A burial chamber with 4 uprights supporting a capstone (measuring 2.75m by 1.6m) with 110 cupmarks on top, another 8 are visible on the east side, at least two grooves also seem to be present. Additional art in the form of chevrons and circular motifs on a stone (aprox 0.7m long) to the east of Bachwen was photographed a few years ago by Tim Prevett and suggested possible chevron designs.

The Portal Dolmen is located approximately 200m from the sea and stands at around 24m AOD. The monument is sited on the lower north-western slopes of Bwlch Mawr. As with the nearby Penarth monument, Bachwen would have had an affinity with the sea and its economic resources. The site was investigated by Barnwell who describes in detail a large number of cupmarks that have been carved into the capstone (Barnwell 1867, 152). Lynch (1969a, 130) has suggested that drystone walling was used to infill the spaces between four uprights. Each upright stands over 1m high and supports a wedge-shaped capstone. The capstone, measuring 2.4m x 2.7m, is aligned east-west in order that the orientation of the capstone shall dip towards the sea. It is probable that the covering mound, measuring approximately 14m x 7.5m was constructed of stone would have been oriented similarly. Probable traces of this mound can still be seen. It has been suggested that further chambers once existed (Hemp 1926, 429), but the idea of a multi-chambered cairn is difficult to sustain. The notes of antiquarian observers, however, do suggest that traces of kerbing were visible in 1783 (Pennant 1783, PAGE NO).

Up to 110 cupmarks, each measuring roughly 5cm in diameter and 2cm in depth, are present on the capstone; similar to those found on the upper section of the capstone at the Trellyffant monument (PEM 2), near Newport, Pembrokeshire (Barker 1992, 19).[1] Two shallow grooves on the upper surface of the capstone link three cupmarks together. Up to eight cupmarks are located on the northern ridge of the capstone, while the rest are on the southern side, facing the mountains to the south and east. Similar groupings are found on Bronze Age rock-art panels in northern Britain. The eroded upper surface of the capstone does give support to the argument that capstones remained exposed during the period of use of the monument. It is even probable that the cupmarks date from the Bronze Age and represent a form of graffiti or ‘statementing’ of an ancestral monument. Alternatively, the cupmarks on the capstone may have been hidden and were for the eyes of the ancestors only, as suggested for art found within the chambers at Barclodiad-y-Gawres in Anglesey (Chippindale & Nash, forthcoming).

[1] Daniel (1950:117) suggests there are 70 cupmarks.