Not sure where it came from but I was under the impression it was a socket (dug into the bedrock I suppose), tightly packed with chalk rubble, sarsen chips, mauls etc. Whether all of the packing was all done after the stone was in, I don't know. i.e. not sure if the stuff under the stone was pre-planned rubble (seems like an ideal impact bed) or if some of it was the result of the sarsen smashing into the bedrock and cracking it up. Knowing which, in itself, would give a clue to how the stone went in, but it would need quite a technical eye to analyse it I would think.