 The Old Red Sandstone is a series of red sandstones , marls and conglomerates . It rises as an anticline in the Mendips and appears in the Avon Gorge and at Portishead . Carboniferous Limestone , of marine origin , covers the sandstone and appears in the Avon Gorge and at Weston @-@ super @-@ Mare where it contains volcanic rocks . There are outlying hills at Worlebury , Middle Hope , the Failand Ridge , Broadfields Down , Portishead Down and Wrington Hill . 
