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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Wednesday, 19 November 2014

Flat Holm Erratic Hunt (7): the geomorphology


 Flat Holm seen from the NW -- in this photo the west side of the island is on the right -- note the overall roche moutonnee form.

As mentioned in earlier posts, Flat Holm is a relatively low island, almost circular, and just over 600m in diameter.  There is a gentle slope from the more exposed western shore up towards the eastern cliffs -- and it is on the east that the highest land on the island is to be found.  The highest point is only 32m above sea-level, but because the tidal range is so large here -- up to 15m on some spring tides -- the island expands and contracts rather spectacularly every day.......

As noted in earlier posts, the island has an ideal roche moutonnee form which matches perfectly the passage of ice across the island, moving from west towards east on several occasions during the Ice Age.  However, it would be a mistake to attribute the form of the island entirely to glacial processes, since the main geological features are a series of pitching anticlines and synclines running broadly NE-SW -- and these features, above all else, explain "the lie of the land."  There are extensive exposed bedrock surfaces on the western side of the island between the tide marks.  Scrambling about beneath the cliffs is more difficult on the south coast, and more difficult still on the east side of the island, where cliffs and thick vegetation make exploring an occasionally hazardous occupation.

The western side of the island is a geological paradise, with abundant fissures, faults, folds, thrusts and anticlinal and synclinal structures easily accessible -- alongside many variations in rock type.  The rocks all belong to the Carboniferous series, but in addition to "classic" hard grey limestones there are also mudstones, cherts, oolites and also veins of calcite and post-Carboniferous weathering and residual products exposed particularly in fissures and gullies in the cliff face. 

The most famous surface features are the large ripple marks exposed on the shore platform to the west of the farmhouse.  They have been exposed as a result of the breaking up and stripping off of overlying strata -- partly by glacial processes and partly by wave action.


Some of the large ripple structures on the west side of the island.  A perfectly preserved Carboniferous sea floor......


 Steeply dipping bedrock (mudstones, shales and sandy layers as well as interbedded limestones) near the SW corner of the island.  The upper 2m or so of the bedrock exposure is broken up by periglacial and slope processes;  there are signs of frost heave features.  At the top of the cliff is a thin sandy loam incorporating windblown material.  This is probably Holocene.  In the gully in the middle distance there is a plug of reddish material which may be of Triassic age; but there is much debate about this.......

Around the whole coast of the island there are traces of a raised beach platform -- or more likely, several of them.  Structural controls are so dominant that these fragments are difficult to identify with any certainty -- but they occur at all sorts of altitudes from mean tide level up to about 3m above extreme spring HWM.  Trying to match these traces with the raised beach platforms of Pembrokeshire (for example) is very difficult, because of the vast tidal range here in the Severn Estuary.  The best preserved raised beach platform is about 2m - 3m above extreme HWM, on the cliffs at the SE corner of Coal Beach.  The platform extends towards Point Bay.  This is what it looks like:


Sid Howells on the raised beach platform which runs for almost 100m along the cliffs near the eastern extremity of the island.  The surface is irregular, but it is a well-pronounced 
feature up to 4m wide.

There is also a modern wave-cut platform which is occasionally more than 100m wide, cut by a combination of marine solutional processes and  abrasion as blocks and pebbles are moved about by waves and tidal rises and falls. Parts of this platform are well covered with rocky debris and pebbles, but occasionally it might be exposed, as in this photo of Point Bay, as seen from the clifftop near the Foghorn.


Because the whole island is made up of calcareous rocks with complex structures and abundant fissures, it is inevitable that there are tunnels, caves and solutional hollows and collapses all over the place.  Some of these have been used for the exploration of past inhabitants for lead and other minerals -- and indeed for the exploitation of mineral finds.    Above the raised beach rock platform between Coal Beach and Point Bay, there are two tunnel entrances.  It's thought that one of them is natural and the other man-made -- but to me it looks as if both might be natural but maybe enlarged by those in search of mineral wealth.  This is the entrance to one cave.  It looks very similar to the caves of Gower, South Pembrokeshire and Caldey Island -- and it is quite possible that prehistoric layers might be present and worthy of examination.


Castle Rock is a spectacular double stack with an old cave system at its centre.  It is being whittled away by marine processes, and at any moment -- probably during some storm bringing big waves in from the north -- the tops of both stacks will slide into the sea, along old bedding planes which can easily be seen from the beach below.


 Finally. a word about the solutional features which are particularly prominent around the southern shores of the island where the massive limestone beds of the Birnbeck Limestone are exposed.  We can refer to these features as "karren".  They incorporate jagged raised surfaces and complex and intertwining channels which are sometimes almost a metre deep.  mechanical abrasion does not seem to be very active in most cases-  these are straightforward solutional features.  The edges of the ridges are often razor-sharp -- capable of ripping boot soles to shreds, not to mention hands, if you are unwise enough to lose your balance when hopping around on the ridges........


Conclusion:  Plenty of geomorphology to get your teeth into here, not to mention classic geological features.  Interestingly enough, Sid, Chris and I did not identify a single in situ glacial or fluvio-glacial deposit on the island cliffs.  That confirms, in my mind, that Devensian ice probably did not reach Flat Holm.  But there are erratics everywhere, and in several places we see deposits that look like very ancient glacial deposits that have been eroded and modified over a long period of time.  The best guess is that they are Anglian in age, and that it was the Anglian Glaciation that carried those thousands of erratics from the far west into the vicinity.












Monday, 17 November 2014

Vyrnwy excavations 1882


This fantastic 1882 photo (from the Hubbard Collection) has just been released by Heritage of Wales News. It shows the sheer scale of the excavation work that went in to the creation of the dam which flooded a whole community and created a huge new water supply lake. But it also shows in amazing detail the glacial geomorphology of the area, with the superficial deposits stripped off and the jagged bedrock surface beneath exposed so as to create a firm foundation for the masonry dam. This is apparently the SW end of the vast excavation trench. The deposits are here about 16m thick and seem to consist of relatively fine-grained materials at the base (fine-grained till?) overlain by a more pebble layer and then by something that looks like a major unconformity, and some flat-lying sediments about 2 m thick above that. That's all a bit speculative. I wonder how these sediments were described by the engineers of the day? And the bedrock surface.. In places it looks distinctly moulded or streamlined -- but there is clear structural control here, so it would not be very wise to speculate too much......


This photo shows the completed dam before the lake was filled.

Friday, 14 November 2014

Oxford Univ East Greenland Expedition 1962

Glacial geomorphology in 3 minutes

Yet another Stonehenge experiment



 A pic from the previous experiment.  Photo: Tim Daw

Another Stonehenge experiment coming up...... The appetite for Stonehenge stories is insatiable.  And yet another BBC Stonehenge spectacular is apparently planned.   You'd have thought people would be fed up by now with "Stonehenge mystery finally solved" stories, but onwards and upwards it all goes.....


Stonehenge experiment to be repeated with ‘lost’ stones

Another attempt is to be made to solve the mystery of how the largest stones used to build Stonehenge were moved.

The experiment was first carried out in a BBC documentary in 1996

In 1996, a BBC TV programme aimed to find out how the stones for the largest trilithon were put into place, and how the lintel was placed on top.

Since then the concrete replicas have remained untouched and forgotten about at an army base on Salisbury Plain in Wiltshire.

They have now been rediscovered and the experiment will be repeated.

Archaeologist Julian Richards is teaming up with farmer Tim Daw see if modern techniques are any more efficient.

Mr Daw, who farms at All Cannings, near Devizes, and who created the first "Neolithic" long barrow to be built in the UK for 5,500 years, also works part-time at Stonehenge.

He said one of the most popular questions asked by visitors is 'how were the giant stones moved?'.

"When Julian Richards mentioned there was a life-sized replica of the largest stones at Stonehenge that were looking for a home that we could do some experiments on I said 'let's do it'."

The 45-tonne replicas were used in the BBC documentary Secrets of Lost Empires: Stonehenge, which was broadcast in 1996.

They have remained at Larkhill Camp, about a mile from Stonehenge ever since.

The experiment was partially successful, but now new theories have emerged about how the stones may have been moved.

"The first thing is to collect the stones from Salisbury Plain where they have been languishing for the past 20 years and get them back to my farm," said Mr Daw.

"Hopefully next year we'll get some teams of people [to take part in the experiment]"

Mr Daw said different theories had now emerged about how the huge stones could have been moved.

"The experts certainly think they know more. Whether they actually do know more is an interesting question.

"Without trying all the wonderful ideas of how you do it Neolithic style, just using man power - no wheels, no draught animals, no machinery - we can't tell what is practical and what is just fantasy."

It is hoped the result of the experiment will be turned into another television programme to air next year.

http://www.bbc.co.uk/news/uk-england-wiltshire-30041330

Saturday, 1 November 2014

Acts of God -- available as from now



Excuse me, ladies and gents, for posting something rather erratic (if Stonehenge and the bluestones are the things that keep you awake at night) -- but herewith a bit of shameless promotion........

The cover is designed, the book is printed, and I expect delivery on Monday.  Here is the blurb:

Eight members of a scientific expedition arrive in the Arctic wilderness of East Greenland at the height of the Cold War. At the same time the inhabitants of a remote Inuit village are evicted from their traditional hunting grounds. As one Act of God follows another, both the explorers and the Greenlanders become the unwitting guinea pigs in a series of grotesque experiments. The death toll mounts. Bit by bit, the victims uncover a global conspiracy, and realize that an implacable enemy with limitless resources will not allow any of them to survive.

And the good news is that if anybody wants to order a copy, either for the Kindle or paperback version, it can be done via this web site:

http://actsofgod.weebly.com/

I have been forced to stick it into a genre, so you'll see it advertised as an action thriller / conspiracy / mystery / adventure book.  But I'm not all that happy with labels like these.   I like to think that this book has rather more depth to it than most modern thrillers, and that characterization and location are as important as frenetic action and violence.

Anyway, if anybody gets to read the story, hope you enjoy it!  And as ever, please like it, rate it, and stick your reviews onto the Amazon web site so that it can struggle into the "top million bestsellers" list........