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The above map and key come from the excellent -- but rather specialised -- British Regional Geology volume for Wales. The igneous rocks here are approx the same age (around 463 million years old) as those belonging to the Fishguard Volcanic series (Llanvirn: Ordovician) on the northern flank of Mynydd Preseli. Of particular interest are the igneous tocks from the southern part of the island -- including porphyritic rhyolites, turbiditic tuffs, ash fall tuffs, ash flow tuffs, and brecciated lavas. Then there is a big microtonalite mass on the NW flank of the island.
For many years it has been known that these rock outcrops have provided entrained material for the overriding Irish Sea Glacier -- on at least two occasions -- and that erratics from Ramsey are widespread. The super-erratics near the Sleek Stone, Broad Haven, have almost certainly come from Ramsey, and from our 2014 trip to Flat Holm we have strong suspicions that there is a lot of Ramsey material there too. Sid Howells is hoping to take a careful look at the collected samples this spring -- so watch this space!