Geomorphology for dogs

Set deep in a narrow valley, the rocky crag known as Rhosyfelin has been credited as the Actual Source of certain of the Stonehenge bluestones that make up the inner circle of pillars.

When archaeologists stripped away much of the turf and vegetation that covered the sides of the crag they revealed vertical sections of rock that appeared to be ripe for removal with wooden wedges. (See extra)
And in digging below the rock face they unearthed this huge slab which, they  decided, was positioned ready to man-haul the 140 miles to Salisbury Plain. Except that it didn't happen, it remained here - perhaps because it's far bigger than the bluestone pillars and to drag it (or indeed any stones) up and out of this deep glacial valley would seem to make no sense when equally suitable stones could be tumbled down from nearby Preseli hills - and carried across land and sea..?

A distinguished local geomorphologist, Dr Brian John, has for years  challenged the notion that the bluestones were transported by human agency from West Wales to Wiltshire.  (A modern attempt to replicate the feat failed.) He believes that the glacial ice sheet that covered much of Wales and stretched across the Bristol Channel could have carried Pembrokeshire bluestones much closer to the site of Stonehenge and, when the ice melted, dumped them within striking distance for the later builders of Stonehenge.
He also disputes the idea that the stones from this site 'match' the stones at Stonehenge, pointing out that such accuracy is impossible when the glacial layering may run for miles in the same formation.

We had an instructive and thought-provoking  visit with Brian John in which he explained the geomorphology of Rhosyfelin and took us through the contentious issues, a debate that has been going on for years. Most of the archaeological 'evidence' is now covered up and grown over as nature  reclaims the site with a living layer of bracken, brambles and heather. The great slab holds on to its secrets:  Stonehenge dropout or a handy picnic table for local quarry workers?

(I see that Brian John has already updated his blog with some observations following our visit.)