in the early afternoon by a visit to
Hugh Miller’s Cottage (National Trust for Scotland; fig. 2) and the local
Courthouse Museum; the day ended with a guided tour around Cromarty with
David Alston of the Courthouse Museum looking at the town architecture
and items of Millerian interest (fig. 3). On Sunday morning, the
party traversed the Cromarty foreshore examining the fish-bearing Middle
Old Red Sandstone succession and the faulted junction with the Moinian;
members then observed a “new” occurrence of Quaternary sediments, apparently
not seen since Miller’s day. Lunch was taken at the old railway
station in Strathpeffer, and the excursion concluded with a visit to a
neohenge megalith and, following in Miller’s footsteps, speculation on
the vitrified blocks at the Iron Age fort of Knockfarril.
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Figure 1. Bifurcated sandstone dyke (also showing orthogonal change in direction) striking away from observer in centre of photo, intruded as wet sand from below into horizontally-bedded Kimmeridgian mudstones, Eathie foreshore. |
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Figure 2. Hugh Miller's cottage. |
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Figure 3. The anachronisms of Cromarty. David Alston (Curator, Courtyard Museum in white on left) answers an architectural question from EGS members in a typical Cromarty lane, little-changed from the 18th century, while beyond an inactive 20th century oil rig sits brooding in the Cromarty Firth. |