UNCONFORMITIES 543 Figure Hutton’s unconformity at North Newton, near Lochranza, Isle of Arran, Scotland The arrows mark the obvious change from steeply dipping Dalradian Schists in the lower part of the photograph to sub horizontal Devonian sandstones in the upper part sight ‘heterolithic’ (Greek for unlike rocks) may appear to be an appropriate term, it could also apply to clastic rocks overlying carbonates or to marine rocks overlying non-marine rocks In some parts of the world heterolith is used to refer to interbedded sandstones, siltstones, and mudstones) In all of the cases discussed above there was an assumption that the surface of unconformity represented a subaerial erosion surface If we look at many parts of the world today, we see that the land surface is not a smooth plane It is therefore not surprising that many unconformities that originated as subaerial surfaces are also irregular This is often referred to as a buried-landscape type of unconformity A good example is the Torridonian unconformably overlying the Lewisian in north-western Scotland In 1910 Bailey Willis (1857–1949) included surfaces of non-deposition in marine sediments as a variety of unconformity Over the next few decades several authors stressed the importance of subaqueously formed breaks, but it was not until 1957 that John Essington Sanders (1926–1999) proposed a complex Greek-based nomenclature that attempted to distinguish clearly between subaerial and subaqueous breaks It is, perhaps, not surprising that most people were put off by the nomenclature or thought that it was all an elaborate joke But in ignoring the terms many people also ignored the attempt to refine geological thinking It was inevitable that the expansion of the use of the term unconformity would give rise to some nomenclatorial confusion (Table 1) The same word was used by different authors to describe different concepts, and the same concept was given different names In recent years there has been some convergence of views on the nomenclature Authors have tended to use the descriptive terms angular unconformity, disconformity, and heterolithic unconformity (Figure 1) This consensus has, however, been challenged by the specific definition of unconformity that has been used by the proponents of seismic and sequence stratigraphy (see below) Lateral Variation One descriptive term is usually adequate to describe an unconformity at a single exposure, but when the surface is traced over wide areas it is common to see the nature of the unconformity vary A good example is the North Sea Unconformity Complex, often called the ‘base-Cretaceous unconformity’ or the ‘Late-Cimmerian unconformity’ This is perhaps the most easily identifiable surface of the Phanerozoic succession of the Norwegian continental shelf It displays great local complexity and great variability on a regional scale, such that in different places it has been classified as a nonconformity (in the sense of heterolithic unconformity), a disconformity, and an angular