"The red sandstone dolmens that stood upright"
Poulnabrone dolmen (Co Clare, Ireland)
Creative Commons AttributionPoulnabrone dolmen (Co Clare, Ireland) - Credit: Steve Ford Elliott

Dolmens are prehistoric monuments usually formed by three or more uprights stones supporting a large flat horizontal capstone.

Most date from the early Neolithic period, and were usually covered with earth or smaller rocks to create a burial mound.