The Loch Cuithir diatomite deposits, Skye - an excursion

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From: Bell, B.R. and Harris, J.W. An excursion guide to the geology of the Isle of Skye : Geological Society of Glasgow, 1986. © 1986 B.R. Bell & J.W. Harris. All rights reserved.
Figure 33 Excursion 19 The Loch Cuithir Diatomite deposits and Kilt Rock, location map

Excursion 19 The Loch Cuithir Diatomite deposits (Figure 33)[edit]

Purpose: To examine the remains of a former diatomite excavation.

Aspects covered: a Quaternary age diatomite deposit; landslipped material; Lower Tertiary plateau lavas.

Route: Lealt–Loch Cuithir–(Lealt).

Distance: 10 kilometres on rough road (return journey from Lealt to Loch Cuithir), plus 1km on foot.

Time: 2 hours.

General comments: The workings of this deposit are still visible, although, unfortunately, the remaining, unextracted diatomite is under several metres of water. This excursion is only suitable for car and minibus parties.

The Lealt turn-off is located 18km (11 miles) north of Portree and 11km (7 miles) south of Staffin on the main (A855) coastal road of north Skye.

Follow the minor road west (inland), through the settlement of Lealt, for 5km (3 miles) to the east side of Loch Cuithir. This road is suitable for vehicles (cars and minibuses only) if care is exercised. Caution is advised in the vicinity of the loch—it is deep and the surrounding area is, in places, marshy.

Locality 1 [NG 4764 5960][edit]

Loch Cuithir is located upon landslipped material (11D), which overlies Upper Jurassic strata (2F). Only parts of these diatomite workings remain. Some of the brick buildings, together with the line of the tramway used to transport the diatomite (11F) to the coast, are still obvious. The diatomite occurred as a 3–6m-thick horizon below a 1m covering of peat. The loch had an original area of 60 hectares (24 acres) and was drained in order to extract the diatomite. Ditches, around the perimeter of the loch, were excavated and the water was drained through a man-made outlet at the northern end of the loch, thus allowing removal of the peat and extraction of the diatomite. East of the drainage outlet are spoil-heaps, mostly of plateau lava boulders, presumably removed from the workings during excavation. The diatomite from this deposit was very pure, with little or no interlayered silt or mud. Macadam (1920) notes that the calcined (heat treated) diatomite contains over 96% SiO2 (reported in Anderson and Dunham 1966), whilst Strahan et al. (1917) gave a value of 98.78%. According to Macadam (1920), the absorptive value of the material from Loch Cuithir was over 3.56 (a good diatomite would have an absorptive value in excess of 4.0). Further details of Skye diatomite deposits are presented in Section (11F) Of Chapter 11.

This locality also provides a spectacular view of the plateau lavas of the Beinn Edra Group (3D). These flows crop out on the Flasvein–Creag a' Lain–Sgurr a' Mhadaidh Ruaidh ridge and dip at a shallow angle to the west.

Return to the main road.


Appendix 1: Glossary of petrological names and terms[edit]

Appendix 2: Glossary of fossil names[edit]

Appendix 3: Glossary of place names and grid references[edit]

At all times follow: The Scottish Access Codeand Code of conduct for geological field work