The Ordovician Rocks of Arvon

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Abstract
Summary: The Ordovician of Arvon lies between that of Anglesey and that of the mountainland of Snowdonia. Brief petrological studies are made. The zonal succession is much restricted, for five of the higher Ordovician zones are missing. An interesting faunal feature is the richness of the Extensus Zone in Azygograptus eivionicus . The total thickness of the succession is rather more than 2500 feet. The base of the Extensus Zone sweeps unconformably across all the older systems. The Ordovician facies is marine, like that of Anglesey, the Volcanic Series of Snowdon having probably thinned away before it reached as far north as Arvon. Reasons also are adduced for supposing that all the higher zones have been swept away by waste. The curvature of the Ordovician boundaries indicates extensive thrusting. One or two thrusts are visible, a section in an abandoned mine adit proving that the Arvonian rhyolite has been driven, horizontally, over Ordovician shales. The several Ordovician tracts are briefly described.

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