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 – Anglesey east 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 Penrhyn Point
Caravan Park (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 water level. Thin chilled margins are observed although there is little
evidence of baking as fossils 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.
Location 1
basalt/dolerite
dyke
bioclastic nodular
limestone
limestone
joints
dyke
dip
dyke margin
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 of the 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 confirms evidence from erratics that the last
direction of ice movement was towards the SW from the Lake District. However, this
direction of movement can not be interpreted from the striations alone.
Penrhyn Point
Location 2
“fossil” potholes
ruined house
Loc.3
till
limestone
pavement
pothole
striation
orientation
Ice direction
Location 4
Location 3
Location 5.
Further south from Location 4, at a small headland before the boulder-strewn cliff
(Loc. 5), the limestone in the cliff and in fallen blocks is dominated by beds of reefbuilding 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.
Loc. 5
Loc. 4
Loc. 3
brachiopods in
“life position”
silicified
coral colony
chert nodules
Return to the parking area via the coastal path.