Cerrig Duon is the name given to an egg shaped stone ring located in the valley of the river
Tawe (SN852206) north of the Dan-yr-Ogof Show caves in the upper Swansea valley. The stones
in this ring are quite small and the ring diameter is approximately 59ft. 10ft north of the ring is a large standing stone called
Maen Mawr 'Great Stone' approximately 6ft high with two smaller stones in alignement to
the north of this stone. I was puzzled as to the purpose of these small stones until I
noticed that Maen Mawr cast a long shadow to the north when the sun was not obscured by
clouds. I calculated that the nearest small stone to Maen Mawr marked the position of the
shadow cast by this menhir when the sun's rays reached the ground at an angle of 23.5
degrees. This marked the northernmost position of the shadow at mid-day on midsummers day.
To the northwest of the ring are a series of spaced out stones which point in the direction of an arrangment of four stones. This alignement coincides with the midsummer sunset. A tapering avenue of small stones
approaches the ring uphill from a NNE direction but stops short of the ring to its west.
(click on image for hi-res photo)
View of ring from west.
View looking n.w. to midsummer sunset and four stones.
View looking northwest to four stones.
Maen Mawr viewed from the south.
Maen Mawr and marker stones viewed from the east.
Maen Mawr viewed from the north west.
