Trwyn Maen Dylan is host to (possibly) the largest erratic boulder that I have ever encountered on the shore of North Wales. Comprising dark-grey mudstone without any obvious features or cleavage, this monster required the serious attention of both my OSSP assistants. This stretch of coast is well-known for its abundant glacial till and glacio-fluvial deposits, and is well studied as part of the reconstruction of the later stages of the Devensian glacial period. (Photo: Jonathan Williams)
Grid ref: SH 425 523, near Aberdesach, Gwynedd -- N flank of Llyn Peninsula and SW of Caernarfon.
It's intriguing that this massive erratic is made of mudstone, rather than an igneous rock, which is generally assumed to be more "survivable" in glacial transport. It is presumably therefore much more difficult to work out where its possible provenance might be.
It's been famous for quite a long time, and indeed it is mentioned in one of the tales of the Mabinogion.
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