"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).