Albert Barnes Commentary Deuteronomy 27:2

"And it shall be on the day when ye shall pass over the Jordan unto the land which Jehovah thy God giveth thee, that thou shalt set thee up great stones, and plaster them with plaster:" — Deuteronomy 27:2 (ASV)

The stones here named are not those of which the altar (Deuteronomy 27:5) was to be built, but are to serve as a separate monument witnessing to the fact that the people took possession of the land by virtue of the Law inscribed on them and with an acknowledgment of its obligations.