An Upper Carboniferous formation of sandstone and dark mudstones and shale. On this part of the coast the axis of the strata runs parallel to the coast. Blackchurch is an arch sea stack composed of layers of sandstone and shale tilted at about 60 degrees. These large folds were formed during a mountain building event about 290 MYA, called the Variscan Orogeny. The rocks were dated by cephalopod fossils called goniatites. Hartland Peninsula west of Clovelly. Devon, UK.