Bwa Ddu SH 26077631

The Bwa Du Fault is of regional importance as one of the best-exposed major faults on Anglesey. It is expressed as a 15m-wide gully between the ruggedly exposed yellow quartziteof the Rhoscolyn Formation to its SE and the lower-lying cliffs of the green-grey schists of the New Harbour Formation to its NW. The fault zone has a trend of 120° and a steep, nearvertical, dip. The complex faulting and juxtaposition of the Rhoscolyn Formation (South Stack Group) with the New Harbour Group of Precambrian rocks, together with later small-scale folds in the schist, make this an exciting research site and excellent teaching material for students.

Graig Wen & Porth Wen SH 40259470 & SH 40009466
This site provides rare exposure of the unconformity between the Precambrian Gwna Group schists and quartzites (Greenly, 1919) and the Ordovician (Arenig) conglomerates of the Porth Cynfor Formation and the sandstones of the Torllwyn Formation above (Bates, 1972). The Ordovician conglomerates are spectacularly exposed on the west shore of Porth Wen (SH40259470), whilst the unconformity between the quartzites of the Gwna Group and the Ordovician conglomerates is well seen on the Graig Wen ridge to the west at SH 40009466. This unconformity is important in understanding the palaeogeography of this area in late Precambrian to early Ordovician times, a time span of around 100 million years. It demonstrates that the sedimentary rocks of the Precambrian must have been involved in uplift and erosion during a tectonic event prior to the area being transgressed by the sea and new sediments being deposited at the onset of the Ordovician Period. Since no intervening Cambrian rocks are to be found in this area, this limits the duration of this tectonic event.

Llanbadrig Point Coast SH 375946-373947
The rocks on the steep northern coast of this headland offer the finest view of the Gwna Mélange in Anglesey. The mélange consists of a jumble of blocks of rock, ranging in size from several tens of metres to a few centimetres across, mainly quartzite, limestone, and phyllite. The mélange is thought to be the product of submarine slumping. As Greenly 1919 remarked “the many-coloured mélange is really indescribable and must be seen in the field to be envisaged”. Apart from the three major rock types, the matrix includes a bewildering mosaic of rock types one within another. Grey limestone over 200 m across forms the largest olistolith in the mélange. This site is a good place to view the Ogof Gynfor unconformity between the Precambrian Gwna Group schists and quartzites (Greenly, 1919) and the overlying Ordovician
(Arenig) conglomerates of the Porth Cynfor Formation. Porth Wen, a short distance to the west exhibits a similar unconformity (see Graig Wen & Porth Wen RIGS). The unconformity is important in understanding the palaeogeography of this area in late Precambrian to early Ordovician times, a time span of around 100 million years and proves that during the
intervening period some great earth movements occurred (orogeny) where the mélange which had accumulated in a deep-sea trench was uplifted prior to the area being transgressed by the sea and new sediments being deposited at the onset of the Ordovician Period.

Porth y Corwgl SH 26077631
Porth-y-Corwgl is of regional importance as one of the best-exposed faults on Anglesey, which was considered by Greenly (1919) to be the eastern extension of the Carmel Head Thrust. This faulted junction between the Ordovician (Llandeilo) sediments to the south and the Precambrian New Harbour Group rocks to the north represents a time difference of more than 100 million years between the rocks to the north and those to the south. Much of the interest at this site lies in the opposing, polarized theories as to the type of fault which exists here and its relationship, or otherwise, to the Carmel Head Thrust plane on the opposite side of the island. The Ordovician rocks comprise conglomeratic slates and grits that have suffered a single deformation, whereas those in the Precambrian to the north of the fault show complex metamorphic changes in the phyllites (schists which retain some sedimentary features) and spilites (pillow lavas). These exposures raise an important question - are they the lateral equivalent of the Carmel Head Thrust as described by Greenly, or are they a reverse fault formed later than the Carmel Head Thrust as described by Barber et.al. (1981)? Greenly based his theory on the discrepancy of age on either side of the fault, but this is now considered to be part of a diachronous transgression (gradual change in sedimentation through time). However, Barber et.al. claim the Porth y Corwgl fault occurred later than the Carmel Head Thrust and it is similar to some 40 other high-dipping faults in the area which would displace the Carmel Head Thrust in this area down on its north side. Bates (1974) and Greenly (1919) note that the low angle dip at Carmel Head is not typical and suggest that Porth-y-Corwgl is merely the Carmel Head Thrust in an unusually steeply dipping section. This controversy ensures the importance of this site for research for some time to come.

Porth y Pwll SH 244793
Porth-y-Pwll is of regional importance as it exemplifies the Precambrian New Harbour Group rocks, in a less complicated manner than other exposures in this formation. The beds are parallel to the schistosity and so are technically called phyllites. The rocks are composed of the minerals, chlorite, muscovite (white mica), and elongated, sub-parallel grains of quartz and felspar which helps to make up a strong schistosity. Quartz veins follow the schistosity around the folds and were either injected along the schistose layers or were formed by removal of the quartz from the original rock before or after the folding. The steep limbs of some folds are marked by an earlier generation of tight folds. Looking back from the seaward side towards the cliffs, this is the best place to observe the characteristic large-scale folds of the New Harbour Formation. The context of the structures in this formation is difficult to understand in relation to the other formations, for example, the South Stack Formation shows one major folding event whereas, the New Harbour Group of rocks exhibits two major folding sequences. It has long been thought that the South Stack Group was the oldest, but some current research reverses the proposed age sequence of the groups and considers the South Stack Group to be the youngest in the stratigraphic sequence. The uncertainties involved in the relative ages of the various groups makes this an important player in the controversy and could help stratigraphers to unravel the true sequence of formation of these groups of rocks.

Rhoscolyn Head SH 25897517
Rhoscolyn Head is of regional importance as it contains the most spectacular de-watering structures on Anglesey. They have not been described elsewhere in the South Stack Formation and are important in demonstrating that the stratigraphic succession of this formation is the right way up. This formation consists of a half-kilometre-thick sequence of slightly metamorphosed sandstones deposited by a fast-moving current on the sea bed (turbidites) which alternate with beds of mudstone. The de-watering structures are displayed in a bed some 70m long at the top of the cliff and can be seen from the seaward side. The water-escape structures have rounded bases which alternate with pointed crests. They represent the escape routes of unstable water-rich beds sandwiched between more coherent layers of mudstone. These rare features are the only known examples in the Precambrian rocks of Anglesey and help us to understand the conditions and environment of deposition at a time around 500 - 600 million years ago. One possible explanation for their existence is that the unit marks a time of instability resulting from an earthquake.