Neogene periods in Wales though they are known to occur offshore. The exceptions are the sequence revealed in the Mochras borehole on the coastal strip south of Harlech and the 'pocket deposits' where sediments fill depressions in the Carboniferous Limestone in northeast Wales. In the south, poorly cemented sands occur sporadically in the central and western part of the Brecon Beacons National Park; these are thought to be a product of the Cenozoic weathering of the underlying Twrch Sandstone. Quaternary The landscape of Wales has assumed its present shape over the last 2.6 million years i.e. during the Quaternary period which reaches to the present day. 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. Glacial legacy 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. The glacial cirques of Snowdonia and to a lesser extent of the Cambrian Mountains and the Brecon Beacons are well known. Many pre-existing valleys were further deepened by glacial ice. Cirque moraines in the mountains and terminal and recessional moraines in the major valleys are the most striking depositional legacy of the glaciation. Three substantial median moraines extend beneath the waters of Cardigan Bay, parts being exposed at low spring tides as Sarn Badrig, Sarn y Bwlch and Sarn Cynfelin. There are too, swarms of drumlins and a widespread plastering of glacial till elsewhere. The greatest concentration of drumlins is in Denbighshire though there are also distinct areas around the Severn valley. Coastal deposits Following the end of the last ice age, sea levels rose to roughly their current levels by around 6000 years ago. Forests which had become established at or below this level were destroyed though the preserved stumps of trees in growth position can still be seen in the intertidal zone in places, as can the remains of peat deposits which again had originally formed above the high-water mark. Redistribution of glacial and fluvial sands has given rise to extensive dune systems around the Welsh coast, notably at Newborough Warren on Anglesey,