Moine geology of West Glenelg and Loch Hourn, West Glenelg - an excursion
| From: Strachan, Rob, Friend, Clark, Alsop, Ian, Miller, Suzanne (Editors). A Geological excursion guide to the Moine geology of the Northern Highlands of Scotland.: Edinburgh Geological Society, Glasgow Geological Society in association with NMS Enterprises, 2010. |
By John Ramsay
Excursion 6 West Glenelg and Loch Hourn is composed of the following articles:
- Excursion 6 West Glenelg and Loch Hourn - introduction
- West Glenelg. Localities 6.1 to 6.3
- Loch Hourn region. Localities 6.4 to 6.7
Excursion 6 West Glenelg and Loch Hourn, West Glenelg
Locality 6.1 North Glenelg Bay [NG 809 205]
North Glenelg Bay (Fig. 6.1), (Fig. 6.3). Tectonically modified unconformity between Western facies Lewisian gneisses and Moine psammites.
From the Y-road junction of the road northeast of Glenelg village, take the road towards the Kylerhea Ferry. After about 1.5km [NG 8094 2049], a small track on the west of the asphalt road and beneath an imposing old sea cliff with raised beach and sea caves leads down to the present beach where there is space to park. These outcrops will take around 0.5-1 hour of study.
Just to the north of a newly-built (2005) house ‘Beach-Haven’, a prominent red-brown lamprophyre dyke cuts grey Moine psammites (Fig. 6.3). South of this dyke, a contact between Moine psammites and Lewisian gneisses is exposed [NG 8080 2036]. The outcrops are best visited at lowtide. The best exposures are to be found just below high water mark. The mineralogical and textural contrasts between the grey Moine psammites and the orange-brown Lewisian gneisses are very marked, and at several localities the actual contact between the two groups of rocks may be investigated. Most of the Western facies Lewisian gneiss consists of coarse-grained biotite-hornblende gneiss with a banding produced by variations in the biotite and hornblende content. The gneisses often show an agmatitic structure with clots and lenses of biotite and hornblende-rich material. There is no evidence of a sedimentary origin. The gneisses locally have boudinaged sheets of amphibolitic rock and, although these are here parallel to the banding in the enclosing biotite gneisses, their contacts are generally sharp. They probably represent amphibolitized basic dykes which at some other localities in the Glenelg region can be seen to be discordant to the gneissic banding. Lenticular masses of ultrabasic material are also common, consisting of green actinolite and diopside, dark brown biotite and black hornblende. At some localities, S-shaped minor folds (D2) are present with their hinge lines parallel to an intense rodding lineation.
The Moine psammites are well-bedded, muscovite-biotite-quartz-feldspar rocks. Gneissic material is completely absent, although some quartz veins are present. Certain beds show their original clastic grain structure and are marked with small pock-marked weathering holes. At several localities, cross-bedding is preserved and truncated foreset bedding is often well preserved especially in the hinge zones of minor folds. These sedimentary structures consistently indicate that the Moine rocks young in a direction away from the Lewisian-Moine contact. There are abundant tight D2 folds with regionally consistent S-forms. These folds generally show well-developed axial-plane schistosity, together with an E-plunging and intense schistosity-bedding intersection parallel to a rodding-lineation and to the fold hinge lines. At one locality (Fig. 6.3) small isoclinal folds are refolded by the dominant D2 structures, but it is uncertain if these structures are tectonic (D1) or sedimentary in origin.
The contrast of rock type between the Lewisian and Moine units is very marked and the contact is sharp with stratigraphic relationships that suggest that the Moine sediments were deposited on a basement that had already undergone upper amphibolite facies metamorphism and had been intruded by basic and ultrabasic sheets. At the actual contact here is no clear evidence of an angular unconformity and some of the finer grained bands of Lewisian gneiss superficially resemble the adjacent Moine psammite. This similarity in appearance is a good example of metamorphic and textural convergence, whereby two groups of quartz-feldspar-mica rocks with initially distinct textures and fabrics converge in grain size and overall appearance as a result of undergoing identical deformational and metamorphic processes. Both Lewisian and Moine have experienced intense common ductile folding and such deformation is well known to generally reduce angles of discordances (Ramsay, 1967; Ramsay & Lisle, 2000). However, certain structural features do suggest that the two groups were originally structurally discordant. Although the orientations of the main folds which affect the Lewisian and Moine are similar, there are slight but consistent differences between the common geometric features. For example, the orientations of the fold hinges and rodding structures in the Lewisian and Moine are not quite the same: those in the Lewisian plunge to the ESE with a generally more varied and steeper plunge than the more constant and easterly plunging structures of the Moines. These differences are probably best interpreted as being the result of differences in the orientations of the Lewisian banding (steeper and more variable) from those of the Moinian bedding surfaces (less steep and more constant).
If the visitor has extra time (10 minutes walk) some nearby coastal outcrops a little further to the south and to the SE of the headland of Creag Mhor at [NG 8086 2013] of Lewisian agmatitic gneisses can be highly recommended, and are especially photogenic (see Clough in Peach et al., 1910, plate VI).
Locality 6.2 Glenelg village [NG 8092 1921]
Glenelg village (Fig. 6.1). Western facies Lewisian gneisses and amphibolite sheets.
At Rudha Mhic Cuinn, on the foreshore just west of the Glenelg War Memorial, banded hornblende-biotite gneisses show hornblende agmatites injected by acid veins [NG 8092 1921]. The gneisses are cut by massive sheets of amphibolite with sharp contacts and with little or no gneissic material. These sheets are interpreted as basic intrusions injected into the gneissic host and subsequently recrystallised in amphibolite facies metamorphic conditions during D2. All the rocks show an intense D2 rodding lineation plunging 15°-25° to the ESE. Just south of the monument are exceptionally clean outcrops of banded gneiss cross-cut by a massive amphibolite with a knife-sharp contact. The gneiss shows an easterly dipping schistosity oblique to the banding and with schistosity/banding intersection parallel to the prominent rodding which is exactly parallel to the hinge line of nearby D2 folds.
Locality 6.3 Sandaig [NG 7680 1463] to [NG 7706 1520]
Sandaig (Fig. 6.1). Western facies Lewisian gneisses and amphibolites, some of which show eclogitic cores; tectonically modified unconformity with cross-bedded Moine psammites.
Drive along the asphalt surfaced road south from Glenelg village, and after 1.5km turn right to cross the Gleann Beag river at Eilanreach. Continue along this road for about 4km where a gravel track branches left into a forestry plantation just north of the reed-filled Loch Drabhaig [NG 7836 1517]. Park by the roadside and proceed through a locked forestry gate into the forestry plantation following the main track (there are various branching tracks) to cross the Allt Mor Shantaig by a bridge. About 600m from the bridge a complex of tracks meets at a cross road: take the track on the right which descends towards the sea. After several bends the track finishes and is replaced by a narrow footpath descending towards the Bay of Sandaig. This descent from the park place will take about 30 minutes. Sandaig [NG 7724 1472] is now a ruined croft, once inhabited by Gavin Maxwell who wrote a well known book describing his stay there (The Ring of Bright Water, Penguin Books, 2001). Pass through a gate and follow a footpath to two small monuments, one with a small bronze plaque in memory of Maxwell, the other to his otter Edal (site marked on the 1:25,000 map ‘meml.’). If the season has been dry it is possible to cross the river (Allt Mor Shantaig) where it enters the sea. More normal Scottish conditions will require wading or by using the shaky double-rope bridge, one rope for the feet the other for the hands.
From the rope bridge a footpath leads westwards to the Sandaig Islands where there are excellent wave-washed outcrops [NG 7680 1463] of quartz-feldspar-hornblende gneiss with a strongly developed, easterly plunging rodding lineation (D2) (Locality 6.3A). The gneissic banding is often crosscut by dark-green hornblende-garbenschiefer spears. Some of these hornblendes are up to 4cm long, while others occur as stellate aggregates following hornblende-rich layers. Because they cross-cut the rodding structures it is clear that the metamorphism which led to their development must be post D2. If tide and time allow it is well worth visiting the more westerly of the islands (although that in the far west where the lighthouse is located is generally inaccessible except by boat). Here the quartz-feldspar rich gneisses contain amphibolite sheets free of gneissic material and are sometimes boudinaged. Their contacts are invariably sharp and locally discordant to the gneissic banding, and they are best interpreted as representing basic intrusions into the gneisses.
From the Sandaig Islands, proceed northward past the headland of An Gurraban. In a small bay (Locality 6.3B, [NG 7696 1506]) a contact between Lewisian gneisses and Moine metasediments is exposed. At the actual contact, the SE-dipping Moinian sediments contain elongated blocks of Lewisian gneiss material which has been interpreted as representing a true basal conglomerate of the Moine Series (Clough in Peach et al., 1910, p. 50). Unfortunately the best and most convincing exposures of this conglomerate are now practically inaccessible, overgrown by moss and situated in a steep forest-covered hillside further to the northeast [NG 7868 1649]. The basal Moine sediments form alternating bands of garnetiferous pelitic mica-schist and semipelitic material (the so-called ‘basal Moine pelite’, Ramsay & Spring, 1962). In contrast to the immediately adjacent Lewisian rocks, the Moine metasediments contain no gneissic material, only a few crosscutting quartz veins. The pelitic Moine sediments contain perfectly idiomorphic dark red-brown garnets up to 8mm in diameter and which, under the microscope, are seen to contain spiral inclusion trails suggesting that they grew during and after the D2 deformation. The pelitic bands nearest to the Lewisian gneisses contain some small hornblende-garbenschiefer spears, and these are believed to have formed at the same period of metamorphism as those seen in the Lewisian gneisses described earlier (post D2). Generally the Moine sediments of the Glenelg region lack amphibole. The origin of the hornblendic material is uncertain: it could be have been derived from the erosion of Lewisian material, or it could be the result of migration of the mineral constituents from the adjacent gneisses during metamorphism.
A broken wire fence comes down to the coast just north of the Moine-Lewisian contact. Just north of this fence the more psammitic parts of the Moine are well exposed at Locality 6.3C [NG 7706 1520] and show good cross-bedding structures indicating that the SE-dipping metasediments are inverted and young away from the Lewisian. The psammitic Moines show beds with well preserved (although tectonically stretched and recrystallised) clastic feldspars. The cross-bedding features are especially well developed as the distance from the Lewisian-Moine contact increases and are especially well preserved in the sea cliffs on the headland of Rubha na h-Airde Beithe ([NG 7785 1640]; and see Peach et al., 1910, p. 53), but a visit to this headland will take at least 20 minutes (low tide conditions necessary).
If the tide is low and the visitor has time (½-1 hour) it is worth visiting the coastal outcrops of the Lewisian south of Sandaig. The Lewisian along this stretch of coast, in contrast to that seen in the Sandaig Islands, has been very highly deformed, and much of the coarse-grained fabrics have been replaced by laminated, very fine grained quartz-feldspar-hornblende-biotite rocks. Because of their fine- and rather even-grain size, these rocks were once described as ‘granulites’, this description applied because of their granular nature and not because of their metamorphic state. The rocks are intensely and finely banded, and the sheets of hornblende schist have been boudinaged into pod-like pieces. At Locality 6.3D [NG 7675 1404] the centres of the largest pods sometimes have an altered eclogitic mineralogy (the eclogite transforming to garnet amphibolite and amphibolite), a feature which is more characteristic of the Eastern Lewisian facies and generally absent in the Western facies. In fact, these outcrops are the only ones seen in the Western facies and the eclogitic remnants here have been dated at ~1700 Ma (Storey et al., 2010), markedly different from the ~1100 Ma eclogites in the Eastern Lewisian facies (Excursion 7; Sanders et al., 1984; Sanders, 1988). The strongly banded Lewisian gneisses show lenticulate blebs of feldspar, while the quartz crystals are found as sinuous streaks around the feldspar porphyroclasts. The timing of this granulation of the rocks is uncertain at this locality. Elsewhere in the Glenelg region, such highly deformed fabrics are also found in Moinian rocks, appear to predate the strong recrystallization that is attributed to D2 and post D2, and are attributed to the first of the deformation in common to both Lewisian and Moine (D1) (Ramsay & Spring, 1962). From south Sandaig return to the road.
References
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