Commentary Inf XIII 16-19

The poet sets Virgil the chore of 'diagramming' the protagonist's present location with regard to his next one: the burning sand ([Inf XIV 13]) that is the stage upon which are punished those who offended 'God's child,' nature, in the third and final 'ring' of the seventh Circle. Here, in the second ring of that circle, the protagonist finds a forest (in which the 'trees' are the souls of the dead) bordered by a river (of burning blood).