All the rocks exposed in the area of Southerndown are sedimentary rocks. They were laid down as deposits of mud, silt, sand and lime (calcium-rich deposits from shells and bones) that, over long periods of time, were compacted and solidified into limestone, shale (compacted mud) and
conglomerate (rock of pebbles of various size). The oldest rocks are hard, grey, often shelly,
Carboniferous Limestone. These were laid down in a warm, shallow sea during the early part of the
Carboniferous Period when the future
Wales lay on or near the equator. Continental drift and earthquakes folded and fractured the Earth's crust, and during the next 100 million years the rocks suffered extensive erosion. The movements left the upfolded Carboniferous Limestone as high ground, the slopes and cliffs of the
Vale of Glamorgan. During the early
Jurassic Period, a shallow sea spread northwards over the land. As the water deepened the higher areas became islands before finally being inundated. Water depth influenced the kinds of sediments that were deposited. Rocks formed close to the shore (Sutton Stone and Southerndown Beds) are strikingly different from the
Blue Lias, which was deposited at the same time but farther offshore. Sutton Stone consists of massive, white, conglomeratic limestones with pebbles of black
chert (silica) and Carboniferous Limestone. The overlying Southerndown Beds are blue/grey conglomeratic limestones and limy sandstones with thin shale partings. The Sutton Stone and Southerndown Beds pass laterally into, and are overlain by, the alternating blue-grey limestones and shales of the Blue Lias. The Jurassic rocks are generally rich in fossils. Shells of
bivalves, and
ammonites, fragments of
crinoids,
corals and pieces of carbonised
fossil wood are quite common. During the last 200 million years further earth movements have seen some folding and fracturing of the rocks and, during the relative stasis but higher waters since the end of the last
Ice Age, the coastline has in most places resumed a fast pace of erosion, retreating in places by several miles by subaerial and marine
erosion. ==References==