The land to the southeast of the Great Glen was the old Celtic region of Dál Riata (Dalriada), and in 1891 Archibald Geikie proposed the name Dalradian as a convenient provisional designation for the complicated set of rocks to which it was then difficult to assign a definite position in the stratigraphical sequence.

[1] In Archibald Geikie's words, "they consist in large proportion of altered sedimentary strata, now found in the form of mica-schist, graphite-schist, andalusite-schist, phyllite, schistose grit, greywacke and conglomerate, quartzite, limestone and other rocks, together with epidiorites, chlorite-schists, hornblende schists and other allied varieties, which probably mark sills, lava-sheets or beds of tuff, intercalated among the sediments.

[1] The Dalradian Supergroup spans the late Tonian through to early Ordovician, however, there is an unfortunate lack of direct geochronological age constraints throughout the succession.

A detrital zircon study provides useful maximum depositional age constraints for the lowermost Argyll group 1.1 km thick glaciogenic Port Askaig Formation.

[4] The age constraints suggest that the Port Askaig Formation correlates with the global ‘Sturtian’ Snowball Earth interval and spans ca.

The oldest part of the group is interpreted to be the Keltie Water Formation, which includes the Leny Limestone and Slate Member from which lower Cambrian trilobite fossils have ben recovered.

The sedimentary rocks that lie unconformably above the ophiolite include the Dounans Limestone Formation that contains a fossil fauna of mid-Arenig age (near the boundary between the lower and middle Ordovician).

The Argyll Group is divided into four subgroups, thus: whilst the Appin Group is divided into three subgroups: In Shetland the Whiteness ‘Division’ forms the core of Mainland whilst the Scatsta ‘Division’ forms the western halves of Unst and Fetlar, all of Whalsay and much of the southeastern part of Mainland, east of the Nesting Fault.