Jamieson, Fausset and Brown on Joshua 4:6-7

6-7. That this may be a sign among you--The erection of cairns, or huge piles of stones, as monuments of remarkable incidents has been common among all people, especially in the early and rude periods of their history. They are the established means of perpetuating the memory of important transactions, especially among the nomadic people of the East. Although there be no inscription engraved on them, the history and object of such simple monuments are traditionally preserved from age to age. Similar was the purpose contemplated by the conveyance of the twelve stones to Gilgal: it was that they might be a standing record to posterity of the miraculous passage of the Jordan.