Clastic sediments: sandstone, conglomerate and arkose,
from Torridonian and overlying Cambrian strata, Liathach, northwest Scotland

Strata across the Precambrian-Cambrian boundary on a Scottish mountain massif

rock 217-1 [359 kb]
rock 217-2-side [342 kb]
rock 217-2-top [325 kb]
rock 217-3 [518 kb]

Fig. 1.: This month we have not one but six samples. The three above are of Cambrian age, the three in Figure 2 are from the underlying Precambrian sequence known as the Torridonian, after nearby Loch Torridon. So the six samples are (I hope, ideally) presented in sequence from highest (youngest) to lowest (oldest) in the sedimentary pile. The references are mostly as dated as the collection of the samples (18 August 1976), but see Con Gillen's (2013) book for a colourful, modern summary of Scottish geology. Liathach makes a brief appearance (Gillen, 2013, pp.6,53), as does Quinag, the two being amongst the most impressive peaks in Sutherland. Caveats: This at least is the story here: the samples were collected long ago on a challenging day in the mountains, and only 217-1 can be taken as effectively in situ. Hence, these samples are really lithological representatives, and we cannot be 100% sure that they constitute a true stratigraphic sequence. Furthermore, each sample may or may not be presented "right side up": way-up criteria such as fining upwards were not applied when the photos were taken. At the least, we can examine sandstone, grit, quartzite, arkose and conglomerate, which are arguably under-represented in these pages.

Above, then, top to bottom, we have samples:

217-1 - Cambrian white basal quartzite, from the path along the summit ridge on the eastern end of Liathach, location circa NG941582 (Stuc a'Choire Dhuibh Bhig, elevation 3,000'). The rock is generally fine-grained (circa 1 mm) with scattered angular detrital grains of grey quartz and pink feldspar. There is a faint lamination in pale, pastel grey and orangey-pink tones. A white, evidently altered feldspar is abundant within the quartz, giving the rock an arkosic composition. From this area the ridge trends west over the highest summit, Spidean a'Choire Leith (3,456');

217-2 (2 photos) - Inferred basal conglomerate in the lower Cambrian strata, with abundant grains and pebbles 3-30 mm in size, in part layered, suggesting that the largest pebbles arrived in times of higher flow that could transport heavier particles (the first photo is a side view, the coarsest pebbles lying atop the sample in this orientation, better seen in the second photo, the top view). The conglomerate is locally clast-supported, with only a modest content of fine granular matrix, pale grey to pale green, except where ironstained. White vein quartz appears to be the most abundant pebble source, with lesser grey quartz and pink rhyolite. Overall, a mature, silica-dominant sediment. Sample taken, like samples 217-3 to 217-6, from loose scree descending a gully on the south side of the summit ridge on the west end of Liathach, location circa NG913576, on the flank of Mullach an Rathain (3,358': see photo in Gillen, 2013, p.53); and

217-3 - more of the Cambrian basal quartzite. A subtly laminated white quartzite with 5% porosity (due to weathering-out of feldspar?), grain size 1-2 mm.

Note: the locality data are from my transcribed 1976 notes, but the readings summarised here suggest that the western peaks are composed of Torridonian strata alone, so I am not sure of the derivation of samples 2,3.

"Rock of the Month # 272, posted for February 2024" ---


is a mighty massif in the far northwest of Scotland, forming an east-west line of summits on the north side of the head of Loch Torridon, west of Beinn Eighe. The summit of the easternmost peak is capped by Cambrian strata, but the bulk of the massif is a thick sequence of Torridonian strata. All the rocks are clastic sediments, described below. Liathach is clearly a favourite of Poucher (see his hill walkers' guide, 1974, pp.228-229 [map], pp.234-243). He says (of the eastern end of the 6-mile, 10-km series of peaks) "On a clear sunny morning you will also notice the glittering cap of white quartzite which has often been mistaken for snow and is the crowning feature of the lofty ridge". The Cambrian quartzite unit (Fig. 1) occurs only on the somewhat lower eastern peak, Stuc a'Choire Dhuibh Bhig (3,000'). Detailed topographic maps include those of the Ordnance Survey (1955, 1975). In a regional map (Geological Survey, 1957), the detail is limited yet suggestive, e.g., up by Liathach and Beinn Eighe, north and then east from Upper Loch Torridon, outcrops of Cambrian basal quartzite can be seen atop the Torridonian, on the easternmost summit of Liathach and more extensively at Beinn Eighe. The Cambrian strata sit unconformably atop the much older Torridonian sediments (Phemister, 1960, pp.41-46 and plate II, opposite p.1).

The Cambrian strata . Contrary to usual practice, we'll look at the younger, Cambrian rocks first, as if we were on the eastern summit of Liathach, and then at the underlying Torridonian sequence, which we would encounter walking to the west, and then on our descent. The basal Cambrian in this area is a thin conglomerate, 1-10 feet (0.3-3 m) thick, with pebbles of quartz, feldspar, jasper, quartzite and felsite (Phemister, 1960, p.47). E.K. Walton described the stratigraphy of Cambrian and Ordovician rocks in the northwest Highlands, including the basal Cambrian strata found eastwards from Liathach (in Craig, 1965, pp.161-164).

The Torridonian . The Torridonian and Moinian rocks in northwest Scotland were described by M.R.W. Johnson, including the Liathach area (in Craig, 1965, pp.79-87). The rocks have been described as the Applecross Group. A more modern synthesis renames them the Applecross Formation within the larger Torridon Group, the youngest of three successive formations composed in the main of sandstone with lesser units of conglomerate, mudstone and shale. The Torridonian represents an unfathomable thickness of products of erosion. The Torridon Group alone totals some 6 km in thickness, the base estimated to be 1000-970 Ma in age (the full Torridonian sequence may be 11.5 km thick, the oldest strata dated at 1200 Ma: Gillen, 2013, p.50). The Torridon Group sandstones were deposited when Scotland was on the margin of the old continent of Laurentia. They underlie, and form locally dramatic landscapes (Suilven, Liathach, Quinag and other mountains), from near Cape Wrath past Loch Torridon to Skye and Rhum in the south.

The unfossiliferous Torridonian strata are little-altered and scarcely deformed, preserving sedimentary structures from the long-ago time of their deposition. Primary features include layers of heavy minerals, ripple marks and the pits formed on wet sediment by rain drops. A constant southeast dip of current bedding indicates that the mainland lay to the north or northwest (Phemister, 1960, p.45). The Applecross rocks are dominated by coarse pebbly grits and sandstones, the arkosic grits containing a very fresh microcline feldspar (Johnson, in Craig, 1965, p.86). The feldspars are thought to be derived from older, underlying Lewisian rocks (the Cambrian cuts unconformably across both Lewisian and Torridonian rocks). The erosion products, copious amounts of sand and pebbles, were largely deposited by rivers, with occasional periods of deposition in shallow basins. The sandstone-dominated succession of the Applecross Formation is predominantly homogeneous, cross-bedded, coarse to pebbly red sandstone. The strata were deposited prior to the onset of land vegetation in the Ordovician. The sedimentary structures imply a braided fluvial system with channels of high width/depth ratio. There is soft sediment deformation related to liquefaction or fluidization. It seems that groundwater levels remained high, explaining the "unusual abundance of soft-sediment deformation in the Applecross Formation" (Owen and Santos, 2014).

The erosion of so much rock from the upper crust, to produce so-much quartz-dominant siliciclastic sediment, surely implies a source region much larger than modern-day Scotland. This could include lands adjacent to Scotland in the Proterozoic (long before the birth of the north Atlantic and the earlier Iapetus ocean), such as Greenland and eastern Canada.

rock 217-4 [344 kb]
rock 217-5 [717 kb]
rock 217-6 [411 kb]

Fig. 2.: Samples representative of the Applecross Formation, uppermost section of the thick pile of Torridonian strata. Above, top to bottom, we have samples:

217-4 - arkose, a very attractive, rather coarse-grained rock (grain size 2-6 mm) with subequal amounts of greasy grey quartz and pale pink feldspar. The latter appears fresh: the rock has only traces of fine, pale greenish-yellow sericitic mica;

217-5 - arkosic grit (=coarse sandstone) with a suggestion of a fining-upwards cycle (seen better in a companion sample). Poorly sorted, grain size 1-6 mm, composed mainly of grey quartz and a pale, evidently somewhat weathered feldspar; and

217-6 - poorly sorted arkosic grit with thin, dark shaly laminae, and with 3-5-mm quartz, feldspar and lithic clasts in a granular matrix, grain size 1 mm.


Craig,GY (editor) (1965) The Geology of Scotland. Oliver & Boyd, Edinburgh and London / Archon Books, Hamden, CT, 556pp. plus map.

Geological Survey (1957) Geological Survey `Ten-Mile' Map, Sheet One. Geol.Surv.Great Britain, Scotland and Northern England, 2nd edition.

Gillen,C (2013) Geology and Landscapes of Scotland. Dunedin Academic Press Limited, Edinburgh and London, 2nd edition, viii+246pp.

Ordnance Survey (1955) Lochcarron. Ordnance Survey one-inch map 26, 1:63,360 scale.

Ordnance Survey (1975) The Cuillin and Torridon Hills. Ordnance Survey, Southampton, England, 1:25,000 scale map.

Owen,G and Santos,MGM (2014) Soft-sediment deformation in a pre-vegetation river system: the Neoproterozoic Torridonian of NW Scotland. Proc.Geol.Assoc. 125, 511-523.

Phemister,J (1960) British Regional Geology. Scotland: the Northern Highlands. Geological Survey and Museum, HMSO, Edinburgh. 3rd edition, 104pp.

Poucher,WA (1974) The Scottish Peaks. Constable and Co. Ltd., London, 4th edition, 456pp.

Graham Wilson, posted on 02-03,07-10 February 2024

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