COUNTRYSIDE COUNCIL FOR WALES
SITE OF SPECIAL SCIENTIFIC INTEREST: CITATION
ANGLESEY TRWYN DWLBAN
National Grid Reference: SH532817
This site is selected for its geological interest.
The rock platform, cliffs and quarry faces of Trwyn Dwlban and Castell-mawr provide important exposures of Carboniferous Limestone strata. The rocks consist of limestone and sandstone layers, part of the Benllech Limestone, formed during the Brigantian Stage of the Lower Carboniferous Period. This is the type or reference locality for several members of the Benllech Limestone. Sedimentary structures of outstanding interest are displayed, including sandstone-filled pipes cutting the limestone layers, assumed to have formed by the collapse of wet sand into fissures opened by earth movements. These are associated with large cylindrical pits, also filled with sandstone, which are amongst the largest of their kind in Britain. They are thought to have formed by palaeokarstic processes, i.e. removal of the limestone in solution. This is a nationally important Carboniferous Limestone site with considerable potential for future research. The Carboniferous Limestone at Trwyn Dwlban is overlain by late Pleistocene deposits of great importance in unravelling the glacial/interglacial history of North Wales. A well-developed raised shore platform is underlain by a mixture of cemented angular limestone head and raised beach sediments which pass upwards into a pure limestone head. Irish Sea till overlies the head deposits and sometimes rests directly on the shore platform which exhibits excellent striations. This is one of only two sites known in North Wales where raised beach sediments of undisputed origin occur. It also records the clearest evidence from any section in North Wales for a sequence of environmental changes ranging from interglacial conditions with high sea-levels, through a period of severe periglacial climate, to full glacial conditions and a major incursion of ice from the Irish Sea. The site is unique in North Wales and provides a crucial link with South Wales, where raised beaches are a common element of the Pleistocene succession. Trwyn Dwlban is therefore a site of exceptional significance in developing a Late Pleistocene chronology for North Wales.