sentences:  Icesheets and valley glaciers which developed during a series of ice ages have significantly altered a landscape which had developed as rivers drained a tilted upland surface which is thought to have emerged from beneath the sea during earlier Cainozoic times.The effects of the last (Devensian) ice age are the most readily understood.An icesheet which at its maximum extent covered virtually all of Wales and reached as far south as Cardiff, Bridgend and Gower left in its wake suites of both erosional and depositional landforms.