Dalradian

Folded Dalradian phyllites of the Ordovician Ben Ledi Grit Formation, part of the Southern Highland Group of the Dalradian Supergroup just below the summit of Ben Lomond - walking pole about 1.2 m long for scale

Andalusite crystals in Dalradian metamorphic rock at Boyndie Bay, north-east Scotland

Dalradian in geology describes a series of metamorphic rocks, typically developed in the high ground which lies southeast of the Great Glen of Scotland. This 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. The total thickness of this assemblage of rocks must be many thousand feet." The Dalradian Series (as then defined) included the "Eastern or Younger schists" of eastern Sutherland, Ross-shire and Inverness-shire, the Moine gneiss, as well as the metamorphosed igneous and sedimentary rocks of the central, eastern and southwestern Scottish Highlands. The series has been traced into the northwestern counties of Ireland. The whole of the Dalradian complex has suffered intense crushing and thrusting.^[1]

This thick rock sequence (excluding the Moine sequence) is now called the Dalradian Supergroup and geologists divide it into four groups: the Grampian, Appin, Argyll, and Southern Highland groups. Some of these groups are themselves divided into subgroups.^[2]

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