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.