Corianton "did go over into the land of Siron" (Alma 39:3), implying that the land of Siron was in somewhat of a valley, and that it was separated from the land of Antionum by an elevated divide or a body of water.
That the land of Siron was "among the borders of the Lamanites" might imply that the land of Siron was either a local land within the land of Antionum near the southern border, or that Siron was the name of the land on the other side of Antionum's southern border.