Bala Series, in geology, a series of dark slates and sandstones with beds of limestone which occurs in the neighbourhood of Bala, Merionethshire, North Wales. It was first described by A. Sedgwick, who considered it to be the upper part of his Cambrian System. The series is now placed at the top of the Ordovician System, above the Llandeilo beds. The Bala limestone is from 20 to 40 ft. thick, and is recognizable over most of North Wales; it is regarded as the equivalent of the Coniston limestone of the Lake District. The series in the type area consists of the Hirnant limestone, a thin inconstant bed, which is separated by 1400 ft. of slates from the Bala limestone, below this are more slates and volcanic rocks. The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick. In South Wales the Bala Series contains the following beds in descending order: - the Trinucleus seticornis beds (Slade beds, Redhill shales and Sholeshook limestone), the Robeston Wathen beds, and the Dicranograptus shales.

The typical graptolites are, in the upper part, Dicellograptus anceps and D. complanatus; in the lower part, Pleurograptus linearis and Dicranograptus Clingani. In Shropshire this series is represented by the Caradoc and Chirbury Series; in southern Scotland by the Hartfell and Ardmillan Series, and by similar rocks in Ireland. See Caradoc Series and Ordovician System.