In the northeast they extend from Hartlepool south through the Vale of Mowbray and the Vale of York, then south through Nottinghamshire into the English Midlands, though are very largely concealed by superficial sediments.
There are further occurrences in Northern Ireland north and east of Limavady, east of Cookstown, between Dungannon and Armagh and along the Lagan Valley beneath Belfast and Newtownards and on the Antrim coast [REF].