On the foreshore just northeast of Carrickfergus Castle there are exposures of the Mercia Mudstone Group (Triassic). Although the Mercia Mudstone Group consists largely of fine-grained sediments, usually mudstones but with some siltstones and silty-mudstones, sandy bands (or skerries) are known to occur and the sandstone beds seen here represent one of these units. The massive dolerite intrusion (Fergus's Rock) on which Carrickfergus Castle is built appears to be a local thickening of a dyke, perhaps associated with a fault. On the foreshore immediately below the castle the contact between the Mercia Mudstone Group and the dolerite intrusion can be examined and the hornfelsed nature of the mudstone is obvious. It is worth pointing out that although the castle itself is built of basalt, the architraves and corner stones are made of Magnesian Limestone (Permian) the closest outcrop of which occurs on the southern side of Belfast Lough at Cultra.
