Abram's Altars

has anyone else noticed the two altars in Genesis 12?

Gen 12.6 Abram traveled through the land as far as the site of the great tree of Moreh at Shechem. But the Canaanites were in the land. 7 The LORD appeared to Abram and said, “To your offspring I will give this land.” So he built an altar there to the LORD, who had appeared to him.

Gen 12.8  From there he went on toward the hills east of Bethel and pitched his tent, with Bethel on the west and Ai on the east. There he built an altar to the LORD and called on the name of the LORD.

Gen 12.9  Then Abram set out and continued toward the Negev.

the first altar seems to be about thanksgiving. upon entering the promised land, but finding it occupied, God promises, and Abraham gives thanks for this promise despite appearances by making the altar (and theoretically sacrificing upon it also).

but then he makes another one, seemingly with no purpose.

my thinking is this:
all of Abram's travels were directed by something specific.

• Ur to Harran because of his dad
  
• Harran to Canaan because of God's word
  
• Shechem to Bethel-Ai because God speaks of future promise
  
• Bethel-Ai to Negev because of [something to do with the altar]?
  
• Negev to Egypt because of famine
  
• Egypt to Negev because of the Pharaoh
  
• Negev to Bethel-Ai because of flock size
  
• and so on

so the missing part of the puzzle seems to be the altar between Bethel and Ai, and my thinking is that this altar is part of entreating God, 'calling on the name of the LORD'. i haven't seen any other evidence for this use of the altar. Saul's pre-emptive sacrificing seems more about sanctifying the battle rather than asking for guidance - the normal most Exodus means for guidance seems to be the Urim and Thummim. i couldn't find anyone who agreed with me (let alone mentioning the possible importance of the second altar), until i came across Calvin on prayer. in Inst. III.xx.9

Hence, under the law it was necessary to consecrate prayers by the expiation of blood, both that they might be accepted, and that the people might be warned that they were unworthy of the high privilege until, being purged from their defilements, they founded their confidence in prayer entirely on the mercy of God.

the Battles edition of the Institutes i have lists Genesis 12.8 as a cross-reference here, that is, he was praying to God for guidance, direction, and proleptically, pointing to Jesus, the high priest who, because of his shed blood, is able to intercede directly with the Father on our behalf.