And when they came unto the borders of Jordan, that are in the land of Canaan, the children of Reuben and the children of Gad and the half tribe of Manasseh built there an altar by Jordan, a great altar to see to. The borders of Jordan, that are in - Canaan - This verse can never mean that they built the altar on the west side of Jordan, for this was not in their territories; nor could it be a place for the purpose of public worship to their own people, if built on the opposite side of Jordan; besides, the next verse says it was built over against the land of Canaan. It appears that when they came to the river they formed the purpose of building the altar; and when they had crossed it they executed their purpose.
A great altar to see to - A vast mass of earth, stones, etc., elevated to a great height, to serve as a memorial of the transactions that had already taken place. Probably it was intended also to serve as a kind of watchtower, being of a stupendous height, altare infinitae magnitudinis, an altar of an immense size, as the Vulgate terms it.