The narrow isthmus on Bardsey Island separates the settled and farmed `mainland? to the north from the low-lying rocky southern peninsula which terminates in Pen Diban, upon which the lighthouse was constructed in 1821. The isthmus is defined by the sweeping bay of Henllwyn on the south-east side, now used for modern landings with the concrete slipway and boathouse located at the eastern end, and the smaller sandy bay of Porth Solfach on the north side together with the rocky stretch of lower-lying coast on the west, Porth Hadog, these two inlets separated by the angular headland of Trwyn Dihiryn.

Coastal erosion is aggressively affecting both sides of this geologically weaker section of Bardsey Island, with Porth Hadog and Porth Solfach bearing the brunt of the most severe westerlies and north-westerly storms respectively, as evidenced by storm-thrown deposits upcast from the beaches, with the soft sediments of the more sheltered bay of Henllwyn still susceptible to southerly gales and storm surges.

The exposed section at Henllwyn changes annually, and has been intermittently recorded for its eroding archaeology since 2003. Basic comparison of the mapped high water mark with First Edition Ordnance Survey mapping from c. 1890 suggest as little as 8m has been lost from the eastern, slipway, end of the bay while up to 26m has been lost from its south-western end.

Recorded for the 2003 Bardsey Island Lithic Scatters project, and for the EU funded Ireland-Wales CHERISH Project 2017-2021.

T. Driver, RCAHMW, 2019.