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Wednesday, 30 January 2013

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!

20 comments:

Jon Morris said...

I don't really understand the difference between the two processes:

Is it a case of rills being more likely to follow natural planes of weakness between chalk beds if those exposed bed-lines follow a trajectory which is close to the natural maximum slope?

BRIAN JOHN said...

Periglacial stone stripes are considered to be "polygenetic" -- ie many different processes are involved. But in the ones I have studies they have always run durecctly downslope, and there is always a difference in particle size between the ridges and the furrows. The ridges and furrows are often of similar width, and the ridges of finer fragments may stand up to 30 cm above the surface of the furrows. but things get complicated, because water is concentrated in the furrows, and that washes out the finer fragments and also favours the growth of vegetation. So the furrows may appear green in the landscape, and the ridges barren by comparison. A high permafrost table has something to do with it, and it may be that there is some sideways or lateral frost-heave going on, because in the furrows the winter ice is deeper and more compact, and therefore capable of greater expansion. it's all a bit of a mystery.....

Solution rills will tend to develop straight down a sloping ground surface which is flat or undifferentiated. But as we can see on sloping sandy beaches when the tide is going out, there is always a tendency to the development of a dendritic pattern, cuased initially by minute surface irregularities and then accentuated or exaggerated to that the "strongest" drainage routes tend to take over or capture the smaller and shallower ones -- and a dendritic pattern is the most efficient one possible for the evacuation of moisture from a sloping surface. So one would expect a pattern of solutional rills to tend towards a dendritic pattern, all other things being equal. Which they clearly are not.....

Constantinos Ragazas said...

Brian,

The plot thickens with every new fact on the ground! So now we know for sure these Avenue stripes run “DIAGONALLY down slope”; while, “all of the periglacial stripes ... run directly down a slope”, as you argue. Clearly, the formation of these stripes was “in some way STRUCTURALLY CONTROLLED”. What can this be? My repeated claims clear the mystery here! Let me reiterate!

The Avenue was a meltwater drain channel with embankments of ice. While Stonehenge was a meltwater retaining basin in the ice sheet that once covered the area. This explains ALL the facts on the ground! Including now the convex shape of the chalk bedrock forming these stripes(so clearly shown in the photo) and the parallel ditches on each side of the Avenue.

Truth has a way of catching up with us, or we with it! Try as we may, it just wont go away!

Kostas

BRIAN JOHN said...

Come off it, Kostas. Your wild theory explains NONE of the facts on the ground. I am not going to allow any further discussion of this -- if you want to carry on arguing, do it elsewhere.

adam said...

there are obviously many clever people writing many clever things, here is a point from a less interlectual side. I see many things in this world that most do not. auras energy bits of the past and future and spirit people. With stonehenge over the years I pass there and look at the energy. About 150 m above stones I see a mass of twirling energy with like a pulsing beat at its center. At different times of the day I see the enegy flowing inwards other times flowing outwards. This I think is just an extension of the leyline theory. Why Im writing this is because I was at silbury hill late last year and realized it was there too, this time with the hill If they had some sort of woodern platfrom they would be near the center of the energy. My point is that seeing silbury hill made me think with some people seeing like me in the distant past this energy why not build your circle there and why not later try to build a means of getting closer to it with a hill, adam,

Constantinos Ragazas said...

Brian,

Of course! So predictable. Leaving my 'wild theory' aside...

What explains the convexity in the excavated strip of the Avenue, so clearly shown in the photo? And why the middle of this strip dips some from the overall convexity. And why the stripes are bigger and deeper in the middle and along the outside ditches?

Just a coincidence you say? Completely reasonable I say.

Kostas

Anonymous said...

With you all the way adam, my man.
I see the same vortex bringing stones from afar and a man of foreign origins sitting on a throne of ice, worshipping a collector of rocks whose ancient wisdom is berated by small evil demon called 'Norton'.

Jesus Christ!

Anonymous said...

Jesus, who is Norton?
Peter

BRIAN JOHN said...

Dangerous question. If Jesus doesn't know, we are all in serious trouble...... so God help us.

adam said...

was only suggesting that if ancient man used ley lines which is energy, then silbury hill may have been made to get closer to that energy. Sorry I offended you

Anonymous said...

Adam,

Unfortunately anything that disagrees with an intelligent glacier conveniently delivering the correct amount of stones, of the required dimensions, to the right area is derided by people who think they have the monopoly on brains.
Best ignorebthem.



BRIAN JOHN said...

Anon, yoiu are welcome to have a go at me or any of the others whp contribute vto our threads on this blog. But since you are obviouslt a brainy person, would you kindly explain what this means: "......an intelligent glacier conveniently delivering the correct amount of stones, of the required dimensions, to the right area."

How many stones? How big would you like them to be? And where do you consider the right area to be located?

No woolly thinking please. And no carts before horses...

Constantinos Ragazas said...

Brian, you write

“This all confirms me in the belief that these [the Avenue stripes] 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.”

Brian, such grasping for straw is unbecoming of a truth seeker. A testament of desperation when lacking sensible explanations. Equaled only by archeologists' made-up stories.

Such fine detail in the (not too deep) stripes made of (soft) chalk and so near to the surface would not have survived 400,000 years of repeated glaciation!

So very disappointing …

Kostas

BRIAN JOHN said...

Grasping for straw, Kostas? As in all situations where we have phenomena to explore, I am looking for explantions. My suggestion is not at all outrageous. I just wish that you would go away and read some geomorphology.

Constantinos Ragazas said...

Brian,

We can't always have our wishes! Corrupts us and makes us lazy and grasping for straw!

Kostas

Jon Morris said...

Thanks Brian

So one would expect a pattern of solutional rills to tend towards a dendritic pattern, all other things being equal. Which they clearly are not.....

Is it more that the formation mechanism makes periglacial even more unlikely? It seemed to me that the comment indicates that they are not rills.

All the best

Jon

BRIAN JOHN said...

Sadly, all I have to go on are the comments made by Charly and Mike -- I have to take their word for it that the stripes do not run straight downslope, but diagonally. I would dearly love to see a proper large-scale map....... but until then I think we'd better keep open the possibility that the stripes are somehow influenced by bedding planes or some other structural features in the chalk bedrock.

Constantinos Ragazas said...

the Avenue “stripes”...
... not “periglacial” and not “solutional rills”. And not “... somehow influenced by bedding planes or some other structural features in the chalk bedrock.”

STRUCTURAL CONTROLS responsible for the diagonal direction of these stripes are more likely to have been over the surface (like embankments of a meltwater stream) than under the surface. If “under” this would be apparent upon excavation and in the photo. And it would not have produced the “stripes” we see.

Kostas

Jon Morris said...

Thanks Brian

Much appreciated: If you get to know any more, can you let us know?

BRIAN JOHN said...

Certainly will do, Jon...