The Mishnah on Sanhedrin 60 discusses idol worship. Mercurius was the patron of the highways and travelers. This position led many to erect statues of him on crossroads. Oftentimes, these representations presented just the head of the idol and passersby would place stones at the foot of the statue. On occasion the representation was simply a pile of rocks, and travelers who passed by the pile would toss their own stone on it as an offering to the god.
(This image was taken from the Hebrew edition of the Steinsaltz Talmud, Tractate Sanhedrin, page 260. To order your copy of the Steinsaltz Talmud, click here (Hebrew site), contact your local Jewish bookstore, or e-mail Milta Books at email@example.com.)