Siccar point Hutton’s Unconformity

Siccar Point is a coastal promontory that beautifully displays in three dimensions the spectacular angular unconformity with Upper Devonian beds resting discordantly on folded, near vertical, Lower Palaeozoic strata. The Lower Palaeozoic beds are turbiditic dark grey, fine grained wacke sandstone and interbedded finely laminated, fissile mudstone of the Gala Group, of early Silurian (Llandovery Epoch) age (Browne and Barclay, 2005). These rocks were strongly deformed, being folded and cleaved, during the Caledonian Orogeny. Subsequent erosion, pre-Late Devonian times, formed an irregular surface with considerable palaeorelief through differential rates of weathering and erosion of individual beds in the Silurian succession; beds of wacke sandstone standing up more sharply and prominently than others (Greig, 1988; Treagus, 1992). Upon the sharp angular unconformity there rests Upper Devonian strata, comprising reddish brown breccia and conglomerate, of the Redheugh Mudstone Formation of the Stratheden Group. In marked contrast to the underlying Silurian strata the Upper Devonian beds have only a gentle inclination produced during the later, here less intense, Variscan Orogeny.

The unconformity played a key role in James Hutton’s insight that geological processes are not just “destructive” but also capable of restoring thus leading to a rock cycle of indefinite duration so “that we find no vestige of a beginning, – no prospect of an end” (Hutton, 1788: 304).