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(also Wales Massif), mountains in Great Britain which occupy most of the peninsula of Wales. Length, approximately 150 km; elevation, to 1,085 m (Mount Snowdon). They are primarily composed of shale, limestone, and sandstone. Broad, deep river valleys divide the Cambrian Mountains into a series of plateaus above which rise individual ridges with mountain-glacial topography. There is a dense river network and numerous small lakes. At altitudes of 400–600 m there are broad-leaved forests of oak, beech, and ash; at higher altitudes there are heaths and peat wastelands. Cambrian deposits were first described in the these mountains.