On elongated erratics

The gallery of glacial erratics pasted in above illustrates the fact that for the most part erratics are referred to as "boulders" since their length:width:depth ratios are not far removed from 1:1:1.  Sometimes the length of the boulder can be twice that of the width, but there are not many cases in the literature where the proportions go up to 3:1:1.  In other words, pillars or columns are rare.  The reason is that during glacial transport, even if a pillar is entrained by the ice,  the stresses placed upon it are so huge that breakage is almost inevitable -- except in rare cases where the debris falls onto the glacier and is transported along on or near the ice surface.  James Scourse goes so far as to argue that pillars the shape of the bluestone pillars at Stonehenge can not possibly have been transported by ice all the way from Pembrokeshire to Stonehenge without being broken down into chunks or boulders.  That is a reasonable argument, and I can see where James is coming from.

However, look at the photos in a bit more detail, and you can see that some erratics are fissured or bedded to such an extent that it is surprising that they have not been broken up into much smaller flattish slabs during entrainment, transport and dumping.  Look at the bedding in the third erratic from the top -- part of the Big Rocks erratic train in North America.  And the bottom two photos -- one from western Ireland and the other from Lofoten, in Norway, show that elongated erratics can be transported and dumped without being broken into small pieces.  Are those occurrences freaky or unusual?  Yes, to some degree they are -- but even if only 5% of erratics of this shape survive, at least we have a reminder that we should never use the word "impossible" in our discussions.

NOW HERE'S MY LATEST BIG IDEA.

What if a number of "giant erratics" were entrained by the Irish Sea Glacier as it crossed Pembrokeshire and were carried to Salisbury Plain during the Anglian Glaciation?  By "giant erratics" I mean room-sized erratics -- or even larger, weighing hundreds or even thousands of tonnes.  The Big Rocks erratic group at Okotoks is the classic example -- but there are plenty of others, some of which I have posted up before on this blog.  Look at these:

Notice that thebottom two erratics are fractured, foliated or bedded -- again it is surprising that they have travelled on or in ice without being broken up.  Maybe they were even bigger to start with.......

So here's my big idea -- several rather big slabs of rock were picked up by overriding ice in Pembrokeshire (for example from the Craig Rhosyfelin area, from Carn Goedog and maybe Carn Meini) and dumped at or near Stonehenge in a cluster.  The Neolithic builders of Stonehenge built the monument there because they had an easy source of raw materials conveniently to hand.  So they "quarried" their slabs and pillars there, on the spot -- not in the "bluestone quarries" in Pembrokeshire that have so obsessed the learned professors.  There might also have been other isolated erratics in the same general area, which were collected up and incorporated into the stone settings.  If there was a lot of stone working on the site, that might go some way to explaining the vast quantity of "debitage" and the abundant fragments in the Stonehenge layer -- and we might also explain the apparent use of rhyolite and "preselite" for the making of stone tools.  (Maybe these stone tools did not come from the destruction of bluestone orthostats, but from the waste materials left behind after the extraction of the "pillars" from the giant erratics.)  Many people have of course speculated about the "Stonehenge axe factory."

Just thinking out loud here -- but until somebody puts me off it, I think this is rather a nice theory........