Adam Clarke on Joshua 4:20

Those twelve stones - It is very likely that a base of mason-work was erected of some considerable height, and then the twelve stones placed on the top of it; and that this was the case both in Jordan and in Gilgal: for twelve such stones as a man could carry a considerable way on his shoulder, see Jos 4:5, could scarcely have made any observable altar, or pillar of memorial: but erected on a high base of mason-work they would be very conspicuous, and thus properly answer the end for which God ordered them to be set up.