Unconformity—An Historical Study

Abstract

It may be assumed that all present-day geologists realise the importance of ‘unconformity’ in its structural and genetic aspects. Although angular unconformity was sketched by Nicolaus Steno in 1669, by John Strachey in 1719 and by Jean Etienne Guettard in 1766, its true significance was not realised until the end of the eighteenth century. The discovery of the real -import of unconformity in geology was made by James Hutton, who ‘needed’ it to substantiate his idea of the cycle of changes in the Earth, the ‘geostrophic cycle’, consisting of a series of processes—denudation, transportation, deposition, consolidation, folding, uplift and so on. Hutton postulated unconformity before observing it, as it was only in 1787 that he discovered a splendid example of angular unconformity on the Isle of Arran, later followed by similar discoveries at Jedburgh and Siccar Point. But Hutton never used the term ‘unconformity’, using instead long descriptive phrases in which the expression ‘conjunction of vertical and horizontal strata’ was widely used. Strange to relate it was the antagonist of Hutton, the celebrated Wernerian, Robert Jameson, who applied the term ‘unconformity’ to the feature discovered by Hutton. This he did in 1805 in his book on the geology of Dumfriesshire, in which he translated the German term proposed by Werner ‘abweichende Lagerung’ (‘deviating bedding’) by an old English ecclesiastical term—‘unconformity’, which originally had the same meaning as ‘nonconformity’. Launched in such a way, the term ‘unconformity’ took a long time to penetrate into the geological vocabulary and it was only after the heroic effort of Charles Lyell and Henry De la Beche in the second quarter of the nineteenth century that the term was universally accepted and understood. Until the beginning of the twentieth century, however, the term ‘unconformity’ meant only ‘angular unconformity’. In 1905 Grabau proposed the term ‘disconformity’, for what was later called ‘parallel unconformity’. In 1909 Blackwelder suggested that ‘the contact between rocks of wholly unlike origin’, such as granite and clay, should also be classed as an unconformity. For this type I would like to suggest the name ‘heterolithic unconformity’. Finally in 1910 Willis proposed that a gap in deposition of sediments should also be considered to represent an unconformity of a type which may be best named ‘non-depositional unconformity’. Thus these four types of unconformity both in their structural aspect and their geochronological aspects are now firmly embedded in the science of geology, as representing a process which played such an important part in the shaping of the visible part of the Earth.