A boulder to the west of Carn Alw (Mynydd Preseli) showing Flow Banding.
Due to the high silica content, Rhyolite Volcanic lava is very viscous and flows slowly.
Flow banding , which manifests as a banded, streaky appearance / swirling layers
of different color or granule size .
Caused by friction of the viscous magma in contact with a solid rock interface,
usually the wall rock to an intrusive chamber or if the magma is erupted,
the surface of the earth across which a lava is flowing.
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Exposed rocks showing flowbanding can also be found at the ends of the promontories
Y Penrhyn and Penfathach , west of Pen Anglas - Goodwick ,
also near the coast path to the west of Cwm Bach - Newgale.
Ref: 12119
Date: 01/08/2013
Location: SN 1375.3371
