Devonian series rock types dominate the area giving the soil its characteristic red hue. The Usk valley bottom consists of St Maughans Formation sand and mud stone combined with river sands and gravels overlaying glacial deposits. Brown Stones of the lower Devonian period form the rising uplands to the north and south of the river. Carboniferous limestone underlies the moorland at Penrhiw Calch forming a more undulating landscape. The U shape of the the Caerfanell valley south of the Talybont reservoir and the extensive moraine mounds bordering the Usk, beyond the Ashford tunnel along the B4558 east of Talybont, are strong evidence of glaciation.