COUNTRYSIDE COUNCIL FOR WALES
SITE OF SPECIAL SCIENTIFIC INTEREST CITATION
ANGLESEY TRAETH LLIGWY
Date of Notification: 1993
National Grid Reference: SH491884, SH494878, SH500872
O.S. Maps: 1:50000 Sheet number: 114
1:25000 Sheet number: SH48, SH58
Site Area: 25.7 ha
Traeth Lligwy lies on the north-east coast of Anglesey between Amlwch and Moelfre. It has been selected for three features of geological interest.
At Traeth yr Ora and Trwyn Forth y Mor. the rocks reveal ancient meandering river-eposits in the Old Red Sandstone. The coastal outcrops here, of the Devonian Porth y Mor Formation (exact age unknown) consist of alternating fluvial sand bodies and verbank shales with calcrete soil-profiles. This was the first section where epsilon ross-stratification was recognised, and shown to be characteristic of point bars of meandering rivers. It remains one of the best sections for observing deposits of eandering. mixed bedload rivers in the British Devonian.
Lligwy Bay contains a very rare example of deformed Old Red Sandstone rocks north of The Hercynian Front. As Anglesey contains the only Old Red Sandstone locality in North Wales, these rocks provide unique information with which the duration of the Caledonian Orogeny in this region can be assessed. The Old Red Sandstone rocks on the north side of Lligwy Bay record polyphase deformation involving folding cleavage formation and thrusting. Two upper formations of the Old Red Sandstone. the Porth-y-Mor Formation and the Traeth Lligwy Formation, lie in a broad synclinal structure. This open structure has a monoclinal fold on its northern limb, while on the southern limb thrusting and light minor folds with axial planar cleavage occur. The intensity of the deformation which affected the Old Red Sandstone sediments of Lligwy Bay was at least as strong as that suffered by the unconformably underlying Ordovician succession. This indicates that Caledonian deformation of some significance extended beyond the end of the Silurian.
From Carreg Ddafad to Trwyn Gribin is the "type" and best locality for the Lligwy Bay conglomerate a diachronous basal unit to the Carboniferous Limestone. This deposit of Asbian (Dl) Brigantian (D2) age marks the transgressive encroachment of the late inantian sea onto the north side of St Georges Land and is an important locality vital to our understanding of palaeogeographic changes and sedimentation in late Lower carboniferous times.
This site has been selected as a result of the former Nature Conservancy Council’s Geological Conservation Review, a national survey and evaluation of sites of geological and physiographical interest.