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....
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Monday, 6 July 2015

Genesis of a giant erratic


Scrambling about on the west side of Rødløga Storskar (in the Stockholm Archipelago) the other day I realised that the prominent rocky knoll I was climbing on, about 25m above the sea, was falling to bits. I have always recognized it as a well-developed roche moutonnee with a smoothed and moulded up-glacier side and a plucked and fractured lee side, facing south. The rocks here are Pre-Cambrian basement rocks, comprising pink and red granite and a wide range of metamorphosed igneous and sedimentary rocks which probably have some very complicated geological names.

The western flank of the roche moutonnee is breaking up, although many of the broken blocks on the cliffs are well smoothed by over-riding ice. So these blocks (some of them as big as caravans) have been entrained, moved and smoothed.  Most of them have not moved very far.

But then at the position shown in the satellite image above I realised that a deep crack about 30m long and at least 20m deep was the first sign of a massive shearing-off of rock on the south-western corner of the hillock.  In other words, the crack indicated the genesis of a giant erratic, since the block of granite to the south of it must have been bodily dragged away (by overriding ice) from a prominent fracture plane.  The gap is now in places up to 50 cms wide.  I calculate the size of the moved block to be about 40m x 30m x 30m -- which gives us a volume of c 36,000 cubic metres and a weight of around 100,000 tonnes.

That would have been rather a hefty erratic, had the Devensian Glaciation continued for longer.  It would have been moved southwards and no doubt broken up further, into lots of smaller erratics.  The large block is very heavily fractured and is petrologically varied too, so it could not have survived transport as a single super-erratic.  The photos below show the crack as it appears on the upper surface of this little promontory, and the broken or plucked southern face.  The bouldery beach is packed with erratics from near and far.




Sunday, 5 July 2015

Men at work


One of my favourite Stonehenge photos!  From the EH Atkinson collection.  Wandering about like that on top of the lintels would never be allowed these days!  Does anybody know who the people are?

Wednesday, 1 July 2015

The ones they left behind.......



The Rhosyfelin "abandoned orthostat" is not alone......

On my recent day wandering about on Rodloga Storskar in the Stockholm Archipelago, I came across  quite a range of pseudo-proto-orthostats  -- here, there and everywhere, resting on rock platforms and almost all of them off the ground -- supported by pillars, props, wedges, fulcrums and even "railway tracks".  (These two large rocks, one weighing about 6 tonnes and the other about 5 tonnes, were obviously destined to be dolmen capstones, because there are many others in the lexicon that look very similar in shape.)   It's clear that all of the propped-up boulders in this area were destined for Neolithic monuments, and that they give us further evidence of the extent of Neolithic quarrying in this area.  They were clearly being dragged down towards the sea for loading onto rafts when they were abandoned.  Somehow or other they just got left behind.  Perhaps the builders of the nearest dolmens had already fulfilled their quota of capstones and supporting stones and free-standing monoliths?  Perhaps building priorities suddenly changed?  Perhaps sudden conflict disrupted a previously stable political alliance between tribes in different parts of Sweden?

Only joking.  These and thousands of other large boulders in this landscape of low islands and washed surfaces are glacial erratics -- still in the places where they were dumped, and still resting on the stones jammed beneath them when they were "let down" from glacial transport.  Most of the fine material (till and fluvioglacial sands and gravels) has been washed away as this area has gradually emerged from the sea as a result of isostatic uplift.  Wave action has played a great part in cleaning up the bedrock surfaces.  The total amount of isostatic uplift here is about 125m, and the land is still rising at a rate of 4mm per year.  During the Neolithic this area was still deeply submerged beneath the sea.


Monday, 29 June 2015

Stockholm Archipelago -- striae everywhere




On the Precambrian basement rocks of the Stockholm Archipelago striae are ubiquitous.  They are almost everywhere, and on the islands they are of course best observed round the coastlines of the islands where vegetation is sparse.  Most of them in the area I'm familiar with are running directly N-S.   It's assumed that most are of Weicheslian (last glaciation) age, but some might be older, dating from a time when the ice flowing across this area was around 2500m thick.  Even during the Weichselian the ice was probably 2000m thick, and that translates into enormous basal ice pressures capable of using tools to scour, scratch and gouge the rock in spectacular fashion.

As I have said before (probably every summer at around this time!) the archipelago is a sort of glacial erosion laboratory.  A text-book's worth of stuff in every square kilometre.........



Most of the grooves are less than 2 cms deep, but now and then one finds deeper grooves or channels which can be more than 10 cms deep.  On any particular rock face one might find deviations from the "normal" orientation by up to 30 degrees, and because the ice has obviously moulded itself to the bedrock topography and has flowed around obstacles, on some sloping or vertical rock faces striae may run as much as 45 degrees away from the norm.  To work out the overall direction of ice movement it is always best to examine more or less horizontal rock surfaces.

Not all rock types carry striae, even here in the Stockholm Archipelago -- some that are very coarse-grained seem to resist polishing and scratching very successfully!

Sorted stone circles


A fabulous photo taken on Nordaustlandet, Svalbard, by Jenny Ross.  It shows sorted stone circles in an area of permafrost.  The sorting takes place in the active layer under the control of rather mysterious processes, which have been argued about by geomorphologists for many years.

It's much more common to see pictures of sorted polygons.  When the density of circles is too great for them to remain as individual entities, they press together -- and the result is that all of them assume polygonal shapes.

Thursday, 25 June 2015

New image of the Ramsey Sound trench

Back in 2012 I posted a piece about those amazing 3D images of the Ramsey Sound trench:

http://brian-mountainman.blogspot.co.uk/2012/04/glacial-landforms-ramsey-island.html

So today it was a pleasant surprise to receive an even better image from Dr Paul Evans of Cardiff University.  Paul can be reached here:  EvansPS3@cardiff.ac.uk

He has kindly offered to make the data files for this new high-res image available to anybody who may be interested.

Here is the new image:


The trench is a real puzzle -- it is most unusual to find a trench or canyon of this type which is closed at both ends.  The excavation of the trench cannot be laid simply at the door of tidal scour.  And glacial erosion in this situation is not all that likely either, given that the prevailing direction of ice movement in both the Anglian and Devensian glaciations is most likely to have been perpendicular to the alignment of the trench, and not parallel with it.  So there must be some structural influences at play.  An interesting geomorphological enigma.

Tim's Jolly Jape

 

 The blue line shows the "axis" of the monument, and the red one shows the "newly discovered alignment."

Stonehenge mystery solved at last........ etc etc.  I've been watching with increasing incredulity the wonderfully gullible coverage in the media of Tim Daw's latest theory about the alignment of the flat face of stone 56 at Stonehenge.  There was even a photo on the BBC web site showing the sunset moment in just the right place, depending on where you stood to take the photo..... but let that pass.  This should of course have been published on April 1st, but somehow they got the dates mixed up at WANHS magazine.  So here we are, with yet another wacky alignment theory, by far the most implausible in a long history of such things.  And people keep on taking them seriously, obsessed with the search for meaning and significance. Almost anything will do, and the media will take it seriously.  So good for you, Tim! 

Er, you can't possibly be serious, can you....??

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Stonehenge's tallest stone 'points at winter sunrise'

http://www.bbc.com/news/uk-england-wiltshire-32356993

The tallest stone at Stonehenge points towards the sunrise on the midwinter solstice, according to a new theory from a steward at the site.
Historians have long known the circle of stones is aligned with the midsummer sunrise but Tim Daw says the tallest one is lined up with the midwinter sun.
It was previously thought the stone had been put back at the wrong angle when it was re-erected in 1901.
But Mr Daw, who works there, says his research shows its angle is deliberate.
Mr Daw said: "The largest stone at Stonehenge is not where it 'should' be, it is twisted.
"This stone, Stone 56, is the tallest one at the end of the inner horseshoe of sarsen stones.
"Because it was put back to the vertical in 1901 it has been assumed that the twist is the result of the modern excavators botching the job.
"My research shows that not only was the standing stone out of symmetry with the central solstice alignment originally, but that its now fallen partner had also been, and so were surrounding stones, including the Altar Stone."
Mr Daw, who last year came up with evidence that the outer stone circle at Stonehenge was once complete, said his newly discovered alignment was at 80 degrees to the line of the axis of the monument, which points to midsummer solstice sunrise and midwinter sunset.
"The stones point to the midwinter solstice sunrise and midsummer sunset," he said.
"This alignment had been missed by previous investigators... as they used an idealised plan rather than an actual plan for their calculations."
"This isn't some nebulous sighting line on a distant star; this is 100 tonnes of stone deliberately pointing to the major event at the other end of the day the rest of the monument celebrates.
"One stone out of line might be a coincidence but that it is five of the major stones, at least, shows it was a designed feature.
"It shows what can be discovered by simple observation even in such a well-researched site as Stonehenge."
Director of the Wiltshire Archaeological and Natural History Society (WANHS), David Dawson, said: "This is an interesting new idea which highlights the "skew" of the Stonehenge trilithons, which has been known for some time.
"It highlights the significance of the summer and winter solstices at Stonehenge, and the 80 degree angle between them.
"We know that the Bush Barrow lozenge, on display at the Wiltshire Museum, hints at this same significant astronomical feature.
"There will now be a debate between archaeologists and a re-examination of the evidence to test this new hypothesis."
Jessica Trethowan from English Heritage said it was "an interesting idea".
Mr Daw's theory has been published in the latest WANHS magazine.
People traditionally gather at Stonehenge for the winter and summer solstices