Fig. 1: Sample of Durness limestone with abundant chert bioclasts, often with sponge-like forms. Being so hard, the chert weathers out, standing proud above the limestone surfaces and lending a rough texture to the rock. Much of the host formation is dolostone, but this sample effervesces rapidly in 10% hydrochloric acid, indicative of calcite (limestone) rather than dolomite (dolostone).
Sample 218.17, 12.5 x 9 x 5 cm, 657 grams, not appreciably magnetic. The sample was collected in situ in Strath Suardal, a valley draining northeastwards to Broadford Bay, on the southeast flank of Beinn na Caillich, eastern sentinel of the Red Cuillin hills. The exact site was the top of a knoll east of the road (A881) on the northwest slope of Ben Suardal, where the limestone appears just southwest of older Torridonian strata, which overthrust the lower Paleozoic sediments, at circa NG627214 (Ordnance Survey, 1971). Collected on 26 March 1979.
"Rock of the Month # 256, posted for October 2022" ---
This sample comes from the southern part of the Isle of Skye in northwest Scotland, at the east end of the rounded granitic hills of the Red Cuillins. This Durness limestone (or dolostone) succession includes six members (Craig, 1965), at the top of which is the Ben Suardal Group, from which this sample was taken. Originally considered all of lower Cambrian age, more recently the upper three members have been deemed lower Ordovician. The lower Paleozoic sediments were buried by Torridonian strata in the overriding Kishorn thrust, only to be exhumed by later erosion (Bell and Harris, 1986, pp.23-26).
There is much siliceous material in the limestone, and as a result, weathered surfaces have a spiky appearance. Note the fine, sub-mm porosity in the host rock. The silica is probably in large part secondary, but some fossil material (bioclasts) remain. The Durness limestone overall is not very fossiliferous, but nautiloids and other fossils are recorded from this area.
Tilley (1948) undertook a classic study of the effect of granite magma on siliceous dolomites in the area, leading to the development of grossular garnet, wollastonite, clinopyroxene and other minerals in skarns at granite contacts.
Not related by more than geography, perhaps, but there is further interesting geology just to the west, on Loch Slapin, an inlet on the south coast of Skye. Limestone breccia, interpreted as paleokarst fill of early Jurassic age, lies above the Durness Limestone Group and below the Broadford Formation (Farris et al., 1999).
Bell,BR and Harris,JW (1986) An excursion guide to the geology of the Isle of Skye. Geological Society of Glasgow, 317pp.
Craig,GY (editor) (1965) The Geology of Scotland. Oliver & Boyd, Edinburgh and London / Archon Books, Hamden, CT, 556pp. plus map.
Farris,MA, Oates,MJ and Torrens,HS (1999) New evidence on the origin and Jurassic age of palaeokarst and limestone breccias, Loch Slapin, Isle of Skye. Scot.J.Geol. 35, 25-29.
Ordnance Survey (1971) Portree. Ordnance Survey one-inch map 25, 1:63,360 scale, revised 1955, major roads revised 1971.
Tilley,CE (1948) Dolomite contact skarns of the Broadford area, Skye: a preliminary note. Geol.Mag. 85, 213-216.
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