Jamieson, Fausset and Brown on Joshua 8:30-31

30-31. Then Joshua built an altar unto the Lord God of Israel in mount Ebal--(See on De 27:11). This spot was little short of twenty miles from Ai. The march through a hostile country and the unmolested performance of the religious ceremonial observed at this mountain, would be greatly facilitated, through the blessing of God, by the disastrous fall of Ai. The solemn duty was to be attended to at the first convenient opportunity after the entrance into Canaan (De 27:2); and with this in view Joshua seems to have conducted the people through the mountainous region that intervened though no details of the journey have been recorded. Ebal was on the north, opposite to Gerizim, which was on the south side of the town Sichem (Nablous).