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Tuesday, 20 December 2016

Linear concentrations of sarsens

 We have talked about this before, particularly with respect to Fyfield Down.  We also had a bit of fun speculating on the situation at Devil's Den, where there is a similar linear arrangement of sarsens at the food of a steep valley side: 

In the latter post we referred to these linear concentrations of sarsens (which are quite common) as "sarsen drifts."

There is of course much more on Tim Daw's site called

I come back to this because I discovered in my pamphlet collection a little geomorphological trail called "Fyfield Down NNR Geomorphological Trail, by MJ Clark.  It was published by NCC in 1976.  A long time ago, but the observations are still valid, and it is mercifully free of speculations about the collection and transport of sarsens destined for Stonehenge!  It simply refers to a sarsen collecting "industry" that has been gong on for a very long time.

Anyway, the key discussion in the booklet is to do with the manner in which, in an asymmetrical chalk valley like Clatford Bottom (with one gently sloping flank and one steep one) the greatest concentration of sarsens is in a line along the foot of the steepest slope.  Could concentrations like these really be morainic remnants?  Could the concentrations be more apparent than real, because the fines on these valley floors have been washed away by intermittent stream flow, leaving sarsens exposed, whereas similar concentrations on the valley sides and interfluves  may still be buried in chalk debris and slope deposits?  Or could it be that the linear concentrations are still there because elsewhere so many have been removed over the centuries for land clearance and building use? (Because it involved more hard work to remove large quantities of stones from valley bottoms....)

Clark's analysis of the situation is summarised in the diagram above.  In phase 1 he suggests that in the early Pleistocene there is a slightly flexed land surface with a fragmented duricrust and with gentle valleys here and there, with sarsens mostly concentrated on the interfluves where they have best been able to survive erosional attack.  From then on, the story is mostly to do with extended phases of periglacial activity.  Permafrost and snowpatches?  Yes.  Glaciers? No.

Gradually (phase 2) the valley becomes more asymmetrical, as a result of more shade on the N or NE-facing slope and more frost action and snowpatch sapping.  On the long gentle slope solifluxion becomes a dominant process, and debris begins to accumulate on the valley floor.  In phase 3, as this process goes on, more and more of the sarsens from the  right-hand interfluve are transported downslope, gradually accumulating in the valley bottom.  Intermittent meltwater action in the valley floor (assisted by the presence of permafrost which prevents infiltration and effectively makes the chalk impermeable) removes the finer sediments in which the sarsens have been carried.

In phase 4, presumably coinciding with the Holocene, periglacial activity comes to an end and there is continued intermittent stream action and downcutting, and continued removal of finer material, leaving the alignment concentration of sarsens more or less as we see them today.

I would concur with Mike Clark that this is essentially a periglacial landscape, affected by hundreds of thousands of years of periglacial conditions during the Quaternary.  For much of that time there would have been permafrost on Salisbury Plain.  But on some of his other points I am not convinced.

For a start, I am not sure why he thinks there would have been greater concentrations of sarsens (and more duricrust or silcrete remnants) on the interfluves rather than in the early Pliestocene valleys.  I am more convinced by the argument (as discussed by Ed Pegler and David Field) that the duricrust (and hence the concentration of sarsens) would have been greater in the valleys, since that is where moisture would have been concentrated in pre-Ice Age times and where silcrete formation would have been accelerated.  I am also not sure why he thinks that periglacial processes and debris accumulation would have been more active on the warmer, gentle slopes of the valleys than on the steeper more shady slopes.  (Because the active layer was mobile for a greater part of the year and because more moisture was available rather than being locked in perennial snowpatches?  Might go with that........)  I am also not very convinced by the idea that downcutting and the washing out of fines was accelerated after the last removal of permafrost.  Surely, once the full permeability of the chalk had been restored, river erosion in these dry valleys would have been reduced, not enhanced?

Anyway, it's a stimulating little booklet, which encourages debate!  And that is what, as a teaching aid, it was intended to do.

1 comment:

Neil Wiseman said...

Nicely fleshed-out idea. Most questions answered by the diagram.
I like it.
I was never happy with the Glacier-Moved-Them scenario, but a much more subtle engine over long intervals makes perfect sense.