NWHG Ref. 031 — Allt nan Uamh

Location, grid reference and photograph

The site is located within the valley of the Allt nan Uamh, about 1.5 km south of Stronchrubie on the east side of the A 837, Grid Ref. [NC 256 179].

(Figure 32) Waterfall over vogesite sill, Allt nan Uamh. BGS Photo P512707 — T Bradwell

GCR site reference, block, volume and notified feature of SSSI?

GCR Ref. 1230, Caledonian Igneous Block, Vol. 17. Notified feature of Ben More Assynt SSSI.

Description and geological significance

This small site lies within the course of the Allt nan Uamh and displays a 20 m thick sill of fresh vogesite (hornblende-rich lamprophyre), containing a high proportion of large prismatic hornblende crystals set in pink feldspar, running across the stream and dipping gently eastwards. Vogesite occurs widely within the Moine Thrust Zone of Assynt and Ullapool but is otherwise a rare rock-type. It is the most widespread hypabyssal intrusive rock-type occurring in Assynt and offers an excellent example of the lamprophyre family for teaching purposes.

Accessibility

The site is easily accessed from the “Bone Caves” car park [NC 253 179] on the east side of the A 837, 2km north of the northern end of Loch Awe. A short walk up the unsurfaced path, past the former fish-farm building, would, unfortunately, prove very difficult for all abilities.

Conservation

Low conservation requirement due to the nature of the exposure.

Visibility and “clarity”

The sill of vogesite is clearly exposed at a conspicuous waterfall feature formed in the course of the Allt nan Uamh.

Interpretation and interpretation potential

The site is important for geologists but probably of limited interest to the general public. Nonetheless, it is excellent for specialist geological teaching purposes as an example of the lamprophyre family. The area of the waterfall, with the vogesite sill underlying the main rock ledge of Salterella Grit, is a very photogenic one and attracts many visitors on their way further up the valley to the Bone Caves. The site is featured as part of a walk in the guidebook “Exploring the landscape of Assynt” Although there is currently no interpretation panel, consideration might be given to erecting one which would explain why the waterfall feature is there (i.e. due to the baking effect of the hot sill on the immediately overlying Salterella Grit strata which were therefore locally hardened and resistant to subsequent erosion). The site should be included in a future Geopark guide.

Key references

PARSONS, I. 1999. Late Ordovician to mid-Silutian alkaline intrusions of the North-west Highlands of Scotland. In Stephenson, D., Bevins, R. E., Milward, D., Highton, A. J., Parsons, I., Stone, P. & Wadsworth, W. J. (eds) Caledonian Igneous rocks of Great Britain. Geological Conservation Review Series, 17, Chapman & Hall, 345–393 (site description page 388).