I have many discussions with experts off the record, and in one recent exchange of opinions with a senior geomorphologist we have agreed to differ on what the "default hypothesis" should be, given that there is no "smoking gun" or "killer fact" in the glaciers versus humans debate. His view is essentially that of Prof James Scourse, who said many years ago that since there is no firm evidence of glaciation on Salisbury Plain we must therefore assume that glacial entrainment and transport of the bluestones was effectively "impossible" -- and that no matter how unlikely it might seem, the human agency thesis has to be the one to go for. (The reason for that is that we know that Neolithic people were rather clever, and if they were clever enough to know quite a lot, they probably knew how to carry large stones over long distances........)
I disagree fundamentally with that. Nobody should ever say that the glaciation of Salisbury Plain was impossible, especially since modelling shows that it was indeed possible, and since there are glacial deposits in the SW counties including Somerset. The Stonehenge bluestones themselves look like a glacial erratic assemblage, and if they look like erratics they may well be erratics! We must also remind ourselves that there is no physical evidence of any sort which supports the idea of either Neolithic quarrying in Pembrokeshire or long-distance human transport of the bluestones, and which withstands scrutiny.
In those circumstances the balance of probability must lie with the glacial entrainment and glacial transport thesis. Some evidence trumps no evidence, any time. In summary:
1. There is no sound evidence from anywhere in the British Neolithic / Bronze Age record of large stones being hauled over long distances (more than 5 km or so) for incorporation in a megalithic monument. The builders of Neolithic monuments across the UK simply used whatever large stones were at hand.
2. If ancestor or tribute stones were being transported to Stonehenge, why have all of the known bluestones come from the west, and not from any other points of the compass? Were belief systems and "local politics" quite different to the north, east and south?
3. There is no evidence either from West Wales or from anywhere else of bluestones (or spotted dolerite or Rhosyfelin rhyolite in particular) being used preferentially in megalithic monuments, or revered in any way. The builders always used whatever was available to them in the vicinity, and it can be argued that stone availability was a prime locational determinant for stone settings.
4. If long-distance stone haulage was "the great thing" for the builders of Stonehenge, why is there no evidence of the development of the appropriate haulage technology leading up to the late Neolithic, and a decline afterwards? It is a complete technological aberration.
5. The evidence for Neolithic quarrying activity in key locations is questionable. No physical evidence has ever been found of ropes, rollers, trackways, sledges, abandoned stones, quarrymen's camps, or anything else that might bolster the hypothesis. The so-called “engineering features” are entirely natural.
6. The sheer variety of bluestone types (near 30 when one includes packing stones and debris) argues against selection and human transport. There cannot possibly have been ten or more "bluestone quarries" scattered across West Wales.
7. Bits and pieces of experimental archaeology on stone haulage techniques (normally in "ideal" conditions) have done nothing to show that our ancestors could cope with the sheer physical difficulty of stone haulage across the heavily-wooded Neolithic terrain of West Wales (characterised by bogs, cataracts, steep slopes and very few clearings) or around the rocky coast. The one reasonably "authentic" project (the moving of the "Millennium Stone" in the year 2000) was a shambles and a disaster.
8. Neither has it been shown that the Stonehenge builders had the geographical awareness and navigational ability to undertake long and highly complex journeys with very heavy loads.
9. And if there was a "proto-Stonehenge" somewhere, built of assorted local stones and then dismantled and taken off to Stonehenge, where was it? The mooted "Preselite" axe factory has never been found, and neither has the mythical Stonehenge precursor.
10. Analyses of bluestone monolith stone shapes does not suggest that elongated “pillars” were preferred. Slabs, stumps and boulders of all shapes and sizes are highly suggestive of a glacial erratic assemblage.