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Friday, 30 April 2010

Bluestone fragments galore

I'm increasingly intrigued by the super-abundance of "bluestone" fragments (ie fragments of rock other than sarsen, chalk and flint) within and around the stone settings at Stonehenge, and the fact that they also pop up elsewhere -- in the Cursus and other digs on which the geologists have been called in to do lithological identifications.

The Stonehenge Layer is rather intriguing.......

With ref to the Darvill / Wainwright dig in April 2008: "The dig also investigated the “Stonehenge Layer”, a significant and varied layer of debris and stone chippings spreading across the whole extent of the stone circle and comprising a high proportion of bluestone fragments. This is the first time that the nature, content and structure of this layer has been properly studied, crucially to determine whether this deposit was derived mainly from the construction or destruction of the Double Bluestone Circle and of Stonehenge as a whole."

We see references to debris, chippings, fragments, flakes etc -- and there is a consensus that many of these bits and pieces have come from the working of the bluestones on the site. But do all of the fragments match up with known stones or stumps on the site? Are some of the rock types identified found only as fragments? Could they therefore be either intact bits of erratic debris or the remains of smaller erratics that have been broken down?

The question "Where are all of the small erratics?" is asked quite frequently -- and I wonder if they have been there all the time, in the Stonehenge Layer and maybe in the undisturbed soils of Salisbury Plain? Does anybody have any clues that they might wish to share?

3 comments:

Anonymous said...

AH THE 10000000000 DOLLAR QUESTION tO BE ANSWERED SOON.
ROB

Kostas said...

Brian, the answer is simple! The Stonehenge Layer is the many deposits of soil and errant stone fragments brought there by melting snow and ice that also brought the sarsen and bluestones to Stonehenge. Imagine ice surface runoff water that starts at Preceli Hills (and elsewhere) and ends as waterfalls over the circular ice edge at Stonehenge accumulating on the bottom.

One thing that also caught my attention as I looked at the photos of the Arkinson excavations (thanks for directing me to them) is that such gravel and stone fragments is so ly well mixed into the soil. This is not what you would expect if these fragments were chipped off the stones while working on them. Such chippings would form a distinct layer and not be so uniformly be mixed in with the soil.

My theory on this stands Brian! Ice did cover Salisbury Plain when Stonehenge was erected and played a key role in its construction! Read all about it here:

http://knol.google.com/k/constantinos-ragazas/the-un-henging-of-stonehenge/ql47o1qdr604/16#

Thanks for your Stonehenge Thoughts blog, Brian. It is becoming a clearinghouse of the latest pertinent information.

Brian said...

What interests me is the shapes of these fragments -- I'm not sure that anyone has ever done a systematic analysis of them. Every geomorphology student learns that there is a continuum of stone shapes from brittle and sharp-edged (which might indicate frost shattering and local accumulation), through to faceting and some rounding -- sometimes called sub-angular (which we might find in glacial deposits like till), through to moderate rounding of edges (in fluvioglacial deposits), through to better rounding of edges (in true fluvial gravels and cobbvles), through to even better rounding (as in beach deposits.) the key relationship is that the roundness index is related above all to the distance the particle has travelled and the degree of attrition caused by fragments grinding and bumping against each other. There are many other variables which we need not explore here -- but there has been quite a lot of debate on whether either the Stonehenge monoliths and smaller stones have glacial facets on them, or striations, or rounded edges that can be put down to glacial transport. The problem is that this sort of analysis is never very definitive -- there is a lot of room for subjective interpretation....