Hutton’s Great Unconformity at Siccar Point, Scotland: Where deep time was revealed and uniformitarianism conceived

Abstract

The classic angular unconformity at Siccar Point became a landmark location in the history of geology after a boat trip to the site by James Hutton and his colleagues Professor John Playfair and Sir James Hall in 1788. Hutton successfully used the unconformity to support his view that the Earth’s landforms and geological record resulted from uniform natural processes such as sedimentation, uplift, erosion, and renewed sedimentation through deep geologic time. At Siccar Point, gently dipping Devonian fluviatile Old Red Sandstones unconformably onlap vertical Silurian deep-water greywackes (schistus) to produce the most classic of angular unconformities that remains a must-see outcrop for geologists to this day. The relevance of this outcrop for the petroleum geologist is that it provides the opportunity to view an end-member stratal relationship in the continuum of unconformable surfaces (i.e., angular unconformity, disconformity, paraconformity, correlative conformity). The locality also aids the visualization of geometries relevant to unconformity (subcrop) traps and to onlap and pinchout (supracrop)–type traps. Although most Scottish geoscientists will have seen this outcrop in their student days, as a teaching aid to illustrate basic principles, a trip later in life can remind us of the importance of the site in a historical geology context. The location continues to inspire and instills a sense of duty in us, as scholars of the Earth’s formation, to communicate and share the principles of deep time with those who have not had the privilege of an education that included the fundamentals of geology.