Williams on the rills and grooves

Above I have reproduced some of the images relevant to the "periglacial stripes" debate. (By the way, these features were not "discovered" by MPP and colleagues -- Martin Trott described them in 1988, and apparently Atkinson knew about them as well. Trott saw no great significance in them, and referred to them and to solution hollows in the chalk surface.  Here is a cutting reproduced from Tim Daw's "Sarsen" blog:

 http://www.sarsen.org/2012/11/stonehenge-periglacial-stripes.html


I think the rills are anastomosing and irregular, with individual channels or gullies extending for maybe just a few metres before joining other rills or splitting -- with the sort of pattern we see on braided rivers on glacial outwash plains.  As I have said before, I do not see any evidence of any rills extending for hundreds, or even tens of metres.  If we look carefully at the second image above, we can see that there is one slight trace of a feature extending for maybe a hundred metres to the left of the Avenue embankments.  To me, that looks man-made.

On the bottom image (which Neil has used on another forum discussion) we can pick up all sorts of details.  Some of the "texture" might be down to old plough marks -- but there are many very subtle features which one can only pick up on if you enlarge the photo.  (Click to enlarge.)  There are even some slightly winding or meandering features, and some things that look like solution hollows.  I would really like to knoe what the features of the solid geology are in this area......

Finally, I came across this chapter by RBG Williams, who has written very extensively about the periglacial landscapes of Britain:

RBG Williams 1986 "Periglacial phenomena in the South Downs."  Ch 17 (pp 161 - 168) in Sieveking and Hart "The Scientific Study of Flint and Chert."

This book is long since out of print, but if you google the chapter, you will find it on Amazon, and it so happens that Ch 17 is on of the chapters you are allowed to read online..........

Here are a couple of short extracts:

 

Williams talks about frost shattering of chalk surfaces during the Devensian and earlier glacial episodes, sheetwash, solution processes, and the formation of rilles under a number of different scenarios.  

This all accords very nicely with what I have been saying.  As Neil has confirmed, these features are everywhere on the chalk downs.  Maybe the features owe part of their origins to surface and subsurface runoff and solution during the Devensian -- maybe when there was an intermittent permafrost table which provided an "impermeable" layer a metre or two under the ground surface.

It may be that the features are still forming today, with water concentrated in the subsurface rills when there are periods of heavy and continuous rainfall -- so that the infiltration capacity of the chalk is exceeded.