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.
The geologists haven't been that specific about the location. The Senni glacier flowed north? Well, it might have done for a very short time, but for most of the time, during the big glaciations, the ice from this part of mid Wales was flowing south, towards the Bristol Channel. By the way, the Senni Beds are not just in the Senni Valley -- they outcrop over a wide area including the lower Towy Valley.
I think the Altar Stone was carried first by a tributary glacier of the Irish Sea Glacier flowing south, then picked up and moved east by the main ice stream. ALL in one glaciation? Maybe. But maybe over the course of several glacial episodes.