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

Sunday, 27 July 2014

More about glacial erosion


Two photos taken on the island of Svartloga the other day. The top one shows the streamlined whaleback forms typical of this part of the Stockholm Archipelago.  The rock surfaces here (heavily metamorphosed Pre-Cambrian granites) are on the up-glacier side of a larger roche moutonnee feature.  What is particularly interesting is the extent of fracturing on the rock surface -- the apparently random fracture pattern is obvious, except that it is not actually random at all, since there is a reason for everything in glaciated terrain.......

I'm intrigued by this top photo, since the fractures are on a rock surface which must have been under heavy compression when it was last glaciated.  Most cracks on roche moutonnee forms are seen where compression gives way to tension, near the highest point on the roche moutonnee surface.  Given a bit more time (ie a few thousand years more, maybe) and this rock surface would have been busted into bits.  Maybe we are looking at the result of pressure release, with these fractures appearing as a result of the removal of the ice load.

The bottom photo shows a moulded, smoothed and striated rock surface with some transverse fractures developing exactly where we would expect them, in readiness for the removal of the next slab or boulder as quarrying of plucking processes come into play.  The clean face at the top right of the photo is on the down-glacier flank of the roche moutonnee -- beautifully smoothed but not striated.  Sometimes these plucked faces are rough, and sometimes smooth.  The reasons for this are complex.  Another very striking feature is the colour difference between the up-glacier and down-glacier faces.  The striated (up-glacier) surface is very weathered, and is coloured grey.  The down-glacier or plucked face is a creamy pink colour -- quite unweathered and fresh.  It would be interesting to do some cosmogenic dating on faces like these, since their physical appearances suggest that on the striated face there has not been a huge amount of erosion since the last interglacial.......

Friday, 25 July 2014

More rock glaciers

Rock glacier, Kenai Mountains, Alaska

Rock glacier, Sourdough Mts, Alaska

Rock Glacier, Selwyn Mts, NWT, Canada

Rock glacier, Talkeetna Mts, Alaska (this one occupies an old cirque in the mountains)

Rock Glaciers

Rock glaciers are strange features which exist only in high latitudes, and for the most part in rugged mountain terrain.  Sometimes they look like glaciers covered in debris, but some occur in such inappropriate locations that they clearly have nothing much to do with glacier ice or glacial processes.  They can be very large and very spectacular -- but they are inadequately understood.  One requirement for them seems to be the presence of permafrost, or at least a long freezing season.  They are composed, internally, of vast amounts of rock debris with the interstices filled by  ground ice formed by the trickling downwards of melting snow which then later freezes.  Another requirement is that they must have relatively modest precipitation -- so they are found for the most part in relatively arid environments, or in places where snow cannot accumulate in sufficient quantities for snow-patches to accumulate and to be converted into firn or glacier ice.  But they certainly flow just as glaciers do -- and this is apparent on many of the surface features.  Here are three photos of rock glaciers found on the web:

The Mount Sneffels rock glacier, Unites States

Snaefell Rock Glacier, southern Iceland

Rock Glacier near McCarthy, Alaska



Tuesday, 22 July 2014

Aurochs find at Whitesands




Thanks to Tony for bringing this to my attention -- from a BBC web page.  It's based on a National Park press release.  Interesting finds -- but it would be good to know more details.  I have no idea how strong the evidence for the footprints actually is -- and I remain to be convinced that they are human, and very old.  Also, I doubt that they are 10,000 years old, unless there is strong radiocarbon dating evidence to support that contention.  From what we know about the submerged forests, their uppermost surfaces are much more likely to be between 7,000 and 5,000 years old -- since that is when the final submergence by a rising sea level is likely to have happened.  The caption to the photo of an aurochs horn calls it an "aurochs tusk"... !!!  Hmmm......

It would be good to know more about the context of the find of the aurochs horn and "the remains" which presumably mean bones.

And as for this:  "the footprints.......suggest the humans may have been tracking a large hoofed animal such as an auroch"  is pure fantasy.  The footprints were at Newgale and the aurochs was found at Whitesands.  There is nothing whatsoever to connect them, and footprints on a peat bed could have been created by anybody wandering through a wooded area for any purpose whatsoever.  But that's what archaeologists apparently do nowadays -- to hell with science; all that matters is creating a good story.  But I suppose in this case it's harmless enough.

 http://www.bbc.co.uk/go/em/fr/-/news/uk-wales-28104756

Storms reveal hunter-gatherer footprints and animal remains

More evidence of prehistoric life in west Wales has been discovered following the severe storms earlier in the year.

Ancient human and animals footprints have been found at Newgale and the remains of extinct cattle at Whitesands in Pembrokeshire.

The finds follow earlier discoveries of forests and other remains along the south and west Wales coastline.

The footprints are believed to date from around 10,000 years ago.

The discoveries were made possible after violent tides stripped large areas of sand away from beaches.

 Phil Bennett from the park authority holding the aurochs tusk
The remains of an aurochs, an extinct breed of cattle, was also found.

Pembrokeshire Coast National Park Authority's culture and heritage manager Phil Bennett said: "The footprints in the exposed peat at Newgale, which are most probably from the Mesolithic period around 10,000 years ago, suggest the humans may have been tracking a large hoofed animal such as an auroch.

"The discovery of the aurochs remains at nearby Whitesands would support this theory and the horns give you an idea of just how large these creatures must have been."

However Mr Bennett said there were two sides to the story, as the weather had also led to the loss of other resources.

The aurochs remains will be conserved, with the aim of putting it on display at Oriel y Parc Gallery in St Davids.

Sunday, 20 July 2014

The Quiraing Landslip, Isle of Skye


Two more fabulous photos of the Quiraing Landslip, on the Totternish Peninsula north of Portree, on the Isle of Skye.   This is the biggest landslip in the British isles,  giving rise to  a bizarre and almost "otherworldly" landscape.....
The Quiraing is a landslip located in the north-east of the Isle of Skye, north of Portree, on the Trotternish Peninsula. It is the largest landslip in Great Britain. - See more at: http://www.natureflip.com/quiraing#sthash.4ymdlGDM.dpuf





The Quiraing is a landslip located in the north-east of the Isle of Skye, north of Portree, on the Trotternish Peninsula. It is the largest landslip in Great Britain. - See more at: http://www.natureflip.com/quiraing#sthash.4ymdlGDM.dpuf

Saturday, 19 July 2014

Ness of Brodgar



There's a nice article, graphic, map and photo gallery featuring the Ness of Brodgar on the National Geographic web site, here:
http://ngm.nationalgeographic.com/2014/08/neolithic-orkney/brodgar-graphic

The article claims that a great complex of buildings and standing stones involved the quarrying and transport of "thousands of tonnes" of sandstone which was transported several miles to the site portrayed in the graphic.  But no evidence is given to support that contention.......

From the "Decoded Past" web site:

http://decodedpast.com/neolithic-scotland-heart-orkney-world-heritage-sites/4413

"The Neolithic drive towards impacting and shaping the landscape, which is evident in slightly later sites across Britain and France, probably began with agriculture. The Neolithic people learned how to move and manipulate rocks in order to make fields to grow crops and rock enclosures to keep cattle from straying.

The Stones of Stenness are true megaliths, in that they are about twenty feet tall. The 23 foot passage into Maeshowe is almost completely walled by two massive megaliths laid on their long edges. There may originally have been sixty stones in the perfect circle that is the Ring of Brodgar, although only twenty or so are still evident. Certainly, a large number of people participated in quarrying, moving and erecting the stones."

There's a quote from Neil Oliver's book:   “A whale-back ridge dominates the middle of the Brodgar peninsula, roughly halfway between the Ring of Brodgar and the Stones of Stenness, and it had always been taken for a natural feature. The fact that it lies right at the centre of this great natural amphitheatre, containing all of Orkney’s most famous ritual monuments, seemed purely a coincidence of geology.”

The award of World Heritage status in 1999 led to plans for a more thorough investigation of the area surrounding the site. This began in 2002 when a geophysical survey was undertaken across the isthmus that links the stone circles, and then the archaeologists got to work. Annual programmes of digging still continue, but by 2012, archaeologists excavated a massive walled enclosure of a dozen mainly ritual buildings.

“We have discovered a Neolithic temple complex that is without parallel in western Europe. Yet for decades we thought it was just a hill made of glacial moraine,” says discoverer Nick Card of the Orkney Research Centre for Archaeology. “In fact the place is entirely manmade, although it covers more than six acres of land.” 

Robert McKie reports in his article ‘Neolithic discovery: why Orkney is the centre of ancient Britain’.

The site contains some of the most perfectly constructed stone walling of any historical era that stood for a thousand years from about 3300 BC. In spite of being ritually ‘decommissioned,’ after excavation, it still stands to two metres in height. Anyone crossing between the two stone circles would have to pass through the site, and its buildings are not residential in character.
-------------------------------------------

There does seem to be a vast amount of stone used in these structures.  Don't have a problem with that -- if you have a big project and you have little timber available, you will use stone, from wherever you can get it.  Much of it will have been quarried or dug up close to the places where it was needed -- indeed the easy availability of stone might have been a determinant for the location of the Ness in the first place.  Other stones will have been glacial erratics,  and some monoliths might have come from places where they were easy to wedge out of the bedrock exposures.

I get a sense from the newer reports that there is a big difference between the interpretations of Colin Richards, who attaches profound significance to the act of quarrying and profound -- maybe spiritual or ritualistic -- significance to the places where the quarrying was done, and the much more utilitarian interpretations of the current digging teams on Orkney.  As far as I can see, they think that the builders of the Ness needed stone, so they went out and got it from wherever they could find it.  Nice and simple.




Tuesday, 15 July 2014

Glen Tilt -- a classic glacial trough


Glen Tilt in Perthshire is a classic U-shaped glacial trough -- except that it isn't really typical of glacial troughs at all.  It's a long, straight through valley, cut along the line of a fault, and for at least some of the time the whole valley was submerged beneath an ice sheet, with ice moving across it at an angle.  So the trough might have contained stagnant ice at that time.  But its shape reveals that it has clearly carried streaming ice at other stages of the Ice Ages, just like some of the through valleys of Greenland.

Monday, 14 July 2014

Tindholmur -- half an island is better than nothing

 I found this amazing image on the web --the island of Tindholmur in the Faroes.  Click to enlarge.  Many of the islands in the Faroes archipelago are really mountain tops or mountain ridges sticking up out of the sea, and marine erosion is so powerful here (having been in operation since these lavas were emplaced in the Tertiary) that many ridges and islands have quite literally been chopped in half.  Coasts facing west and north have been particularly vulnerable.

Tindholmur is a classic example of "half an island", with the old mountainside on the left in the photo, and a vertical sea cliff to the right.

Here is another photo, from the other side of the island.


Sunday, 13 July 2014

Glaciation of the Faroe Islands




Here is a link to a very interesting presentation found on the web on the glaciation of the Faroe Islands.  There are no author's names, so I don't know who to thank.  But some photos are pasted in below.

www.uio.no/.../GEG2130%20HHC%20Faroe%20Islands%20Geomorph...

 Cirque wall, cut into layered Tertiary volcanic rocks typical of the islands

 An old land surface abruptly truncated by a typically precipitous oceanic cliff line

Spectacular oceanic cliffs, typical of the west-facing coasts
 Landslide on a mountainside.  Usually the trigger is moisture overload, linked to the heavy rainfall experienced by the islands -- but in some cases pressure release comes into play

 Soil stripes on a mountain slope -- formed here just on the edge of periglacial conditions

Friday, 11 July 2014

Chevron fold -- Kong Oscars Fjord

This is for Rob and any other geologist who visits this site.  I found this amazing photo of a chevron fold in the colourful sedimentary strata exposed on the shore of Kong Oscars Fjord, East Greenland.

A text-book example indeed....... click to enlarge.

Tuesday, 8 July 2014

The Old Man of Storr, Skye

 The Old Man of Storr and other pinnacles on the Isle of Skye -- a bizarre landscape made up of the shattered debris of lavas which have slid downslope on a base of sedimentary rocks.

Another for our long list of amazing landscapes affected by ice.  The Old Man of Storr and the Totternish Landslides in the north of the Isle of Skye.  Here is what the walking guide says:

The Trotternish peninsula in the north east of Skye is dominated by a spectacular ridge of hills that runs for over 30km along its backbone. There are 13 named summits, from Beinn Dearg in the south to Meal na Suiramach in the north. The ridge rises to its highest point at the 719m summit of the Storr, above the tortured landslip topography that includes the iconic pinnacle - The Old Man of Storr. The ridge is home also to the Quiraing, another landslip area of pinnacles and gullies, this time below the summit of Meal na Suiramach. The hills here are composed of horizontal flows of basaltic lavas, which built up on top of each other to a depth of around 800m. On the east side of the peninsula the underlying sedimentary rocks have collapsed under the weight of the basalt, tipping everything sideways to form the distinctive landslips. The result is a wonderful combination of unique scenery, outstanding views and first-rate walking terrain along the crest of this undulating escarpment.

When we stayed in Portree recently, we could see the Old Man of Storr in the distance, but sadly we had no time to visit it.  The landslides are generally assumed to be post-glacial -- and indeed some sections of the slides are still moving.  I assume that the trigger mechanism was pressure release following the removal of glacier ice; this is what triggered off scores of landslides in Iceland, and my best guess is that the big landslides on the North Pembrokeshire coast west of the Witches Cauldron were also set off when Devensian ice that had been pressed against the coast finally melted away.



Lundy Island and the LGM dilemma


Herewith some more info from that interesting paper by Chris Rolfe et al -- relating to the glaciation of Lundy Island.  As the authors say, this is a critical location for sorting out the Pleistocene history of the Bristol Channel area -- and it should be possible to tie in the evidence from Lundy with that from the Scilly Isles, where the LGM (Last Glacial Maximum) is now securely dated to the Late Devensian, around 20,000 years ago. Securely dated?  Or so we thought........

The link to the paper:
"Paired 26Al and 10Be exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles"
C.J. Rolfe, P.D. Hughes, C.R. Fenton, C. Schnabel, S. Xu, A.G. Brown
Quaternary Science Reviews 43 (2012) 61e73
https://www.researchgate.net/profile/Christopher_Rolfe/publications

This is the map of glacial and other features -- much more accurate than the old map made by Frank Mitchell in 1968.  Most of the important features are at the northern end of the island.


 The cosmogenic dates are shown on the map, together with the locations from which the rock samples were taken.

This is the Abstract for the paper:
Lundy lies in a strategic geographical position for understanding the glacial history of the British Isles. The island bears evidence of glaciation, largely in the form of ice-moulded bedrock and glacially- transported boulders e an unusual occurrence this far south in the British Isles. Irish Sea ice pene- trated the western Bristol Channel overriding Lundy from the northwest during the last phase of glaciation in this area. The results of paired terrestrial cosmogenic nuclide analyses (26Al/10Be) constrain the timing of this extensive glaciation and provide, for the first time, an age for the exposure of Lundy granite following deglaciation. The results from nine paired samples yield 26Al/10Be exposure ages of 31.4e48.8 ka (10Be) and 31.7e60.0 ka (26Al). This challenges the view that any glaciation this far south must belong to Middle Pleistocene glaciations, such as the Anglian Stage (c. 480e420 ka) and a Devensian age for the last glaciation is consistent with findings from the Isles of Scilly further south. However, the findings suggest early-mid Devensian (marine isotope stage (MIS) 4e3) glaciation of Lundy. It also implies that the island was exposed or covered for a short time by non-erosive cold-based ice at the global Last Glacial Maximum (LGM) during MIS 2 (26e21 ka). The potential exposure of the island throughout MIS 2 contrasts with the evidence from the Isles of Scilly and the Celtic Sea, which were glaciated at the LGM.

Nine locations on ice-scoured surfaces were sampled, with two samples from each site subjected to different cosmogenic analyses, to give 26Al and 10Be exposure ages.  There was a considerable consistency in the results, suggesting exposure ages of between 40,000 yrs BP and 30,000 yrs BP.  Of the 18 analyses completed, only one was anomalous.

The authors consider in detail the possibility that the dates are consistently over-estimating the amount of time that has elapsed since the ice retreat from the summit of the island -- and that the real exposure time is c 20,000 yrs BP, which would be more or less in tune with the results from the Scilly Isles.  But they can find no real reason to disbelieve them, so they stick to the conclusion that Lundy was glaciated during the Early or Middle Devensian, at a time when most other authors have assumed cold but non-glacial conditions in western Britain.  Indeed, that has been my conclusion ever since I worked in Pembrokeshire in the 1960's and 1970's.  The stratigraphy there points to a long period of periglacial / permafrost activity in the Eary and Middle Devensian, followed by a short glacial episode in the Late Devensian, followed by an oscillating but generally warmer climate for the period 20,000 - 10,000 years ago, culminating in the cold snap of the Younger Dryas.

So what is the truth of the matter?  Chris Rolfe and his colleagues now think that a long ice lobe might have crossed the Celtic Sea from Southern Ireland, flowing broadly NW towards SE.  It crossed Lundy and might have reached the coasts of Cornwall and Devon.  But they speculate that it might have missed Pembrokeshire.  This is their map of suggested ice limits:


Faithful readers of this blog will see a strong similarity with this map, which I first published in 2011:


The difference is that Rolfe and colleagues have more or less accepted the flowlines of Scourse, Clark, Evans and others, whereas I have not, since I cannot see the sense of a long surge down the centre of the Celtic Sea when there has been nothing to constrain its lateral expansion.  That is glaciologically inherently unlikely......  so the answer has to be that ice crossed the Celtic Sea from an ice-shed somewhere near the coast of SE Ireland, hitting the coasts of SW England and Wales and leaving traces fairly close to present sea-level at many places along an ice front more than 300 km long.

I cannot see that ice from such a piedmont glacier can have overridden Lundy Island while leaving Pembrokeshire unaffected by ice -- that would imply very limited ice extent on the part of the Irish Sea Glacier and the Welsh Ice Cap. 

For the moment, I'll stick with the tentative suggestion that there might be a consistent error somewhere in the dating of the "glacial event" at Lundy, since an early / Middle Devensian age is not actually supported by glacial stratigraphy.  But the debate is blown wide open, and for this, Chris Rolfe and his colleagues deserve much credit.


Monday, 7 July 2014

Pembrokeshire erratics on Lundy Island?




 Lundy Island traces of glaciation.  Above -- two photos of ice moulded (whaleback) forms in the north of the island.  Below:  Perched blocks / erratic boulders resting on glaciated pavements in the northern part of the island.


In a previous post, I speculated about the possibility that glacial features on Lundy Island might date from the Devensian:
http://brian-mountainman.blogspot.se/2011/01/did-devensian-ice-reach-lundy.html

Well, that has now been confirmed in an article that somehow escaped my attention -- published in 2012.  It's a hugely important paper, from Chris Rolfe and colleagues.  I'll devote more space to it in due course, but the key things are these:

(a)  detailed mapping of the glacial features on the island show that they are varied and relatively fresh, confirming the conclusions of Frank Mitchell many years ago that there was indeed glacial ice at one time even on the highest parts of the island;

(b)  cosmogenic dating shows that the last ice cover here was during the Devensian -- although the suggestion is that the ice was present in the Early to Mid-Devensian, and had nothing to do with the Late Devensian ice advance which brought ice to the northern shores of the Scilly Isles.  This is an anomaly that needs to be sorted out....

(c)  Erratic clasts in the glacial deposits or scattered across some ice-scoured pavements on the island suggest that the ice came from Pembrokeshire.

Now we need some help, folks.  Richard, Rob, Olwen and any other geologist reading this -- can you please give us your considered opinions on the rhyolite erratics?  Below are the geochemical analyses presented in Table 1 (click to enlarge).  There are two rhyolite boulders:

The quartzites might also be interesting.........
Anyway, here is the text description:

 =================================

"Paired 26Al and 10Be exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles"
C.J. Rolfe, P.D. Hughes, C.R. Fenton, C. Schnabel, S. Xu, A.G. Brown
Quaternary Science Reviews 43 (2012) 61e73

Extract:
3.3.2. Erratic gravels in the north island watershed
The gravels that are widely spread over the northern island watershed include numerous clasts that are foreign to the local lithologies. Mitchell (1968) identified 100 rounded clasts. The methods used by Mitchell to identify these clasts is not stated in his paper only that they “were collected and identified as follows”:
“Possibly of island origin” 35 clasts including quartz: 22; miscellaneous igneous: 9; mica schist: 2; haematite: 1; limonite: 1 “Probably not of island origin” 38 clasts including grey quartzite: 25; pink quartzite: 13
“Certainly not of island origin” 27 clasts including coarse sand- stone: 9; feint: 8; chert: 3; micaceous sandstone: 2; sandstone with carbonaceous debris: 1; greywacke: 1; ignota: 3

The physical and geochemical differences between the erratics and the Lundy granite are clear from the XRF analysis of 8 clasts in this study (Table 1). The sample of Lundy granite had the following major element composition: 71% SiO2; 16% Al2O3; 6% K2O, and; 3% Na2O. This is representative of Lundy granite and is broadly consistent with major element data from Lundy granite presented in Stone (1990). Three clasts had very different SiO2 contents of 94e98% (PH 2, 3 & 7). These are interpreted as quartz or quartzite erratics. All of the clasts were fine-grained but varied in colour and appearance (PH 2 and 3: light grey; PH 7: pink with quartz veins). Some of the most extensive quartzite formations in the southern Irish Sea area are present on the coasts of both northwest Wales in Anglesey and the Lleyn Peninsula (Phillips, 1992) whilst quartz pebbles are widespread in the Millstone Grit of the Pembrokeshire coalfield (Archer, 1965). The very high SiO2 content of these clasts is higher than some quartzites in Wales (e.g. Phillips, 1992) and it may be that these clasts are quartz erratics reworked from conglomerates such as Millstone Grit. Pink quartz veins are also present in Morte slate, which is present in the SE corner of the island and some of the quartz-rich samples may be of local origin. Nevertheless, there are no quartz or quartzite outcrops in the area of the gravel spreads on Lundy and these clasts are clearly erratic to where they are found. Two other clasts had SiO2 contents of 82% (PH 5) and 85% (PH 1). This suggests that the clasts are not volcanic since even the most quartz-rich rhyolites rarely exceed 80% SiO2. The clasts are more siliceous with lower aluminium oxide contents than most sandstones and mudstones of the southern Welsh basin, for example (cf. McCann, 1991). The clasts have no comparable equivalent within the Lundy volcanics. As noted in the previous paragraph, the southeast tip of Lundy is formed of Morte Slate and this lithology is likely to be present offshore. However, Upper Devonian slates of this region contain much less SiO2 (c. 55e60%) and much more aluminium oxides (>20%) and potassium oxides (!4%) (Cattell, 1998). Thus, these clasts have not been derived from the Lundy country rocks and can be considered to be erratics.

Two samples had similar physical properties (fine-grained light blue-grey clasts) with similar geochemistry with SiO2 contents of 75.8% and 79.5% and combined Na2O and K2O of 2.6% and 3.3%. These samples are similar to some rhyolites. On Lundy, rhyolite dykes are known to be present e including in the north of the island near the gravel spreads where the clasts were found (Thorpe and Tindall, 1992). However, the local rhyolites have much higher values of combined Na2O and K2O (7.8e10.5%). Furthermore, the clasts have much higher iron oxide content (Fe2O3: 6%) than most of the Lundy rhyolites that have otherwise comparable geochemistries (Fe2O3: 1e2%). The clasts have a closer geochemical composition to rhyolites from the thick rhyolite sequences of the Skomer Volcanic Group off the westernmost coast of Pembrokeshire (e.g. Thorpe et al., 1989) although the precise origin remains unknown.

In addition to the siliceous clasts analysed using XRF, several limestone clasts collected from the gravel spreads are clearly erratics since no carbonate bedrock lithologies are found on or near Lundy. Given that rhyolites, quartz/quartzites, siliceous sedimen- tary and carbonate limestones are common lithologies, a wide range of sources are possible as noted in the previous paragraphs. Nevertheless, the presence of these erratic clasts is consistent with transport and deposition in association with an Irish Sea Ice Sheet. The fact that the clasts are well-sorted into cobbles and gravels and predominantly rounded suggests deposition by water. The presence of these deposits on the watershed is consistent with sedi- ment release at the apex of bedrock obstacles in subglacial channels (cf. Lesemann and Brennand, 2009).


=============================

Now the question is this:  do any of the rhyolite erratics show any strong similarities with the rhyolites from Pembrokeshire, or more specifically from the Fishguard Volcanics, or more specifically from the Pont Saeson / Craig Rhosyfelin area?

And another question:  do any of these rhyolite erratics show similarities with any of the rhyolite orthostats or debitage  found at Stonehenge?




Sunday, 6 July 2014

Glen Valtos -- erratic meltwater flow?


On the Isle of Lewis, not far from Callanish, there is the most spectacular meltwater channel on the island -- called Glen Valtos.  It's about 2.5 km long, and flows from west to east, and while it is quite shallow and insignificant at its intake end, it is about 45m deep at the eastern end, where it decants into Loch Miavaig.

So far so good.  The trouble is that the orientation of this channel is quite at odds with the other evidence from the neighbourhood which shows Devensian ice movement broadly from south towards north.  The orientation of the channel is about 90 degrees out.  This is shown on the map below, from a paper by DG Sutherland:


The channel is shown in the top right corner of the map.  The only way to make sense of this map is to have a large glacier mass offshore, to the west -- that would be no problem during a major glaciation, since the coastline of the time would have been far to the west.  In general, subglacial meltwater flow (which this must have been) accords with ice movement as recorded through other features, and with the overall ice gradient.  But nonetheless, I am somewhat perplexed as to why Sutherland, Peacock and other authors have insisted that this channel is of Late Devensian age -- causing them all to get into a bit of a tangle in trying to make sense of the field evidence relating to moraines and fluvio-glacial and other features.

To me, it would make much more sense to argue that the channel is a very old feature,  dating from an earlier glaciation, and maybe used again and freshened up during the Devensian.   It doesn;t make a great deal of sense to me to assume that nearly all of the glacial landforms in the UK are only about 20,000 years old, when we know that there have been several earlier glacial episodes which must have resulted in great landscape modification.

Talking of meltwater channels going the wrong way, this reminds us of the Gwaun-Jordanston meltwater channel system in Pembrokeshire, which has an overall orientation which is very difficult to explain by reference to the last movement of the Irish Sea Glacier, which was broadly NW towards SE.  The channels, in contrast, run east - west and then NE towards SW.  They MUST be very old...... and formed at a time when ice from the Welsh Ice Cap must have been in the ascendancy.



The Valley of the Rocks




Close to Lynmouth in Devon the cliff scenery is very spectacular, with one dry valley in particular making the grade as a "must see" tourist attraction.  This is the Valley of the Rocks -- a deep dry valley with steep sides, one of which has been partly eroded away by cliff retreat.  Many attempts have been made to explain this feature away as a "normal valley" -- but it is anything but normal, and more and more geomorphologists nowadays are interpreting it as a meltwater spillway, formed when the ice of the Irish Sea Glacier was pressed up against the coast.

There are other similar valleys -- but not as spectacular -- at Hartland Quay, Dalehole Point, Speke's Mill Mouth and Bideford.  These also suggest considerable erosion by meltwater that has come from melting glacier ice.

How old are the features?  It's very difficult to say, but I think I would go for an Anglian age, since that is the date of most of the glacial features in the South-West.  That having been said, we know that the Devensian ice reached the Scilly Isles, and I am open to the idea that the Devensian ice of the Irish Sea Glacier also pressed up against the coasts of Cornwall, Devon and maybe Somerset as well.  Watch this space......

Saturday, 5 July 2014

No anonymous posts please!

Just a reminder, in case one is needed, that this blog does not accept comments from anonymous contributors.  Lots of them do come in -- I don't even see them, since they go straight into the trash can and get deleted.  If you would like to take part in any of our discussions, just make sure you use your name or a pseudonym, so that we know you are a human being.......

Thursday, 3 July 2014

How did Palaeozoic building stones get to Glastonbury?


Glastonbury Tribunal -- reputedly built in the 1400's

I came across this interesting snippet in Hugh Prudden's excellent notes on the geology of Somerset -- suggesting that some of the building stone used in the older buildings of Glastonbury has come from Palaeozoic outcrops -- maybe from the Quantocks, Brendon Hills and Exmoor.  These of course lie to the west of Glastonbury, and if the Irish Sea Glacier ever did reach Glastonbury (as I have speculated more than once), it might -- just might -- have picked up Palaeozoic erratics on the way through.  I don't know whether any more accurate provenancing has ever been done -- can anybody enlighten us?

18.  GLASTONBURY ST 5138 
Junction Bed-Yeovil Sands-Holocene formations-landforms

Erosion has separated this outlier of Yeovil Sands from the main escarpment which can be seen some 26 km to the east.  The intervening vale is Lower Lias clays.  To the north are the Mendip Hills and, to the west, the Rhaetic Beds and Blue Lias of the Polden Hills.  On a clear day the Palaeozoic massifs of the Quantocks, Brendon Hills and Exmoor can be seen.  Fluvial and marine deposits of sand, clay, gravel and peat underlie the Somerset Moors and Levels which extend to the Bristol Channel.  This is, perhaps, the best place for a comprehensive view of Somerset. The Junction Bed limestone, which includes the marlstone Rock Bed, forms a marked bench.  The main street in Glastonbury has a rich variety of Palaeozoic and Jurassic building stones.  The Abbey ruins have Doulting Stone facings with a core of Blue Lias.and Marlstone.  



The Quantocks

 The Brendon Hills

Exmoor


Were the earliest megalithic stone settings simply imitations of nature?

 

The Devil's Den, Clatford Bottom -- built because this was exactly where the big stones were?

 

I have been looking again at David Field's chapter from this book on Avebury.  I have made some earlier posts about his comments (in 2010) but have been struck again by how straightforward and practical his interpretations are.  No mystical mumbo-jumbo here -- just entirely reasonable analysis and interpretation of the situation as he sees it.  And the interesting thing is that he seems to interpret most of the motives of the early builders of stone settings as strictly utilitarian -- if there are lots of stones jumbled together in a particular place, either clear them away or use them for something.

As I have said before, this seems very much in tune with my assertion that Stonehenge was simply built where the stones were.  And it also explains why the landscape around Stonehenge is relatively "stone free" today --  they simply used up all the stones they could find (sarsens and "foreign" stones) and when they were all used up, the building work skidded to a halt........

Another interesting thought relates to the past occurrence of sarsens in such great abundance in some areas that they were piled up on top of one another, some teetering in precarious positions.  So might the early use of capstones on dolmens have been an imitation of nature?

It's also tempting to think that these jumbles or piles of stones might not have been strictly periglacial, as Field assumes, but may be even the last traces of moraines?  Long ago Maskelyne speculated that there might have been a moraine at or near Stonehenge.... and although Isobel Geddes and Helen walkington (in the same book) assert that there is no evidence of glaciation on the Marlborough Downs, nothing in this life is certain........

====================

The Avebury landscape: Aspects of the field archaeology of the Marlborough Downs

by Graham Brown, David Field, David McOmish and Deborah Cunliffe

Oxbow Books, 2005,  240pp, 91 b/w illus, 15 tabs

8. Some observations on perception, consolidation and change in a land of stones.
David Field


Abstract
The incidence of sarsen stones on the Marlborough Downs is outlined and methods of clearance and destruction outlined. Implications of prehistoric removal of stones, the land and vegetation and the way in which boulder dominated land might be perceived are also considered.

=======================

David Field on minimisation of effort:  "Many of these monuments may originally, however, have been constructed in areas of dense sarsen accumulation and there are implications in terms of the practical engineering involved in monument construction and in the way that these places were thought of by early travellers and settlers. The lack of boulders lying around parts of the landscape is entirely a result of cultural activity over several thousand years."

David Field on dolmens in stony landscapes:
"Why was it necessary to construct such dolmens? The Devil's Den, a massive capstone placed on two uprights situated just above the floor of Clatford Bottom, was not constructed as a prominent isolated feature in the countryside. On the contrary, it was hidden away on the floor of a valley that, records suggest, contained one of the densest concentrations of sarsen boulders in the area; so dense in fact that it was allegedly possible to step from boulder to boulder (Brentnall 1946). The monument would have been largely camouflaged by the sheer concentration of natural boulders. By the middle of the 19th century, however, most of these had been cleared away leaving Merewether to lament that it was now surrounded by nothing but turnips (Merewether 1851a, 74).
Conceivably the process began in imitation of groupings of stones that were present as natural phenomena. Groups of naturally occurring boulders formed into mounds, or even placed boulder on top of boulder by periglacial action. Brentnall (1946, 427), for example, recorded the presence of naturally occurring boulders 'heaped on each other' in a plantation west of Overton Delling. Set amongst such a density of stones, the dolmen could hardly be described as monumental. It would not be distinguished unduly from the naturally placed clitter all around it, except by the area of removed boulders immediately about it.
A further example once lay on the valley floor of Temple Bottom, Rockley. This was also situated amongst an enormous accumulation of sarsen, though like Clatford Bottom it has long since been cleared, a process evidently already under way when Smith (1884, 195) visited the site."

David Field on Avebury:
"The origin of the stones in the Avebury stone circles is often considered to derive from one of the accumulations that still exist on Overton or Fyfield Downs to the east of the site, each stone being dragged a distance of several kilometres up and over Hackpen Hill. Such suggestions are merely a response to where boulders can be found in abundance today. However, as noted above stone was certainly available locally and may have been present on the site itself. There is no reason for the stones to have been hauled such a distance. Local accumulations of sarsen are or were certainly present here and it is likely that the valleys were once covered with boulders even though only isolated examples now survive. The farmer of land east of the West Kennet Avenue confirmed that a ridge c1.0m high and 30m wide `consists of a seam of sarsen that runs through the field' (Barker 1985, 21). Thus the land for this massive construction would need to be cleared first and any boulders within the immediate vicinity removed. Given this scenario the stones for the circle may well have come from the immediate vicinity of the henge, some potentially from the interior."