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!

The Rhyolitic "debitage" around Stonehenge

 A thin section numbered SH80 -- acknowledgements to the authors of the paper cited below, and to Wilts Arch and Nat Hist magazine.  This rhyolite is very different from the Rhosyfelin ones -- so where does it come from?

There is a new paper by Ixer and Bevins, with a comment by Mike Pitts, in the latest edition of the Wilts Arch and Nat Hist magazine.   Thanks to Rob for passing it on.   It's very difficult to follow, since the authors spend most of the time  discussing very subtle differences between the 14 samples which they looked at, and deciding which category or sub-category to place them in.  The essential conclusion seems to be that the rhyolite fragments examined (which have still not been tied to any standing stones at Stonehenge) are mostly from the Rhosyfelin area -- the similarities with the samples taken from the crag are close enough for "micro-provenancing" in just one case, but in the others the authors have to admit to provenancing from the general area.  They do not speculate as to how wide this area might be -- so it could be covered by a grid 500m x 500m in extent, or it could be even larger.

One general problem that I have with the paper is that it tends to talk of the "Stonehenge rhyolitic debitage" as if it is homogenous or as if they have examined enough of it to be sure that any conclusions drawn are incontrovertible.  This is the problem that all scientists face -- when we have a sample of limited size, how wide are the conclusions we can draw?  It's not a bad idea to bear in mind that the parts of the Stonehenge "stone floor" or regolith or debris layer examined and sampled thus far is very small -- and as I have said before, there may be several "debitage" groups with quite different petrographic characteristics on other parts of the Stonehenge site or in other parts of the Stonehenge landscape. 

I wasn't entirely convinced by the attempt to push samples from SH 80 into the Rhosyfelin assemblage, and would like to know what the characteristics of rhyolites from other outcrops in the region might look like, and how they might compare with the samples examined.

Mike Pitts says:  "It is notable that all the samples matched in this study to Craig Rhos-y-felin come from debitage and not from megaliths (although Ixer and Bevins (201111a and b) have suggested that buried megalith SH32e may also come from Craig Rhos-y-felin). One of the distinctive features of the rhyolitic rocks is that they are flinty – they have a good conchoidal fracture. That makes them relatively easy to break up, if they are standing as monoliths at Stonehenge. But it also makes them suitable for making portable artefacts. There are flaked bluestone ‘tools’ from Stonehenge (including some from the stone floor). Which of these are made from debris created when stones were dressed on site? Which are made from broken up megaliths? And which were made in Wales and brought to Stonehenge by people visiting, perhaps on a pilgrimage of some kind? Clearly the distinction has important implications for how we understand Stonehenge."  Mike is seeking to open up the debate here and to avoid sticking to the standard story, but he is still seeing the world through the same tunnel as all the other archaeologists.  He fails to recognize that there is a further possibility, and he should also have asked this question:  "Which flaked bluestone tools -- and indeed which bits of debris -- might have come from smaller rhyolite stones found in the neighbourhood and which might be assumed to be small glacial erratics?"  I am referring here to stones which might have been too small to use as orthostats, or which might just have been the wrong shape, or which might have been too badly damaged during glacial transport and subsequent frost shattering.



Wilts Arch & Nat Hist Mag vol 106 (2013) pp 1-15

"A re-examination of rhyolitic bluestone ‘debitage’ from the Heelstone and other areas within the Stonehenge Landscape"
by Rob A. Ixer and Richard E. Bevins, with a contribution from Mike Pitts

ABSTRACT
Recently it has been proposed that the Stonehenge rhyolitic debitage can be distributed into five petrographical groups (A-E) (and that at least three of them (A-C) are from rocks cropping out at Craig Rhos-y-felin). This supersedes an earlier classification scheme of this important category of Stonehenge material. The earlier 1980s scheme, based on lithics found close to the Heelstone, divided the rhyolites into two groups (A and B) and sub-divided the larger into two further sub-groups (Bi and Bii). Re-examination of this earlier material together with other Stonehenge rhyolites has allowed the two schemes to be compared and integrated.
The original 1980s Group A lithics are identical to the present Group B, (both are small groups). This group is described in detail so completing the petrographical descriptions of the Stonehenge rhyolitic debitage. Despite bearing feldspar megacrysts this group shares sufficient petrographical characteristics with rocks from Craig Rhos-y-felin to support the view that that location is the geographical origin of the group.
Lithics belonging to the 1980’s groups Bi and Bii, however, are randomly distributed amongst the present A and C groups and there are no strict correspondences. The designation Bi and Bii should therefore be abandoned.
Using the new scheme it should now be possible to map more precisely the distribution of the rhyolitic debitage in the Stonehenge landscape to inform such questions as to the number of rhyolite orthostats originally present and their fate.

More South Pembrokeshire Erratics

A splendid photo by Adrian of the Flimston Churchyard and some of the seven recorded erratics.

My thanks to Adrian James for a batch of splendid photos of other South Pembrokeshire erratics to go with the one at Loveston.   I won't reproduce all of them here -- but encourage you to take a look at Adrian's site:

http://pdboyinsuffolk.blogspot.com/2013/01/other-erratics-around-castlemartin.html

This is an interesting comment from Adrian about the Flimston Collection:

Flimston Chapel churchyard (SR92399558). There is a substantial collection of erratics in this churchyard. Some have been used as headstones for the graves of members of the Lambton Family who died in military service. Others have been left sitting in one corner of the enclosure. All of these stones arrived in the churchyard when the chapel was renovated and re-consecrated in 1903. There are 7 of them. A pamphlet, which describes the features and memorials in the yard, printed at the time of the opening of the chapel in about 1914, gives us these vague details:
No. 1 Boulder, at the head of Lady Victoria Lambton's grave was taken from just opposite Flimston Cottage. A 'brecciated spherultio, albite, trachyte or rhyolite.' Many occur in Pembrokeshire. This one 'seems to fit best with those of Romans'Castle in the character of its spherulites and groundmass.' Most of these appear to have travelled over 30 miles from the N. West separated from their parent rocks by St Brides Bay and Milford Haven, and by a considerable mass of high ground.........
[Flimston Cottage stood at SR927955, about 0.3 km ESE of the chapel and just north of the old clay pits.]
......No. 2 Boulder, from Pwlslaughter, which stands in the opposite North corner. [Bullslaughter, SR942944 - approximately 2.25 km SE]
No. 3 Boulder from Bulliber Farm [About 2.25 km WNW, at SR905968]
No. 3 Boulder from Merrion pond. [ About 2 km NE].
No. 5 Boulder from Lyserry Farm
No. 6 Boulder from Lyserry Farm.
No.7 Boulder from Lyserry Farm.
[Lyserry is about 3.4 km ENE of Flimston chapel, at SR9556967]

 

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Another erratic -- also from Loveston Farm?  The nice thing about the erratics of this area is that they are pretty obvious -- the only local rock type is Carboniferous Limestone -- so anything that isn't limestone is an erratic......

The Devensian Cilgwyn Moraine

This is a false colour image of the Cilgwyn area -- courtesy Henry Patton -- which shows the small irregularities in the land surface because "false lighting" has been introduced from the NE.  On the top image you can see the mounds and terraces of fluvioglacial and glacial material; on the bottom image I have added a line to highlight where I think the edges of a glacier lobe might have been, at the peak of the Devensian Glaciation about 20,000 years ago.

The high ground on the right is Carningli -- a rocky summit which has signs of till and glaciated rock slabs on its northern and north-eastern flanks.  I'm still open to the idea that ice may have flowed right over Carningli during the Devensian -- but dating is very difficult.

At bottom left we see the inlet of the Gwaun Channel meltwater drainage system -- assumed by the authors of the big glacial lake paper about Glacial Lake Teifi to have been a spillway for Glacial Lake Brynberian.  See the previous posts for details.

I live slap in the middle of this area, and I have to say that I have not yet seen any trace of glacial lake deposits -- and am rather unconvinced about the spillway too.  But there are certainly pretty impressive terraces of sand and gravel that look to me like kame terraces -- along the right edge of the photo.

At the southernmost extent of the lobe, as shown on the lower photo, the morainic clutter is very impressive too, with a number of distinct mounds, vary large erratics scattered all over the place, and generally a very rough land surface which makes farming difficult -- very different from the areas where fluvioglacial materials have been dumped.

A lobe here would accord well with the idea of a highly irregular ice edge, with many lobes and embayments coinciding with areas of lower and more hilly land.

I'm still working on this, and will keep you informed...

Ice marginal lakes come and go....

A photomosaic of ice fronts, marginal lakes and outwash plain on the south coast of Iceland.  The sea is right at the bottom edge of the photo.  This is a very high definition image -- click to enlarge as much as possible, and you will pick out great detail.  Note that some meltwater streams follow the ice edge before decanting into marginal lakes, which in turn decant into the sea.  The meltwater routes are well marked -- here and there we see distinct channels being formed, but because almost all of the features beyond the ice edge are made of unconsolidated sediments in this case, the channels are ephemeral.

South Greenland ice sheet edge.  I suspect that this photo was taken in the autumn, after the fall of the first snows of winter.  the lakes are still ice-free.  there are a lot of them -- and if you zoom in you can see some spillways snaking away from the ice front, along depressions or natural cols in the undulating bedrock terrain.

Another South Greenland ice sheet edge, with abundant meltwater filling low-lying parts of the landscape but not actually impounded by the ice edge.  The overall slope of the land surface is away from the higher foreground towards the top right corner of the photo.  If you look carefully you can see that most streams are flowing away from the camera position.  there are some rock-cut spillways or channels.  The lakes in the foreground are full, whereas those in the distance have had higher water levels than those of today -- you can see the light-coloured strips along the waters edge -- these are strandlines, maybe just a few metres above present lake levels.  This is a very dynamic environment.  Each lake has a different history and a different duration.

Glaciated terrain in Svalbard

Two nice satellite images from Svalbard.  The top one is from the NW of the island, and the lower one is from the N part.  The top image shows typical jagged upland terrain, with many knife-edged ridges and steep peaks -- the shadows of these peaks on the snow is what gives the terrain a ragged or jagged appearance.  Don't know what the geology is.......

The lower photo shows an area with more extensive snowfields and plateaux, and fewer jagged peaks.  More interesting for glaciologists, maybe, but not for mountaineers.

Another glacial lake

One of my favourite photos of a glacial lake margin.  This is from South Georgia -- photo by Chalmers Clapperton.  Note that this lake has a wildly oscillating surface -- it's quite probable that these shorelines are not in a tidy time sequence at all.  Also, note that the strandlines are well marked where the rocky slopes are relatively gentle, but less well marked at the far end of the lake where there are quite active scree slopes.