The Anglesey Geodiversity Project

“Rocky Scrambles”

in the

Anglesey Geopark

By Pete Loader

Teachers’ notes

Itinerary 1:Traeth Bychan and Penrhyn Point

“Hot rock, warm seas

and an icy past”

Aim – To study

  • structure and composition of the Carboniferous Limestone
  • a small igneous intrusion
  • palaeoenvironments – tropical coral sea and evidence of ice.
  • Other teaching opportunities – true and apparent dip, way-up criteria, coastal processes.

Location and general description of sections

  • Small bay and headland at the southern end of Traeth Bychan (between Moelfre and Benllech – Angleseyeast coast).
  • Distance from parking to shore: 600m.
  • Distance covered along section: 500m
  • Time: approximate 2.5 hours.
  • Terrain –gently dipping rocky shore and beach, accessible by footpath. Loose rock underfoot. Cliff sections–only high near location 1.

Equipment required

  • Field work sheets
  • Compass/clinometer
  • Hand lens
  • Measuring equipment

Safety considerations

  • Cliff sections – avoid where possible, hard hats required.
  • Locations 1 and 5 difficult at high tide- work on falling tide.
  • Rock scrambling on sharp wet rock surfaces – good footwear, care.

How to get there:

Parking is available in the lay-by on A5025, SW of Traeth Bychan (GR 513842) -large enough for a coach. Walk down the single track, bearing left at PenrhynPointCaravanPark(no access sign). Bear right at track to another caravan park following the coastal footpath sign (Dinas Farm). At signpost go through the gate on the right to the next sign for the coastal footpath (within 20 metres). Bear sharp right (towards garages) and continue left through gate along the coastal path beside the caravan park (on right). Access the shore at the slipway to location 1.

Itinerary 1: Traeth Bychan and Penrhyn Point

Location 1.

Immediately to the west of the slipway the high cliff (care!) can be seen to comprise nodular limestones, interbedded with thin shale, dipping gently (<10o) to SE. The limestone is bioclastic (mainly brachiopods, crinoid stems and other shell debris)with bioturbation (mixing of sediment by boring and burrowing animals) providing evidence of its shallow water origin, which fluctuated with periodic lower energy (deeper?) conditions forming the shale.

This can also be studied on the low cliffs further west of the boulders (Loc.1) where a small basalt/dolerite dyke, assumed from its NW-SE orientation to be of Tertiary age, follows major joint planes in the limestone which locally deflects the intrusion at mean high waterlevel. Thin chilled marginsare observed although there is little evidence of baking asfossils are found right next to the contact. Rough horizontal cooling joints in the dolerite can be observed. The dyke may be traced over the low promontory to the NW where it is has been eroded leaving a significant crevice suggesting it is less resistant than the limestone. This is a good area to demonstrate the difference between true and apparent dips on adjacent cliff faces.

Return to the coastal path at the top of the slipway. Turn left and a short way up the hill bear left to follow the grassy cliff-edge path towards Penrhyn Point, passing holiday chalets on right. Access the rocky shore to Locations 2 – 5 by unmarked tracks north of the ruined house.

Locations 2, 3 & 4

Here the rocky shore again dips gently to the SE with bedding surfaces showing features typical ofthe weathering and erosion of limestone. One main bedding surface (Loc. 2)well above mean high water is seen to be covered in “fossil” potholes formed by water during a break in sedimentation during the Carboniferous when the surface above water. These palaeokarstic features are up to 1m deep in places and often contain contemporary beach pebbles reminiscent of their past processes in their formation. In addition, the prominent bedding surface that crops out in the bay SE of the ruined house (Loc. 4)shows the development of a “limestone pavement” that may represent another palaeokarstic surface that owes much of its formation to a past, rather than present, erosive period. These features can be used effectively as “way-up criteria” to show the beds have not been overturned.

The unconsolidated drift (superficial) deposits overlying the limestone are of glacial origin. The contact between the limestone bedding surface at Location 4 and this till shows very clear evidence of glacial striations caused by the abrasive action of pebbles in the till grinding over the underlying limestone(Loc.3). The orientation of the striations is generally NE-SW and confirmsevidence from erratics that the last direction of ice movement was towards the SWfrom the Lake District. However, this direction of movement can not be interpreted from the striations alone.

Penrhyn Point Location 2

Location 4 Location 3

Location 5.

Further south from Location 4, at a small headland before the boulder-strewn cliff(Loc. 5), the limestonein the cliff and in fallen blocksis dominated by beds of reef-building corals (Lithistrotian and Dibunophyllum) typical of warm, shallow-marine conditions found today. These coral colonies have been beautifully preserved by silica during diagenesis and individual corals protrude from the limestone matrix which is less resistant.Locally chert nodules stand out as darker irregular patches on the limestone. Associated with these is a range of other “reef” fossils, particularly large brachiopods (Productus) many of which appear from their orientation to be in “life position” indicating little disturbance. This is another indicator that the beds are the “right way up”.

The effects of faulting/jointing and silicification of the limestone can be used to suggest an influence on the formation of the Penrhyn Point headland by coastal processes.

Return to the parking area via the coastal path.