Jamieson, Fausset and Brown on 2 Samuel 18:18

18. Absalom in his lifetime had reared up for himself a pillar--literally, "hand." In the valley of Jehoshaphat, on the east of Jerusalem, is a tomb or cenotaph, said to be this "pillar" or monument: it is twenty-four feet square, dome-topped, and reaches forty feet in height. This may occupy the spot, but cannot itself be the work of Absalom, as it evidently bears the style of a later architecture.