Rocher de Tréal (Rock of Tréal) is a passage grave close to the megalithic complex of St. Just, dating back to around 4000BC. The main chamber is approximately 13 metres in length, with a width of just over a metre. The monument is covered by eight large capstones that decrease in size towards the eastern end.
Rocher de Tréal can be slightly awkward to find, and isn't brilliantly signposted. In fact, the whole area around Saint-Just is a signposting nightmare. Nevertheless, it's worth making the effort to track down. Access is easy, and it's a glorious monument, very unlike anything you'll see in the UK, despite superficial similarities.