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

Liathach

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.

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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.