Joshua 4:7

Then ye shall answer them
By informing them of the design and use of them:

that the waters of Jordan were cut off before the ark of the covenant
of the Lord when it passed over Jordan;
the waters below from those that were, above, which stood up on an heap; so that they were divided and separated from each other, and made dry land for a passage of the children of Israel; and this was done before, and in the presence of the ark of the covenant, to show that is was owing to the power of God, of whose presence the ark was a symbol:

the waters of Jordan were cut off;
which is repeated for the confirmation of it, and that it might be taken notice of as a very marvellous event, and to be ascribed to the divine omnipotence and goodness:

and these stones shall be for a memorial unto the children of Israel
for ever;
which, whenever seen and observed by them, would put them in mind of this wonderful appearance of God for them; and Jerom ^F8 speaks of them as if seen by Paula in his time, whose life and travels in those parts are written by him.