Commentaries:
Adam Clarke
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.
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