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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Friday, 13 January 2017

Rocks and landscape: Pembrokeshire

Simplified geological map of Pembrokeshire, showing the clear split into two geological "provinces" -- Lower Palaeozoic in the north, and Upper Palaeozoic in the south

Structural trends -- generalised.  The trend lines do not coincide precisely with either fractures or anticlinal and synclinal axes

Topography of Pembrokeshire -- based upon satellite imagery and showing clearly the structural trends in south Pembrokeshire

I've been doing some work for the new Historical Atlas of Pembrokeshire, and came upon these three images which show rather nicely what a close relationship there is between landscape, coastal configuration, geology and structure.

Time to bring the Wolstonian in from the cold?

 Suggested ice cover for the Wolstonian / Riss / Saalian glaciation of Wales and adjoining areas -- showing relations between Welsh ice and ice flowing from other source areas.  The location of the Gwaun-Jordanston meltwater channel system is also shown.  The limits of the ice cap are taken from one of the models run by Henry Patton and colleagues, assuming a steep precipitation 
gradient from west to east.

 The subglacial meltwater channel complex to the south of Fishguard and the Carningli upland.  I have now come to the view that these channels cannot have been formed during the wastage of an Irish Sea Glacier that came in from the NW, flowing towards SE.  The orientations and characteristics of the channels are all wrong.  The channels must have been formed beneath ice flowing broadly from east to west -- in other words, under a segment of an expanded Welsh ice cap.

When  I was a research student working towards my doctorate, the prevailing view was theat there was a rather complex glacial history (involving many glacial phases) on the eastern side of Britain, but that there had been just two discernible glacial phases in the west.  These were called the "Older Drift" and "Newer Drift" glaciations, and most people in the 1960s and 1970s equated them with the Riss/Saalian / Wolstonian on the one hand and the Weichselian / Vistulian / Wisconsin / Wurm / Devensian on the other.  The assumption was that glacial activity had been much more concentrated and therefore vigorous in the west than in the east -- and this was an explanation for the lack of any coherent glacial deposits beneath the raised beach dated almost everywhere to the last interglacial (Ipswichian).  Older glacial deposits had simply been eroded away by succeeding advances (we thought), whereas in eastern England they has simply piled up like layers of a cake. Then things started to get a bit more complicated, as field workers started to find things that did not quite fit and as more and more dating evidence started to pour in from all over the UK and Ireland.  Eventually the most recent big glaciation (the LGM) was assigned to the Late Devensian (at least I got that bit right, in spite of much opposition) and the Older Drift Glaciation was reassigned to the Anglian glacial episode.  That pushed it back from c 250,000 years ago to around 500,000 years ago.  And that left a very big gap between 500,000 BP and 20,000 BP.........

  The Wolstonian glaciation was not abandoned, because there is clear evidence of it in East Anglia and south of Birmingham, for example -- but because many of its deposits were overridden by Devensian ice and because its outer limit lay within the greatest extent of Devensian ice here and there, researchers were very reluctant to portray its maximum extent on a map.  This is one suggestion, from Gibbard and Clark:

The dashed line on the map shows the tentative Wolstonian limit, but as I have suggested before, it is not sensible across south Wales.  There is no sound reason for assuming glaciation across mid Wales but not across the Brecon Beacons and the Coalfield uplands -- so the southern limit in Wales must have been quite close to that shown for the Devensian.  What about Pembrokeshire, the Bristol Channel and the Celtic Sea?  Difficulties galore........

There are several features in Pembrokeshire and Ceredigion which cause me now to think that there was a greatly expanded Welsh ice cap during Wolstonian times, around 250,000 years ago.

First of all, there is the matter of the orientation of the Gwaun-Jordanston meltwater channel system, as explained at the top of this post.  I think these channels might actually have been cut during the Wolstonian Glaciation, and later modified during the Devensian.  It's possible that some of the other coastal meltwater channels, and the spectacular channel in Ramsey Sound, are also of Wolstonian age.

Second, there are apparent double and triple till sequences in northern Cardigan Bay with -- in some cases -- weathering horizons between them, suggesting that interstadial or maybe interglacial conditions prevailed between lower and upper tills.

Third, I am still mystified by what went on in the Pleistocene on the coasts of Ceredigion, where thick diamictons are variously interpreted as till laid down under ice moving westwards from the Cambrian Mountains, as periglacial slope deposits (by Eddie and Sybil Watson) or as redeposited tills.  The glacial / periglacial sequence at New Quay is also a puzzle, as I explained in an earler post.  There is a reasonable chance that some of these deposits are Wolstonian in age, laid down by ice flowing from an expanded Welsh Ice Cap.

Fourth, I now think that the frequency of Silurian gritstone erratics in the Devensian till at Newport  must have come from previously deposited Welsh till far out in Cardigan Bay.  That till is most likely either Early Devensian or Wolstonian.

Fifth, we have the matter of the till patches of central and south Pembrokeshire.  They look old, but are they old enough to have been associated with the Anglian glacial episode, when almost all other coherent till deposits associated with it have been worn away?

 Extract from the BGS geology viewer, showing fluvioglacial sands and gravels (pink) and till (light blue) outside the supposed Devensian limit in SE Pembrokeshire

Sixth, the map of rock troughs and tunnel valleys (which we have discussed earlier) suggests formation at least in part beneath a big Welsh ice cap -- much bigger than the LGM ice cap modelled by Henry Patton and colleagues.

So here is a proposal.  During the Wolstonian Glaciation Wales was completely inundated by ice from the Welsh Ice Cap, which was powerful enough to flow right across Cardigan Bay and into St George's Channel. It may be that during this glaciation the Irish Sea Glacier was less powerful and less extensive than it was during the Anglian and Devensian glacial episodes.

There we are -- a working hypothesis........ comments please!  I would be rather interested to know how the evidence from coastal sections in North Wales and South Wales fits -- or does not fit.......

Thursday, 12 January 2017

Putorana Piedmont Glaciers?

Now and then, when one is trundling cross-country, courtesy of Google Earth, one spots something rather peculiar.  Above, we see the northern edge of the Putorana plateau in Central Siberia.  The plateau edge is quite sharp, running tight to left across the photo.  In the mountain area we see the modified dendritic pattern of troughs that were transporting glacier ice northwards at various times during the Quaternary.  To the north we see the tundra of the North Siberian Plain, with patchy woodland and many small meltwater lakes in the permafrost.

But the really impressive features are the bulbous lobes pushing out from the mountain front onto the plain.  What on earth are they?  The most logical explanation is that they are morainic loops formed by piedmont glaciers pushing northwards from the ice sheet -- rather like the features we see in Greenland, Ellesmere Island, Alaska and elsewhere today.  The top photo is from Peary Land (Greenland), the middle one is the Malaspina Glacier in Alaska, and the lower one is from Ellesmere Island.

It's often stated that pedmont glaciers form most readily in high Arctic situations where conditions are quite arid, with continuous permafrost and very little glacier bed melting.  This means that basal melting and bed sliding are minimal, and that most glacier movement is byb internal deformation.  This does not alwats hold -- but it is a fair assumption that those conditions would have prevailed in the Weichselian (Devensian) glacial episode on the northern flank of the Putorana Plateau.

According to Svendsen and his colleagues (2004) there were some early glaciations during which both the plateau and the adjacent coastal plain were heavily inundated beneath the Eurasian Ice Sheet.  The Late Saalian Glaciation (160 ka - 140 ka) is the one about quite a lot is known.  During the Weichselian things got complicated.  There appear to have been three glacial episodes, each one less extensive than the last.  In the early Weichselian (related to the Onyoka Moraine) -- about 90 ka ago -- the Putorana ice sheet was incorporated into the Eurasian ice sheet on its western edge, but on the northern and north-eastern flanks of the plateau the local segment of the ice sheet terminated just beyond the plateau edge.  After an ice retreat there was another expansion -- referred to as the mid-Weichselian glacial phase -- where the glaciers in this area terminated in more or less the same positions.  This is called the "Norilsk Glaciation" by Russian workers.  After that there was another retreat, and during the LGM "last glacial maximum) dated to c 20,000 years ago, there was apparently no ice sheet on the Putorama Plateau -- just an assortment of small cirque glaciers, local ice caps and small valley glaciers.

Therefore it is possible that the piedmont glaciers decanting onto the North Siberian Plain were active twice, in the early Weichselian and again in the mid Weichselian. This means that the features seen in the satellite image at the top of this post might be composite in age.

Nonetheless, they are pretty impressive -- and I know of hardly any other examples of Weichselian piedmont glacier maraine loops that are so well exposed in an area currently devoid of glacier ice.  It will be interesting to see whether field observations tally with what the satellite image suggests!


Late Quaternary ice sheet history of Northern Eurasia
Quaternary Science Reviews 23 · January 2004, pp 1229-1271
John Inge Svendsen et al


This appears to be a crucial paper, if only one could get at it:

Isayeva, L.L., Kind, N.V., Kraush, M.A., Sulerzhitsky, L.D., 1976.
On the age and structure of the marginal formations along the northern foot of the Putorana Plateau. Bulleten Komissii poizucheniyu chevertichnogo perioda,
USSR Academy of Sciences
45, 117–123 (in Russian)

Large-scale glacial erosional features in Vestfirdir, Iceland

On looking back through some of my old research materials, I discovered that our short research notes written at the end of the Durham University Vestfirdir Project in 1975 have never been published digitally.  Here is one of them.  It's very short, but it does make some valid points that other researchers might wish to consider.  Here is the DOI if you want to take a look:

DOI: 10.13140/RG.2.2.14187.34080

And here are the maps which show quite well what the research was all about.

Wednesday, 11 January 2017

Brutalised glacial discharge dendritic patterns

For many years I have been fascinated by the difference between dendritic drainage patterns developed by rivers and those developed by glaciers.  Here is a past post:

The illustration above is a satellite image from the northern coast of Iceland, in the uplands west of Akureyri.  (David Sugden and I used a similar image, with more snow, and false colours, on the front cover of our geomorphology text.)

On the main troughs in these images, there has been so much ice to be discharged that all of the smaller and more delicate fluvial valleys that existed in pre-glacial times have been "brutalised" out of existence, and many of the larger interfluves have also been removed.  So the troughs are wide and deep, with relatively few feeder valleys -- and even these latter features are very wide, with prominent trough heads.

Contrast this sort of landscape with that of Sognefjord:

This is one of the biggest fjords in the world (see recent posts), and yet it has retained much more of a delicate dendritic pattern which looks as if it has not changed all that much since the time when there was a fluvial landscape of hills and valleys on the western slope of the Scandinavian mountain range.   Some people use the term "fractals" when looking at an image like this.  Why the difference?  It's a bit of a puzzle.........

Now let's look at two segments of the Putorana Plateau, in central Siberia.  This one is from the NE segment of the plateau.  It was heavily glaciated at least twice, but there are no cirques in this landscape, and the trap plateauaway from the valleys appears almost unmodified by glacial processes.  The dendritic valley pattern is delicate -- and reminiscent of that of Sognefjord.

The plateau edge in the above image is just off the top edge of the photo.  So it appears that close to the point at which the glacier in question was decanting onto the adjacent lowland, the trough actually narrowed.  Was that because ice was spilling out of the trough and flowing across the slopes on either side?  Very intriguing.  My assumption is that in this part of the Putorana ice sheet the ice was cold-based and frozen toits bed -- except in the deep valleys where bottom melting occurred, and where glacial processes could operate.  Let s callthis a classic example of a "glacially protected" landscape......

If we then look at the NW segment of the plateau, only about 200 km away, we see a completely different type of glaciated landscape:

There is almost a rectilinear pattern here (maybe controlled to some degree by fractures in the plateau basalts) but the pattern is much more akin to that of the landscape west of Akureyri in Iceland.  The old river valley pattern has been simplified and brutalised, with smaller valleys and interfluves obliterated.  The ice that operated here is much more likely to have been warm-based, and presumably it flowed rapidly in these deep, wide troughs.

And if we zoom in we see a landscape dominated by glacial erosional features.  There are literally thousands of cirques, the majority of them opening northwards -- these must date from glacial phases before and after the episodes of fully-fledged ice sheet cover.  We are looking at more or less the centre of the photo above.

I must seek some glaciological opinion on all of this.........

History of Stonehenge digs

This is a short history of Stonehenge digs -- sent in by Garry Denke.  I'm not going to publish stuff about the hunt for coal on Salisbury Plain,  since those who are interested in such things can find it elsewhere.  But this is of broader interest, so here we are.


Complete History of Stonehenge Excavations

1611. King James I investigated Stonehenge "to see 'The stone which the builders refused.'"
King James Version, 1611

1616. Doctor William Harvey, Gilbert North, and Inigo Jones find horns of stags and oxen, coals, charcoals, batter-dashers, heads of arrows, pieces of rusted armour, rotten bones, thuribulum (censer) pottery, and a large nail.
Long, William, 1876, Stonehenge and its Barrows. The Wiltshire Archaeological and Natural History Magazine, Volume 16

1620. George Villiers, 1st Duke of Buckingham, dug a large hole in the ground at the center of Stonehenge looking for buried treasure. (Diary)

1633-52. Inigo Jones conducted the first 'scientific' surveys of Stonehenge.
Jones, I, and Webb, J, 1655, The most notable antiquity of Great Britain vulgarly called Stone-Heng on Salisbury plain. London: J Flesher for D Pakeman and L Chapman

1640. Sir Lawrence Washington, knight, owner of Stonehenge, fished around Bear's Stone (named after Washington's hound dog). Bear's Stone profile portrait a local 17th century attraction. (G-Diary)
The Wiltshire Archaeological and Natural History Magazine, Volumes 15-16

1652. Reverend Lawrence Washington, heir of Stonehenge, commissions Doctor Garry Denke to dig below Bear's Stone, reveals lion, calf (ox), face as a man, flying eagle, bear (dog), leopard, and hidden relics. Bear's Stone (96) renamed Hele 'to conceal, cover, hide'. (G-Diary)

1653-6. Doctor Garry Denke auger cored below Hele Stone 'The stone which the builders rejected' on various occasions. Gold, silver, brass, iron, wood, bone, concrete discovered at 1-1/3 'yardsticks' (under flying eagle). Elizabeth Washington, heir of Stonehenge.
Denke, G, 1699, G-Diary (German to English by Erodelphian Literary Society of Sigma Chi Fraternity). GDG, 1-666

1666. John Aubrey surveyed Stonehenge and made a 'Review'. Described the Avenue's prehistoric pits. (the 'Aubrey Holes' discovered by Hawley, not Aubrey).
Aubrey, J, 1693 (edited by J Fowles 1982), Monumenta Britannica. Sherborne, Dorset: Dorset Publishing Co

1716. Thomas Hayward, owner of Stonehenge, dug heads of oxen and other beasts. (Diary)

1721-4. William Stukeley surveyed and excavated Stonehenge and its field monuments. Surveyed the Avenue in 1721 extending beyond Stonehenge Bottom to King Barrow Ridge. Surveyed the Cursus in 1723 and excavated.
Stukeley, W, 1740, Stonehenge: a temple restor'd to the British druids. London: W Innys and R Manby

1757. Benjamin Franklin observes Bear's Stone (96) lion, calf (ox), face as a man, flying eagle, bear (dog), leopard, and Hele Stone 'hidden' relics below them. (Diary)

1798. Sir Richard Hoare and William Cunnington dug at Stonehenge under the fallen Slaughter Stone 95 and under fallen Stones 56 and 57.
The Ancient History of Wiltshire, Volume 1, 1812

1805-10. William Cunnington dug at Stonehenge on various occasions.
Cunnington, W, 1884, Guide to the stones of Stonehenge. Devizes: Bull Printer

1839. Captain Beamish excavated within Stonehenge. (Diary)

1874-7. Professor Flinders Petrie produced a plan of Stonehenge and numbered the stones.
Petrie, W M F, 1880, Stonehenge: plans, description, and theories. London: Edward Stanford

1877. Charles Darwin digs at Stonehenge to study 'Sinking of great Stones through the Action of Worms'.
Darwin, Charles,1881, The Formation of Vegetable Mould, Through the Action of Worms, with Observations on Their Habits. London: John Murray

1901. Professor William Gowland meticulously recorded and excavated around stone number 56 at Stonehenge.
Gowland, W, 1902, Recent excavations at Stonehenge. Archaeologia, 58, 37-82

1919-26. Colonel William Hawley extensively excavated in advance of restoration programmes at Stonehenge for the Office of Works and later for the Society of Antiquaries. Hawley excavated ditch sections of the Avenue, conducted an investigation of the Slaughter Stone and other stones at Stonehenge, and discovered the 'Aubrey Holes' (misnamed) through excavation.
Hawley, W, 1921, Stonehenge: interim report on the exploration.
Antiquaries Journal, 1, 19-41
Hawley, W, 1922, Second report on the excavations at Stonehenge.
Antiquaries Journal, 2, 36-52
Hawley, W, 1923, Third report on the excavations at Stonehenge.
Antiquaries Journal, 3, 13-20
Hawley, W, 1924, Fourth report on the excavations at Stonehenge, 1922.
Antiquaries Journal, 4, 30-9
Hawley, W, 1925, Report on the excavations at Stonehenge during the season of 1923.
Antiquaries Journal, 5, 21-50
Hawley, W, 1926, Report on the excavations at Stonehenge during the season of 1924.
Antiquaries Journal, 6, 1-25
Hawley, W, 1928, Report on the excavations at Stonehenge during 1925 and 1926.
Antiquaries Journal, 8, 149-76
Pitts, M, Bayliss, A, McKinley, J, Boylston, A, Budd, P, Evans, J, Chenery, C, Reynolds, A, and Semple, S, 2002, An Anglo-Saxon decapitation and burial at Stonehenge. Wiltshire Archaeological and Natural History Magazine, 95, 131-46

1929. Robert Newall excavated Stone 36.
Newall, R S, 1929, Stonehenge. Antiquity, 3, 75-88
Newall, R S, 1929, Stonehenge, the recent excavations. Wiltshire Archaeological and Natural History Magazine, 44, 348-59

1935. Young, W E V, The Stonehenge car park excavation. (Diary)

1950. Robert Newall excavated Stone 66.
Newall, R S, 1952, Stonehenge stone no. 66. Antiquaries Journal, 32, 65-7

1952. Robert Newall excavated Stones 71 and 72. (Diary)

1950-64. A major campaign of excavations by Richard Atkinson, Stuart Piggott, and Marcus Stone involving the re-excavation of some of Hawley’s trenches as well as previously undisturbed areas within Stonehenge.
Atkinson, R J C, Piggott, S, and Stone, J F S, 1952, The excavations of two additional holes at Stonehenge, and new evidence for the date of the monument. Antiquaries Journal, 32, 14-20
Atkinson, R J C, 1956, Stonehenge. London. Penguin Books in association with Hamish Hamilton. (second revised edition 1979: Penguin Books)

1966. Faith and Lance Vatcher excavated 3 Mesolithic Stonehenge postholes.
Vatcher, F de M and Vatcher, H L, 1973, Excavation of three postholes in Stonehenge car park. Wiltshire Archaeological and Natural History Magazine, 68, 57-63

1968. Faith and Lance Vatcher dug geophone and floodlight cable trenches. (Diary)

1974. Garry Denke and Ralph Ferdinand set out to confirm Sir Lawrence Washington, knight and Reverend Lawrence Washington's revelation (G-Diary). Auger cores 1.2m (4ft) below Heel Stone 96 (under face as a man). Gold, silver, brass, iron, wood, bone, concrete confirmed. No coal in cores. Stonehenge Free Festival.
Denke, G W, 1974, Stonehenge Phase I: An Open-pit Coalfield Model; The First Geologic Mining School (Indiana University of Pennsylvania). GDG, 74, 1-56

1978. John Evans re-excavated a 1954 cutting through the Stonehenge ditch and bank to take samples for snail analysis and radiocarbon dating. A well-preserved human burial lay within the ditch fill. Three fine flint arrowheads were found amongst the bones, with a fourth embedded in the sternum.
Atkinson, R J C and Evans, J G, 1978, Recent excavations at Stonehenge. Antiquity, 52, 235-6
Evans, J G, 1984, Stonehenge: the environment in the late Neolithic and early Bronze Age, and a Beaker burial. Wiltshire Archaeological and Natural History Magazine, 78, 7-30

1978. Alexander Thorn and Richard Atkinson. NE side of Station Stone 94. (Diary)

1979-80. George Smith excavated in the Stonehenge car park on behalf of the Central Excavation Unit.
Smith, G, 1980, Excavations in Stonehenge car park. Wiltshire Archaeological and Natural History Magazine, 74/75 (1979-80), 181

1979-80. Mike Pitts excavated along south side of A344 in advance of cable-laying and pipe-trenching. In 1979, discovered the Heel Stone 97 original pit (96 original Altar Stone pit). Survey along the Avenue course identified more pits. In 1980, excavated beside the A344 and discovered a stone floor (a complete prehistoric artifact assemblage retained from the monument).
Pitts, M W, 1982, On the road to Stonehenge: Report on investigations beside the A344 in 1968, 1979, and 1980. Proceedings of the Prehistoric Society, 48, 75-132

1981. The Central Excavation Unit excavated in advance of the construction of the footpath through Stonehenge.
Bond, D, 1983, An excavation at Stonehenge, 1981. Wiltshire Archaeological and Natural History Magazine, 77, 39-43.

1984. Garry Denke (and Hell's Angels) seismic survey. Auger cores 1.2m (4ft) below Heel Stone 96 (under lion head). Gold, silver, brass, iron, wood, bone, concrete reconfirmed. No coal in cores. Stonehenge Free Festival.
Denke, G, 1984, Magnetic and Electromagnetic Surveys at Heelstone, Stonehenge, United Kingdom (Indiana University of Pennsylvania). GDG, 84, 1-42

1990-6. A series of assessments and field evaluations in advance of the Stonehenge Conservation and Management Programme.
Darvill, T C, 1997, Stonehenge Conservation and Management Programme: a summary of archaeological assessments and field evaluations undertaken 1990-1996. London: English Heritage

1994. Wessex Archaeology. Limited Auger Survey.
Cleal, R M J, Walker, K E, and Montague, R, 1995, Stonehenge and its landscape: twentieth-century excavations (English Heritage Archaeological Report 10). London: English Heritage.

2008. Timothy Darvill and Geoffrey Wainwright set out to date the construction of the Double Bluestone Circle at Stonehenge and to chart the history of the Bluestones, and their use.
Darvill, T, and Wainwright, G, 2008, Stonehenge excavations 2008. The Antiquaries Journal, Volume 89, September 2009, 1-19
Mike Parker Pearson, Julian Richards, and Mike Pitts further the excavation of 'Aubrey Hole' 7 discovered by William Hawley, 1920.
Willis, C, Marshall, P, McKinley, J, Pitts, M, Pollard, J, Richards, C, Richards, J, Thomas, J, Waldron, T, Welham, K, and Parker Pearson, M, 2016, The dead of Stonehenge. Antiquity, Volume 90, Issue 350, April 2016, 337-356

2012-3. Stonehenge A344 road excavated and removed. (Diary)

Complete History of Stonehenge Excavations

Any missing Digs?

Friday, 6 January 2017

Gallery -- Putorana and Vestfirdir

There are remarkable similarities between the "glaciated trap landscapes" of Putorana in Siberia and Vestfirdir in Iceland.  Flat-bedded basalt layers, flattish plateau surfaces, and deeply incised troughs cut by powerful ice streams.  The main difference is that Vestfirdir is at the coast, with many of the deepest troughs now containing fjords or arms of the sea.  Putorana is far inland, and the troughs are still dry, apart from a few which contain long finger lakes.

Five from Vestfirdir (thanks to Mats Wibe Lund and Murray Foote):

And four from Putorana: