A compact but dramatic outcrop known as Ros-y-felin, with eye-catchingly exposed slivers of white rock, is situated on the floor of a deep and narrow valley [at 51.991686N, 4.74468W] near Pont Saeson (“Saxons Bridge”), immediately at the foot of the northern flank of the Carn Menyn ridge.
The bluestones at Stonehenge are not just one homogenous type. A large number are spotted dolorite from the Carn Menyn outcrops, often referred as the “Preselite” bluestones, but the rest are a mixture of types, the provenance of which have not all been tightly geographically identified, only that they are generally from the north Pembrokeshire area of Wales. A paper in the March 2011 issue of the Journal of Archaeological Science by Richard Bevins, Rob Ixer, and Nick Pearce summarised some years of work in which they had conducted petrological and chemical analyses of rocks along the Carn Menyn ridge and immediate environs. They have identified the Pont Saeson outcrop as almost certainly the source of at least one Stonehenge bluestone, a stump identified on Stonehenge plans as 32e -- it is a particular kind of rhyolite that comprises Ros-y-felin. Moreover, this type of stone seems to account for much of the so-called “debitage” at Stonehenge – i.e. bits and pieces and flakes of rock that have become buried in and around the monument.
We made a preliminary examination of the Pont Saeson rhyolite outcrop. This did not reveal any unusual acoustic properties of the rocks, though there were clear echoes when rocks were percussed, but the outcrop is decidedly visually distinctive, as the images on this webpage show.