LLANDEILO GROUP

This article appears in Volume V16, Page 828 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: LEO-LOB

LLANDEILO GROUP , in geology, the middle subdivision of the British Ordovician rocks. It was first described and named by Sir. R. I. Murchison from the neighbourhood of Llandeilo in

Carmarthenshire

CARMARTHENSHIRE

. In the type area it consists of a

series

SERIES (a Latin word from serere, to join)

of slaty rocks, shales, calcareous flagstones and sandstones; the calcareous middle portion is sometimes termed the " Llandeilo limestone "; and in the upper portion volcanic rocks are intercalated. 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 Cader Idris, and the Arenigs in Wales, and Helvellyn and Scafell in Westmorland and Cumberland. The

series

SERIES (a Latin word from serere, to join)

is also found at

Builth

BUILTH, or BUILTH WELLS

and in Pembroke-shire. The

average

AVERAGE

thickness in Wales is about 2000 ft. The group is usually divided in this area into three sub-divisions. In the Corndon

district

DISTRICT

of Shropshire the Middleton Series represents the Llandeilo group; it includes, in descending order, the Rorrington black shales, the Meadowtown limestones and flags, and the western grits and shales. In the Lake

District

DISTRICT

the

great

GREAT

volcanic series of Borrowdale, green slates and porphyries, 8000 to 9000 ft. in thickness, lies on this horizon; and in the

Cross

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 lime-stone and Kirkland sandstones; and by the Glenapp conglomerates and Tappins'mudstones and grits

south

SOUTH
SOUTH, ROBERT (1634–1716)

of Stinchar. Graptolitic shales, similar to those of southern

Scotland

, are traceable into the north-

east

EAST
EAST, ALFRED (1849- )

of Ireland.
The fossils of the Llandeilo group include numerous graptolites, Coenograplus gracilis being taken as the zonal fossil of the upper portion, Didymograptus Murchisoni of the lower. Other forms are Climacograptus Scharenbergi and Diplograplus foliaceus. Many trilobites are found in these rocks, e.g. Ogygia Buchi, Asaphus tyrannus, Calymene cambrensis, Cheirurus Sedgwickii. Among the brachiopods are Crania, Leptaena, Lingula, Strophomena; Cardiola and Modiolopsis occur among the Pelecypods; Euomphalus, Bellerophon, Murchisonia among the Gasteropods; Conularia and Hyolithes among the Pteropods; the Cephalopods are represented by Orthoceras and Cyrtoceras. The green roofing slates and plumbago (graphite) of the Lake District are obtained from this group of rocks. (see ORDOVICIAN).

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