A remarkable feature in the history of the Llandeilo rocks in Britain, more especially in North Wales and Cumberland, was the outbreak of volcanic action; vast piles of Llandeilo lava and ashes form such hills as Cadair Idris, and the Arenigs in Wales, and Helvellyn and Scafell in Westmorland and Cumberland.
in thickness, lies on this horizon; and in the Cross Fell area the Milburn beds of the Skiddaw slates (see Arenig) appear to be of the same age.
In Scotland the Llandeilo group is represented by the Glenkiln shales, black shales and yellowish mudstones with radiolarian cherts and volcanic tuffs; by the Barr Series, including the Benan conglomerates, Stinchar limestone and Kirkland sandstones; and by the Glenapp conglomerates and Tappinsrnudstones and grits south of Stinchar.
Cardiola and Modiolopsis occur among the pelecypods; Euomphalus, Bellerophon, Murchisonia among the gastropods; and Conularia and Hyolithes among the pteropods.
The green roofing slates and plumbago (graphite) of the Lake District are obtained from this group of rocks.