Periglacial stripes (again)

 The stripes exposed in the Avenue excavation, across the road from Stonehenge.  (Acknowledgement:  Tim Daw's web site)

I have had a few interesting exchanges with Charly French and Mike Allen on the subject of those famous periglacial stripes -- which were, according to Prof MPP, obvious enough features in the Neolithic landscape to have caused the builders of Stonehenge to build the monument here rather than somewhere else.  I have never found that explanation at all convincing -- and indeed I still consider it fanciful in the extreme. 

However, these ridges are indubitably interesting.  Charly and Mike think that they are Devensian in age and periglacial in origin.  This is Mike's latest message, which I hope he will not mind me sharing:


The rest of the slope (ie outside the alignments in the Avenue) contains a number of parallel discontinuous stripes as do several other areas in the same fields. These all run diagonally down slope and are on average 10 to 15cm across and about the same depth with irregular V shaped profiles. The are filled with typically buff to reddish brown silts to silty clays usually stone-free but sometimes containing well patinated flint.

The molluscan assemblages (when preserved) contain typical restricted open country (cold stage) assemblages - but from memory I cannot remember any of the large Pupilla muscorum sometimes found in cold-stage assemblages.  The assemblages were depauperate and species-poor so no diagnostic rarities were recorded from this area. These 'stripes' are typical or and similar to those I've seen and recorded in Sussex, Dorset, Hants etc.

With the Avenue stripes the orientation is the same as the adjacent periglacial stripes and the spacing about the same. Hence our contention that these deeper linear gullies originated as shallower periglacial stripes forming probably in the Late Devensian. The samples produced no shells ... or too few to take further to analysis but I will look through the archived material.

Obviously the origin of these is of interest if not fascination. If purely run-off and solution it does seem odd to have a series of parallel and straight linear grooves. That being said, I don't comprehend the precise formation process that created periglacial stripes as such strong parallel straight features ubiquitous across the southern chalk.




 What fascinates me about this info kindly provided by Mike is that the grooves run DIAGONALLY down slope.  If that is the case they cannot, I think, be periglacial in origin, since all of the periglacial stripes I have ever seen run directly down a slope on the maximum gradient.  And we are just as much in the dark with respect to the precise processes involved.

This all confirms me in the belief that these are solutional rills, possibly of very great age (ie not Devensian but maybe much older, forming and deepening over several glacial / interglacial cycles) and in some way STRUCTURALLY CONTROLLED.  I have no idea what the bedding of the chalk is in the vicinity of Stonehenge, but I think I might speculate that the chalk beds are dipping and that the solutional rills are developed on the strike of these beds as they outcrop at the surface.  A glance at the detailed BGS geological map might give us guidance on the matter.......

All further comments (from Mike or Charly or anybody else) will be welcome!