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Thursday, 8 November 2018

Cemented raised beaches in Pembrokeshire

The raised beach resting on its rock platform at Poppit in the Teifi estuary.  here the beach is stained with iron oxide and manganese oxide, and is partly cemented.

Another exposure of the raised beach at Poppit, showing a basal boulder bed and an overlying beach made of smaller cobbles.  Does this sequence represent a transgression? All the deposits are stained and partly cemented.

This Poppit section of the raised beach shows distinct layering of pebble beds and beds of sand and shingle.  Drapes of colluvium and sandrock above.  There are strong similarities here with some of the raised beach exposures on the Isles of Scilly.

Poppit — parts of the raised beach are cemented with iron and manganese oxides — up to 1.7m thick. Much lateral variation — traceable along the coast for almost 1 km. Towards E extremity, the raised beach is associated with up to 2m of stratified sands and silts, although elsewhere it is overlain by blocky lower head. Above that, Irish Sea till and thinner upper head.

Here are some notes on other important localities:

North Pembrokeshire

A small exposure of cemented raised beach is plastered against wall of a cove to E of Ogof Golchfa — 75 cm of beach pebbles, overlain by 50 cm of shingle and then 45 cm of sand. All stained and cemented with iron oxide and manganese oxide.

The raised beach exposed at the main Ogof Golchfa exposure is NOT cemented. Leach described cemented raised beach deposits here, but they are no longer visible.

See: The Pleistocene Drift Succession at Porth-Clais, Pembrokeshire. Brian John.

At Whitesands Bay (SM733 273), south of the stream, large igneous boulders are embedded in cemented raised beach shingle and are seen resting on a low RB platform. Above that there is main head (uncemented) and then a cemented sandrock.

Cemented and stained raised beach shingle also exposed sometimes at the base of the section in Druidston Haven.

Caerbwdy — fluvioglacial deposits are seen beneath c 3m of stony local till. No raised beach seen, but could well be present.


South Pembrokeshire

Broad Haven South — calcite cemented shelly raised beach is seen on the platform, overlain by cemented lower head. Sandrock in places over that, and then more cemented head. The sandrock may be a localised patch or lense. Then we see non-cemented fresh head on top of that, and finally up to 2 m of unconsolidated blown sands and sandloess and then soil.

West Angle — a stained raised beach (50 cm) is seldom seen, but is overlain by a sandy deposit with raised beach pebbles and mixed with head. Above that, the enigmatic silt and clay series…….

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