How did Peach and Horne map it?
How did Peach and Horne map it?

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The work of the Geological Survey in NW Scotland during the 1880s, led by Ben Peach and John Horne, is widely regarded as the first attempt to obtain a 3D description of a thrust belt. With the notable exception of Charles Lapworth, all previous investigators adopted less ambitious approaches and as a consequence, understanding of the geological structure was at best over-simplified and at worst down-right misleading. However, the mapping was strongly regulated by standard operating procedures of the Survey. Consequently the geological maps are very much products of their time.

The Survey geologists were greatly helped by the availability of base maps created as part of the military triangulation of Scotland in the mid 19th century. The base maps were created at a scale of six inches to one mile (1:10,560) - a staggering detail given the nature of the terrain and the difficulty in moving around. AssyntHowever, while the latitude/longitudinal of hill-tops, rivers and lakes were well located, there was little or no elevation data. The geologists had to draw on their own topographic contours as they went along.

A quick look at any of the field slips of the Survey geologists reveals the wealth of information they were able to collect. While Ben Peach has commonly been regarded as the star structural geologist who could quickly deduce the geometry of folds and thrusts in complex ground, the most highly regarded mapper was Charles Clough. His field map of Loch More is provided hereAssynt. A couple of details are revealing. While he has much detail where thrusts repeat different rock units, there is rather little information on the structure of imbricates developed in single formations. An entry in Clough's colleague, Henry Cadell's field notebook reveals why. AssyntThe performance of the surveyors was tracked by monitoring two things: the number of square miles mapped in a season and the length of geological boundary mapped. Faults within individual formations didn't feature in the audit!

Auditing boundary length had a spectacular side-effect. AssyntThe imbricate structures in the thinner Cambrian units were traced out in painstaking detail. So too were the igneous intrusions (which have a boundary on each side!). And in the Lewisian this approach shines through, each dyke and thin screen of different gneiss type were mapped out, providing a stunning description of a basement terrain. AssyntThis formed the springboard, half a century later, for the studies Janet Watson and John Sutton. Meanwhile, the Survey maps in the Moine Thrust Belt have barely been surpassed for detail and provided the springboard for 1980s mapping in NW Scotland.

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