Things are not always what they seem

The edge of the Esplanade Platform in the Grand Canyon -- a photo from Wayne Ranney's blog.  This is fascinating, because it shows scarring and dissection on the talus apron beneath the high buff-coloured cliffs.  When I first saw this photo I thought we would see scores or even hundreds of metres of stratified talus,  built up layer by layer over thousands of not millions of years.  But no -- what we see instead, if we click to enlarge the photo, is red sandstone, almost horizontally bedded, exposed on the side of the cuttings and gullies, in some cases almost projecting through the talus surface.  The talus from the high cliffs is coloured quite differently, which is what makes this photo so spectacular.  The recent gullying has not only swept the talus veneer away, but has started to cut fresh gullies in the red sandstone as well.

So the talus apron or set of intersecting cones is not an impressively thick accumulation at all -- but just a very thin veneer over a sloping bedrock surface.  Wayne tells me that the thickest talus (for example exposed in the cutting towards the right hand edge of the photo) is 15-18m thick,  and in many places it appears to be under 5m in thickness. 

I'm very surprised!!  This looks nothing like the little theoretical talus cliff profile I posted on the blog the other day.......

Wayne Ranney's blog:
http://earthly-musings.blogspot.co.uk/2011/02/searching-origins-of-esplanade-platform.html

Indicating compatibility.......

Latest news from the National Park (which is still, by the look of it, intent upon flagging up the vast importance of Rhosyfelin as one of Pembrokeshire's key "heritage sites"):

"............ archaeologists from a wide range of academic institutions have excavated the site and recovered material indicating it was quarried in prehistory compatible with the geological evidence....."

The National Park interpretation of the site is very much based on Prof MPP's story, which includes the bit about "one Stonehenge bluestone" having been quarried at Rhosyfelin and carted all the way to Stonehenge.  That, presumably, is the one that was "taken" from that crack in the rock face that features in all those photos, and which was later smashed up to give all those little fragments in the debitage.  As we have said before, the NPA policy seems to be to flag up the huge importance of the National Park in terms of its prehistoric "heritage", as part of an overall destination marketing strategy, regardless of how strong the evidence may be on the ground.   We also get the clear impression that the NPA does not understand the concept of scientific uncertainty -- or if it does, it thinks it is too complex for normal tourists and locals to understand.

Next week, all being well, we will have the new and much-trumpeted Antiquity paper on file, in which we will be able to examine the evidence presented by Prof MPP and his team.  What will the "material indicating it was quarried" actually be?  At least 20 radiocarbon dates, by the sound of it; but bear in mind that bits of charcoal and radiocarbon dates tell us something about occupation (maybe) and nothing about quarrying.  And this phrase is interesting:  "compatible with the geological evidence".    Well, the theory that the stones were taken from Wales by prehistoric giants is compatible with the geological evidence, and so is the theory that the stones were carried by aliens using space ships, and so is the theory that Merlin the magician was responsible for this great feat.

Next week, all being well, we might also be able to reveal the contents of our second paper, from Archaeology in Wales journal.  Could be an interesting week....

EH plan of Stonehenge

Thought this was worth sharing -- a nice clear plan of the Stonehenge stone settings, from the EH web site with full acknowledgement.  Click to enlarge.

Coming soon........ a look at those "engineering features"

 Rhosyfelin 2014.  Some see a monolith delivery route, loading platform, and revetment or quayside on an old river bank.  Sorry chaps, it's a jolly tale, but we see nothing of the sort........

Coming soon:
Brian John, Dyfed Elis-Gruffydd and John Downes.  2015. OBSERVATIONS ON THE SUPPOSED “NEOLITHIC BLUESTONE QUARRY” AT CRAIG RHOSYFELIN, PEMBROKESHIRE.  Archaeology in Wales 54, pp 139-148.

This is a more detailed paper than that published at the beginning of November:

https://www.researchgate.net/publication/283643851_QUATERNARY_EVENTS_AT_CRAIG_RHOSYFELIN_PEMBROKESHIRE

That paper (62 reads so far) was essentially a short note directed at a readership of geomorhologists, geologists and other Quaternary experts.  The new paper is more detailed, and is aimed at a readership of archaeologists.  Following positive and creative comments from the referees and from the Editor Jemma Bezant, some rewriting was done, and on their advice the paper now addresses head-on some of the issues relating to so-called "engineering features" at Rhosyfelin.

Issue 54 of the journal is in the press, and should be available within the next two weeks. (The same journal has published a number of the papers written by Rob Ixer and Richard Bevins, and we applaud the willingness of the editor and publishers to stray outside the normal archaeological "comfort zone" and to publish material which is likely to be rather controversial!)

Watch this space..........

Cliff faces and talus cones

Iceland -- a cliff face partly buried beneath talus cones.  These cones are nowadays not very active -- and so vegetation has been able to establish itself.

How do cliff faces in high latitudes and high mountain environments get destroyed and incorporated into a new landscape of rolling hills and sediment-filled valleys?  Well, there are very many processes that come into play, but here is a model of what should happen in an "ideal" world in which we have a coherent and geologically uniform cliff, an environment where frost-shattering is able to operate, and a foreground in which scree or talus accumulation is able to operate unhindered.  And, of course, we need a stable environment for many thousands of years, since the average rate of cliff retreat is unlikely to be more than a few centimetres per year.

The diagram shows how cliff retreat is accompanied by talus buildup and the gradual evolution of a convex rock slope.  This convexity develops because that part of the rock face buried beneath talus is effectively protected from further frost shattering, while erosional processes continue to operate on the rock face still exposed to the atmosphere.  So the highest part of the cliff retreats over a far greater distance than the base of the cliff.  Until this process is complete, the overall profile of the surface will be concave, with a steep cliff above grading to a shallower gradient on the talus slope below.

But things are never this simple.  Take a look at some of these photos:

Victor Point, Baffin Island.

Lake Louise, Canada.

 Lechtaler Alps.

In the real world rock faces tend to break up into buttresses and gullies, so that instead of a real apron of talus developing along a rock face we see the development of cones or fans, which coalesce either high up or lower down.  Often they overlap in a very complex way, with the deeper gullies feeding the biggest fans, which overwhelm or incorporate those which develop more slowly.  Some fans may be made largely of large blocks and boulders from rockfalls, while others may be made of smaller fragments or clasts which may have shallower "resting angles" -- so quite violent rearrangements might occur across a series of linked fans.  If you have ever walked across a talus slope and set it in motion, you will know what I mean...........

In some cases, as we see in the photos above, talus surface angles may remain more or less constant from tip to toe, but in others (as in the Iceland example above) there is a clear concave profile.  That is caused by the lodgement of the largest blocks at the top of the cone at a relatively high angle, grading down to the settlement of the finest fragments on the outermost part of the cone.  This implies that there is a certain amount of lubrication and water transport involved within the cone -- and this is what happens, for example, in the UK, in Norway and in Iceland.  In the arid high arctic, scree slopes may be essentially dry internally, or else stabilised by permafrost.

And in the real world it would be very rare indeed for an environment -- even in the high arctic -- to remain stable for 10,000 years or more.  A warming or a cooling, or an increase or decrease in precipitation,  will set off a string of changes which will affect sediment supply, the talus angle of rest, and the operation of biological processes.  

And what happens at the foot of the talus cone is also of crucial importance.  If an adjacent valley floor is effectively filled with talus, with virtually nothing being removed, the rate of talus accumulation and cliff face burial will be enhanced.  If there is a river or coastline which effectively saps away at the base of the come or talus apron, then the top of the talus slope may remain remarkably static for many thousands of years, with the rate is sediment supply matched by the rate of sediment removal by rivers or waves.

Then it gets even more complicated........ but you get the general idea.

Prehistoric Quarrying in South America

 Some of the big shaped stones at Puma Punku, Bolivia -- one of the most famous sites where large stones have been used a long way from their places of origin -- or so it is assumed.....

Thanks to Rob for providing this reference.  Haven't been able to look at it myself, and I don't know if it is online -- any info gratefully received!
 
J.W Janusek, P.R.Williams,M.Golitko and C.L.Aguirre 2013.  Building Taypikala: Telluric transformations in the lithic production of Tiwanaku. In Mining and Quarrying in the Ancient Andes,  N. Tripcevich and K.J.Vaughn (Eds), 65-98 Springer.

The Bluestone Enigma

Following that glitsch in ordering and supply, I'm happy to report that a stock of "The Bluestone Enigma" (the book that tells the other story) is back in the EH warehouse, and should shortly be back on the shelves at the Stonehenge Visitor Centre.  So even though there is a powerful emphasis on the "historical orthodoxy" of the human transport story as related by EH, there is at least a willingness to accept that there are alternatives........

Strangely, there appears to be much less willingness on the part of other museums / information centres in Wiltshire to stock and sell the book, in spite of numerous approaches my myself and others.