A valley near Avebury is filled with immense sarsen blocks, resembling a 'river of stone', and perhaps laid there by prehistoric architects. There are also menhirs, dolmens and cromlechs. Surrounded as they were by forests and marshy hollows, it is clear that the downs were densely peopled at a very early period. Circles, formed by a ditch within a bank, are common, as are grave-mounds or barrows. These have been classified according to their shape as bell barrows, bowl barrows and long barrows. Bones, ash, tools, weapons and ornaments have been dug up from such mounds, many of which contain kistvaens or chambers of stone. The lynchets or terraces which score some of the hillsides are said to be the work of primitive early farmers and agriculturists.