Pillow lavas of the Porth Maenmelyn Volcanic Formation (Ordovician, Llanvirn age) are well exposed adjacent to the Pembrokeshire Coastal Path just above and to the north of the eponymous bay.

These pillow lavas were formed by a submarine eruption, probably in an island arc setting, probably in not particularly deep water. The southerly dip (to the right in the photo) is mostly due to subsequent uplift and folding.