COUNTRYSIDE COUNCIL FOR WALES
SITE OF SPECIAL SCIENTIFIC INTEREST: CITATION
ANGLESEY LLANBADIG/DINAS GYNFOR
National Grid Reference: SH372941; SH372945 to SH379949; SH388951 to SH396949; SH383945
Site Area: 27.2 ha
This site is designated for its geological interest.
This coastal section is justly famous for the well exposed contact between the ancient Mona Complex and its younger cover of fossiliferous rocks (which are of "Lower Ordovician" age - about 490 million years old). This contact is a fine example of what is known as a geological unconformity and demonstrates the passing of an enormous period of time between the formation of the Mona Complex and the deposition of the over-lying Ordovician rocks. The exposure of this unconformity is unique and is a vital piece of evidence proving the Mona Complex to be the oldest rock group exposed in Anglesey. Beneath the unconformity are splendid exposures of the peculiar rock unit known as the Gwna Melange: a chaotic jumble of hard rock fragments immersed in a softer, green, slatey matrix. The Gwna Melange is only found in the Mona Complex and this is one of the best and most accessible exposures.
At Penrhyn Mawr Quarry a huge lump of limestone in the melange contains fossilised algae over 500 million years old -the only such fossils ever found in the Mona Complex. This site also provides Key exposures of the Lower Ordovician rocks of Anglesey. Here the basal breccias, conglomerates and sandstones of the Arenig - Llanvirn rest on the Precambrian Gwna Melange. These sediments represent the thickest conglomeratic development in the Welsh Basin, and it has been suggested that the basal units may represent an early Ordovician terrestrial facies deposited on the northern margin of that depositional basin. These unique exposures showing basin edge facies are critical to palaeogeographic studies of the Ordovician. The great diversity of interest provided by these magnificent exposures makes this site one of the most important geological localities in North Wales.