A stone cairn lies over the burial chamber itself. Some examples have a cup-shaped entrance similar to the Court Cairn tombs of Ireland.
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In the so-called "slab type", uncut slabs are buried on end in the ground, and are arranged side-by-side. There is usually a central stele, which is the largest (up to 4 m in height) slab and has a doorway cut through it. The sepulchres have a characteristic rectangular plan with an apse. The burial chamber is usually 5 to 15 meters long and 1 to 2 meters high. The structures were originally covered by a mound resembling the shape of an overturned ship. Near the entrance was an obelisk (betile in Sardinian), which symbolizes the gods or ancestors who watched over the dead.
http://en.wikipedia.org/wiki/Giant%27s_grave