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Thursday, 16 December 2010

Salisbury Plain periglacial stripes





Here are a few analogies which might help us to understand the strange "grooves or striations" which have been reported from Bluehenge and the Avenue.  

Top: a low-level (200m) air photo from Banks Island, Arctic Canada.  The stripes are running down a gentle slope of 2-5 degrees.  The terrain is slightly hummocky moraine.  The dark colours are lines of mosses and lichens, concentrated in "runnels" of concentrated surface wash -- where moisture helps them to grow.  The spacing between the stripes in 1 - 2 m.

Picture 2:  Large non-sorted stripes on Banks Island.  The spacing between stripes is about 1m.  The figure is standing on one of the very slight ridges.  Note that the debris is similar on the ridges and in the hollows -- the only distinguishing factor is the vegetation in the hollows.

Picture 3:  Periglacial stripes near Thule, NW Greenland.  Here the ridges are wider (up to 2m) and the runnels or grooves narrower -- generally under 50 cm.  There is no apparent sorting of material.

Picture 4:  Periglacial soil stripes on Svalbard.  Here there is some sorting -- there are no real ridges and grooves, but the material in the darker stripes has more contained sand, silt and clay.  Note that the flattened stones appear to be thrusting out of the ground -- there is clearly a process of cryoturbation (frost heave) going on here.

The essential rule seems to be that the stripes run directly downslope -- never diagonally.  Where they appear to bend on photos, that is usually because the angle and orientation of the slope has changed.  Sometimes there is a sorting of material, with finer material in the grooves and coarser material on the ridges.  Where there is a distinct difference in height -- up to 50cm -- between ridge crests and groove bases, that appears to be the result of water action, with snowmelt and rainfall (and maybe permafrost meltwater as well) escaping downslope in the simplest way possible.  These features seldom appear on slopes less than 3 degrees, and are most common on slopes between 5 degrees and 15 degrees.  On steeper slopes, more rapid forms of solifluction and even slumping will come into play.

  Because plants thrive in these damper grooves, there may be enhanced erosion associated with biological processes.  In the case of chalk, because chalk is notoriously susceptible to freeze-thaw processes, water concentrated in runnels or grooves will enhance freeze-thaw and will also remove the debris more readily than on the ridge crests.  Also, solution comes into play.  Wherever water is concentrated, as in the grooves, solution processes will further exaggerate the differences in height between the ridge crests and the groove bases.  So I would not be surprised to find, in some cases, ridges up to 3 or 4 m apart, and grooves between them that are up to 1m deep.  The precise details may be related to the amount of surface water available, the surface slope, the nature of the regolith (layer of broken rock and soil close to the surface), and indeed to the precise nature of the chalk at a particular site.

4 comments:

Anonymous said...

Four very nice pictures, but without one being of or from the Salisbury Plain, I find it hard to see a pattern involving the Salisbury Plain. Which I do believe is the subject of you muttering. Am I missing something ?

BRIAN JOHN said...

No. I don't think you are missing anything! Having seemn many mentions of the so-called gullies, striations and grooves, I'm sort of thinking aloud about whether these things are really periglacial. I found this re the Avenue:

"The focus of the excavations in 2008 and 2009 was on locating the ends of the Avenue leading away from Stonehenge. The edge closest to Stonehenge has been identified on the far side of the A344 next to the heel-stone, in the form of two parallel ditches approximately 20-22 metres wide. When this area was excavated two environmental archaeologists on the team pointed out that these ditches were in fact natural gullies dating back to the ice-age. These natural features were in perfect alignment with the solstice which were then utilised by the builders of Stonehenge as the location of the Avenue."

Now clearly these ditches cannot possibly be linked to the examples I cited from the Arctic. For a start, they are far too wide. Goodness knows what the two environmental archaeologists think about the processes that created them........

I saw an illustration somewhere of ridges and grooves that purported to be a reconstruction of the features MPP and others have talked about -- but I can't find them anywhere.

If anybody out there has more info on what these ridges and grooves look like, I will be happy to receive it!

Robert Langdon said...

A couple of good picture are available at:
http://www.flickr.com/photos/pauljcripps/3587361988/in/photostream/

RJL

BRIAN JOHN said...

Ah -- thank you, Robert. Very helpful. I'll bang them up, with due acknowledgement.....