Abstract

The mountainous headland of Penmaenbach has long been an obstacle to communications west of the River Conway in North Wales. Before 1826 the headland was passed either by crossing the beach at low tide (Fig. 1), near where the photograph was taken, or by way of the more difficult inland route over the Sychnant Pass between Conway and Penmaenmawr. Also shown in Fig. 1 are the 1826 Telford Road, the tunnel through the headland for the Chester and Holyhead Railway, built in 1846, and the short tunnel for the existing A55 road constructed in 1932.

As an important part of its plan to improve the road system in this area HM Welsh office are implementing a substantial improvement scheme for the A55 coast road that will require a second road tunnel through the headland.

The rock seen in the photograph is a rhyolite, thought to be the vent of a volcano penecontemporaneous with the Ordovician rock in the area. Being significantly more resistant to erosion than the surrounding sediments the vent is now exposed as the headland with near vertical cliffs to the beach.

The rhyolite is exposed in many place allowing examination of the jointing pattern. Measurement of discontinuities reveal no generally preferred orientation, although in localized areas discreet joint sets are readily identifiable. In the area of the photograph three sets can be clearly recognized plus some random jointing. The sets are designated S1S2 and S3 are highlighted in Fig. 2 by omitting all but one set

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