Joshua 4:9

9.And Joshua set up twelve stones, etc Apparently there was no use of stones under the water, and it may therefore seem to have been absurd to bury stones at a depth. The others which were placed in Gilgal being publicly visible, furnished occasion for inquiry; but stones hidden from the eyes of men at the bottom of the water could have no effect in inciting their minds. I admit that a monument altogether buried in silence would have been useless. (51) But when they talked among themselves of the evidence of the passage left there, the hearing even of what they did not see, strongly tended to confirm their faith. The ark of the covenant was shut up in the sanctuary and covered by a veil placed over against it, and yet its hidden splendor was not without benefit, when they learned from the Law that the covenant of God was deposited in it. It might also happen, that when the river was low, the tops of the heap would sometimes appear. But what I have already said is more probable, that though Joshua buried the stones in the middle of the stream, he did a useful act by establishing a testimony in presence of the people, which would afterwards become the subject of general conversation.