 Wentwood is underlain by sandstones which are assigned to the Brownstones Formation of the Old Red Sandstone, a suite of sedimentary rocks laid down during the Devonian period.The beds dip gently to moderately in a south-easterly direction.It is the southernmost part of a range of hills formed by the relatively hard-wearing Brownstones sandstones which stretch in a rough arc northwards through eastern Monmouthshire, the broadly west-facing scarps of which are generally well wooded.