Archaeology of the site indicates an original construction date from
the fifth millennium (1), (2). The original 'cairn'
was extended in both directions into its final shape and a pavement
surrounded the stepped tumulus.
The capstone dates the final construction phase for the tumulus at
approximately 3,300 BC along with the aforesaid structures.
Whittle says of this:
'In front of the south facade of the primary phase of the long cairn of Er
Grah, close to where the menhir Brise originally stood...a pair of
domesticated cattle were found in a pit (Tresset and Vigne, forthcoming;
Anne Tresset, pers. comm.; cf. Tresset 2000). Radiocarbon determinations
suggest a date in the late sixth and early fifth millennium