Most of us are weaned on the notion that the first unconformity described by someone who knew what they were looking at, took place when Hutton, Hall and Playfair visited Siccar Point on the southeast coast of Scotland in 1788. In fact Hutton had seen and understood unconformities at two sites in the previous year - Jedbergh and, a little earlier, Newton Point on the Island of Arran. All three unconformities juxtapose steeply dipping low- to mid-grade metamorphic rocks, deformed by the Caledonian orogeny, and Upper Paleozoic (Devono-Carboniferous) alluvium. Hutton had long recognized that these two rock types were distinct and commonplace in the central part of Scotland. He found the Arran contact by accident, while looking for evidence of the intrusive nature of granite: "Here the schistus and the sandstone strata both rise inclined at an angle of about 45°; but these primary and secondary strata were inclined in almost opposite directions; and thus they met together like the two sides of a lambda, or the rigging of a house, being a little in disorder at the angle of their junction".

The "disorder" at the unconformity surface is the result of intensive fragmentation of the "schistus" by caliche. Displacive and replacive calcrete textures are strongly developed along schistosity planes. The overlying strata are conglomerates and sandstones that also contain evidence of calcareous pedogenesis.

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