Education and Careers

Q: I have put some pictures of interesting features among rocks on Bennan near Merrick in the Galloway Hills onto a web gallery of images of a hill walk done in this area. These pictures may perhaps be of some interest to you. I certainly would be very grateful if someone could take the time to point me in the direction of an explanation of the geological cause of these features so that this could be added to the web page. You can see what I am talking about at www.walkscotland.plus.com/Galloway/pages/mrcktour/08.htm.

From Mr Duncan Devlin (August 2009)

These are some most interesting geological features which I have not yet visited myself (though I hope to do so before long).

The country rocks hereabouts are greywacke sandstones (sedimentary rocks) of the Kirkcolm Formation, of Ordovician age. Though these rocks are shown as lying outside the visible thermal aureole of the nearby Loch Doon granite (i.e. baked by heat from the molten rock - normally within 1-2km of the margin), the cryptic aureole extends much further out, depending on such factors as the orientation of the igneous contact (vertical or inclined).

Although always rather reluctant to comment on rocks I have not examined myself, I suggest that these are weathered-out carbonate concretions. The latter are natural features which form within sedimentary rocks by the migration and aggregation of minerals (in this case calcium carbonate) into roughly spherical, egg-shaped or sometimes irregular concentrations. The minerals are dissolved by circulating groundwater and subsequently re-precipitated, commonly around a seed or core of some mineral (a fossil or shell fragment perhaps). Concretions are quite common in parts of the Kirkcolm Formation but they do not normally show such enhanced weathering differential with the host rock. It is possible that the gentle baking from the granite has changed their chemical and physical properties and may also explain the unusual raised rim to the holes.

Flint nodules in chalk are a better-known example of the concretion phenomenon, where silica concentrates into irregular fist-sized lumps. In the case of flint, the silica concretions are much harder than the chalk and therefore weather our as pebbles. In the Bennan examples, the reverse is the case and the carbonate concretion is more susceptible to dissolution by rainwater (especially acid rain) than the surrounding sandstone. The latter has also been rendered more resistant to dissolution by having been depleted in carbonate during the formation of the concretion, thus enhancing the differential in chemical erosion.

Note: For the sake of completeness, another possible explanation for such erosional features is that they are boulders of a softer (carbonate-rich) sandstone within the greywacks sandstone (in effect a conglomerate). However, a proven example of this near Carsphairn on the east side of the Loch Doon granite is quite different in appearance and the carbonate concretion explanation remains my best suggestion.