The impressive Blackchurch Rock, which is a large arch stack found at Mouthmill, provides exposures of the Hartland Quay Shale that contains goniatite fossils. Goniatites are also found at two sites where there are exposures of Gull Shale Rock – two anticline structures (folds with the fold axis at the top) 300m and 400m southeast of Blackchurch Rock. It is the types of goniatite found in the ‘marker horizons’ of these features that have helped to date the rocks.