In April 1996, the Vale of Glamorgan became a county borough (unitary authority) of Wales, after forming part of South Glamorgan county. Located immediately to the west of Cardiff between the M4 motorway and the Severn Estuary, the Vale of Glamorgan covers 33,097 hectares (130 square miles) and has of coastline. The largest centre of population is Barry (51,502 inhabitants). Other towns include Cowbridge (6,180), Dinas Powys (7,799), Llantwit Major (10,621) and Penarth (22,083). Much of the population inhabits villages, hamlets and individual farms. The area is low-lying, with a maximum height of above sea level at Tair Onen to the east of Cowbridge. The borough borders Cardiff to the north east, Rhondda Cynon Taf to the north, Bridgend to the north west and the Bristol Channel to the south. The yellow-grey cliffs on the Glamorgan Heritage Coast (which stretches between Gileston and Ogmore-by-Sea) are unique on the Celtic Sea coastline (i.e. Cornwall, Wales, Ireland and Brittany) as they are formed of a combination of liassic limestone, shale and carboniferous sandstone/limestone. They were formed 200 million years ago when the whole area lay underneath a warm, shallow, equatorial sea at the start of the Jurassic Era. Thus today the cliffs contain traces of Jurassic sea creatures, such as ammonites. The stratification of overlapping shale, sandstone and limestone was caused by a geological upheaval known as the Variscan orogeny, which pushed the cliffs out of the sea, contorting them as they did so. This stratification can also be found on other parts of the Celtic seaboard, such as Bude in Cornwall, across the Bristol Channel. The calcium carbonate (limestone) in the soil allows crops to be grown which would be difficult elsewhere in Wales or the West Country: most of the West Country has poor quality and mainly acidic Devonian soils). The liassic limestone and carboniferous sandstone are also used in the Vale as building materials; in previous centuries it was taken by sloops across the Bristol Channel to North Cornish ports such as Bude, Boscastle and Port Isaac to fertilise Cornwall's poor slate soils; the hard Devonian slate was brought back from Cornwall as a roofing material for houses in the Vale. As the Glamorgan Heritage Coast faces westwards out to the Atlantic, it bears the brunt of onshore (westerly and south-westerly) winds: ideal for surfing, but a nuisance for ships sailing up the Bristol Channel to Cardiff.