The Altar stone , according to the latest info, came from the Brecon Beacons, and not Pembrokeshire as originally thought. This was one of those facts that were made to fit the sea route theory. It was thought to have come from Milford Haven, the embarcation point for the crafts that carried the Bluestones.
As it probably comes from the Senni valley south west of Brecon,there is a problem with both the sea route and the Glacier theory as far as the Altar stone is concerned. The Senni valley glacier flowed north, not south east to Somerset.
So, how did it get to Stonehenge.