THE BOOK
Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Friday, 16 February 2018

World of Ice photo gallery





For those friends who are interested in ice in all its forms, and its effects on landscape, you might like to take a look at my Pinterest Gallery.  Enjoy!!

https://www.pinterest.se/brianjohn526/world-of-ice/


Wednesday, 14 February 2018

Abermawr Holocene sediments


Over all the years I have been studying the sediment sequence at Abermawr (on the North Pembrokeshire coast) I have never before seen so much of the storm beach stripped away.  And it has been a big surprise.

I had assumed that the storm beach - which is quite spectacular -- was very thick indeed -- maybe 3m or 4m thick, and that it was resting on a deep notch cut into the underlying Devensian and Holocene sediments.

However, we can see in the above photo (showing the northern drift cliff)  that there is no deep erosional notch, and that the pebbles rest on a steeply sloping surface of till and underlying periglacial slope deposits (the lower head).  The volume of the pebble beach is maybe just 50% of what I had assumed.........

If we assume a similar situation for the middle part of the bay, this is realistic:

In other words, the storm beach in the middle of the bay does not rest on a deeply cut notch but on a slope of organic silts and clays dating from the Holocene.  I am not sure whether there is actually a stratigraphic break between the submerged forest layer and the Holocene organic layers -- maybe they are both part of a continuum.

I now think that the front face of the stormbeach is not very thick at all -- I shall keep it under observation.

The sandy beach rises and falls according to the severity of winter storms -- so sometimes the submerged forest is visible, and sometimes not.

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Abermawr is one of the "top 50" Pleistocene sites in the UK -- details here:





Tuesday, 13 February 2018

Submerged forest at Abermawr




After the winter storms (or some of them -- they probably haven't finished yet) there are some nice exposures of the submerged forest at the bottom edge of the storm beach.  It was a bit too stormy today, and the tide wasn't low enough to examine the exposures properly -- but as usual we can see fallen trees and branches, old tree roots and patches of peat.


Wednesday, 24 January 2018

Perched block near Lake Superior


This is rather splendid a perched erratic block left at the end of the Wisconsin glaciation not far from the shore of Lake Superior.  it was published on Pinterest -- not sure who the photographer was.....

Monday, 22 January 2018

Field et al 2015: the Stones


EH Plan of the stones, using the Petrie numbering system

Following on from our previous post:
https://brian-mountainman.blogspot.co.uk/2018/01/field-et-al-2014-landscape-and.html

we now come to

Reference:

David Field, Hugo Anderson-Whymark, Neil Linford, Martyn Barber, Mark Bowden, Paul Linford, Peter Topping, , Marcus Abbott, Paul Bryan, Deborah Cunliffe, Caroline Hardie, Louise Martin, Andy Payne, Trevor Pearson, Fiona Small, Nicky Smith, Sharon Soutar and Helen Winton (2015). Analytical Surveys of Stonehenge and its Environs, 2009–2013:
Part 2 – the Stones. 
Proceedings of the Prehistoric Society, 81, pp 125-148 

Abstract

Non-invasive survey in the Stonehenge ‘Triangle’, Amesbury, Wiltshire, has highlighted a number of features that have a significant bearing on the interpretation of the site. Geophysical anomalies may signal the position of buried stones adding to the possibility of former stone arrangements, while laser scanning has provided detail on the manner in which the stones have been dressed; some subsequently carved with axe and dagger symbols. The probability that a lintelled bluestone trilithon formed an entrance in the north-east is signposted. This work has added detail that allows discussion on the question of whether the sarsen circle was a completed structure, although it is by no means conclusive in this respect. Instead, it is suggested that it was built as a fa├žade, with other parts of the circuit added and with an entrance in the south.

Quote:  

"Eighty-six stones are visible at Stonehenge, including four stumps (Fig. 2); the 20th century excavations revealed a further ten buried stumps making 96 in all. These stones are coarsely divided between the sarsens, a hard sedimentary silcrete, and the ‘bluestones’, a catch-all term covering a variety of rock types of distant origin including spotted/unspotted dolerite, rhyolites/rhyolitic tuffs (Ixer & Bevins 2011; Bevins et al. 2012), volcanic ashes, a Devonian sandstone (the Altar Stone), and other sandstones (Thorpe et al. 1991)."

Concerning the sarsens and the recent attempts to designate "origins" on the Marlborough Downs and other distant localities: Quote:  "Flinders Petrie in particular, who carefully surveyed the stones and whose numbering system is used here, considered that the very position of Stonehenge may have been determined by the presence of a quantity of sarsen (Petrie 1880), while William Gowland (1902, 75, 115), who excavated at Stonehenge at the outset of the 20th century, similarly thought the stones were brought from ‘no great distance from the spot where the structure stands’. The geologist Prof. J. W. Judd (1902, 115–6) considered that they had been moved ‘only a few hundred yards’, while H. H. Thomas (1923, 242) also thought that they may have come from ‘the site of Stonehenge itself’. Johnson (2008, 121) has suggested that the Heelstone is too awkward and bulky a shape to move on rollers and it, at least, is unlikely to have travelled far. Equally the much smaller Station Stones could easily have a local origin: it is, after all, possible to find larger stones on Salisbury Plain without having to travel to the Marlborough Downs for them." 

The authors suggest that (on the basis of field examination coupled with analysis of the laser scan data) there are at least three different types of sarsen present, potentially indicating that the stones originate from several sources. On different colours: "the majority of the stones are grey, but Stones 54, 55, 101, and 156 exhibit an orange hue while Stones 53, 56, and 154 are purple-grey. "  So the interpretation seems to be that the sarsen stones have been collected up from all over the place, and have not come from a "targetted" quarrying or collecting area in the Vale of Pewsey or somewhere else far to the north.

This is interesting too, regarding stone weights:

"Stone 56 has been cited as weighing 30–50 tons yet, assuming a specific gravity of 2.4, this study calculates the above ground weight to be 23.06 tons, while the known below-ground ele- ment (2.52 m: 22% of the stone) would increase this to c. 28.1 tons, ie, substantially less than the previous estimate. The visible parts of sarsens in the outer circuit vary between 11.1 tons (Stone 21) and 23.5 tons (Stone 16), with most stones bracketed between 13 and 17 tons. Assuming that some 25% of these lie below ground, it can be estimated that the stones in the Sarsen Circle weigh between c. 14.75 and 31.5 tons, with an average of c. 20 tons."

"Assuming that dolerite has a specific gravity of 3, the above-ground weight of the pillars in the Bluestone Horseshoe ranges between 0.96 (Stone 61) and 2.16 tons (Stone 69). As excavation of stones 68, 69 and 70 revealed that between 33% and 40% lay below ground; their estimated weight might be 3.35 tons, 3.24 tons and 2.05 tons, respectively. Stones in the Bluestone Circle are typically not as tall as those in the Bluestone Horseshoe and, due to the numbers that are broken or fallen, it is not possibly to calculate their average weight. In any case they decrease in size towards the north-east. Stone 33, however, has an above-ground weight of 0.51 tons and excavation revealed that c. 1.03 m of it was below ground, allowing its total weight to be estimated at 0.82 tons. In contrast, the above-ground portions of Stones 49 and 31 weigh 1.11 tons and 2.04 tons respectively, and excavated profiles indicate that 39% and 45% of the respective stones was below ground, allowing their total weights to be estimated at 1.82 tons and 3.72 tons."

The great majority of stones (both bluestones and sarsens) have traces of working on them -- mostly associated with pounding with hammerstones of various sizes.    There is a long discussion of the evidence for tooling or working, but there isn't much discussion at all on which surface features may be natural and which are tooled, and on WHY a lot of time has apparently gone into bashing large stones with smaller stones.  Was the idea simply to turn irregular stones into rectangular ones which would be easier to build with?  But of course the evidence of mortice and tenon joints  and tongue and groove joints is compelling, and these are clear evidence that the builders were seeking to use woodworking techniques in a world of stone.  The feeling seems to be that the sarsens have for the most part been shaped for use at Stonehenge -- maybe involving different settings at different times, but that the bluestones may have been used and shaped elsewhere before being brought in to their present positions:  "All of the extant stones in the Bluestone Horseshoe and three in the Bluestone Circle (Stones 150, 36, and 45) have been finely dressed. However, scars resulting from the removal of tenons from the tops of Stones 67, 69, 70, and 72, and the repositioning of Lintels 150 and 36 as uprights, indicate that these stones have been reused from an earlier bluestone structure, probably once located in the Q and R Holes, though possibly elsewhere."  Where was this "elsewhere"?  As we know, MPP and various other authors are very attracted by the idea of a "proto-Stonehenge" made of bluestones  somewhere in the Preseli district which was dismantled and carted off, lock, stock and barrel to be incorporated into Stonehenge..........  And as incorrigibly as ever, after years of trying, they are still hunting for the mysterious site.

One thing which is disappointing about this paper is that the authors do not consider the extent of surface weathering on the bluestones, and what that might mean for their origins and dates of emplacement.  Neither do they consider stone SHAPES -- instead simply carrying on the promotion of the idea that the preferred shape for the "ideal bluestone monolith" was a pillar or column.  In doing that, they ignore the fact that most of the bluestones are not pillars at all, but boulders and slabs.  This is hugely important, suggesting that the stones are more likely to be a glacial erratic assemblage rather than a set of carefully selected monoliths.

Coming next, in another post:  interpreting the stone settings. (p 134)

Submerged forest Abereiddi



Here is a great photo of a fallen tree in the submerged forest -- exposed in 2014 at Abereiddi on the N. Pembs coast.  Taken by Dan Soper and posted on Facebook.....

Some nice glacial geomorphology




Many thanks to Dave for bringing this to my attention.  It shows the value of LIDAR imagery, and a huge amount of detail showing the relations between various types of glacial erosional and depositional landscapes.  Puget Sound area?  Now that looks like the sort of place I'd really like to visit.......

http://www.dnr.wa.gov/publications/ger_presentations_coe_glacial_landforms_puget_lowland.pdf




Close-up of part of the map, showing drumlins and fluted moraine --classic streamlining features.   Not much doubt about the direction of ice flow here.........