It looks like to me the only purpose of the rock between the joist is to plug the hole.
I’d get rid of enough of the rock so you can block in between the joist, then continue the sheathing down to the bottom of the plate. Then you can lag a ledger to go through the sheathing into the blocks.
Be sure to get some tar paper or ice and water shield over the sheathing before you install the ledger.
Agreed... you definitely want to make sure to get a watershield over the sheathing.