Spotted dolerite on Carningli

I have mentioned before that I have seen spotted dolerite boulders on Carningli, near Newport -- but I an not at all sure where the outcrops are.  Well, today, on my walk on the mountain, I came across several very large erratics which seemed to me to be typical spotted dolerite not dissimilar to the spotted dolerites of the Carn Meini area in eastern Preseli.  Click on the photo above to enlarge.  The stone is a bit dusty and dirty, but note the distinctive white flecks or crystals scattered through the grey-blue matrix.  I must go back when there has been some rain, so that the surface is wet and clean -- and next time I'll go with my hammer as well as my camera.......

Cambrian sandstone erratics on Carningli?

This has got me seriously flummoxed.  On a walk on the side of Carningli today , I found these two erratics.  (Robert, note that I do not say "glacial erratics")  They were in an area of a field where a neighbour is levelling out the ground.  These erratics were among many others of all shapes and sizes.  The one of on the right here is about the size of a human skull.  There's a lot of old building rubble in the vicinity, including bits of slate and old mortar -- so I think there may have been a building somewhere nearby, at some stage.

Both of these -- the smaller reddish pink one and the larger purple one -- look remarkably like the Cambrian sandstones around Caerfai and Caerbwdi near St David's. Shall we assume that the stones in question were incorporated into a cottage at some stage?   Nobody who puts up buildings in this area -- whether in the 1700s or today -- carries lumps of stone this size from St David's to the flanks of Carningli.  There is quite enough in the way of building material in the vicinity as it is.  Moraine and fluvioglacial material all over the place.

But for the ice to have carried this material from St David's to Carningli, it would have been travelling from the SW towards the NE -- I have never seen any evidence to support the idea that the ice has ever travelled in that direction.  Ordovician conglomerates incorporating Cambrian sandstone cobbles and boulders?  As far as I know, there are no such things in the Newport area.

The only thing I can think of is that there might be outcrops of Cambrian sandstones somewhere out in Cardigan Bay or St George's Channel -- and that these sea-bed outcrops might be the places from which these erratics have been plucked by over-riding ice.

Are there any rocks with this colour and texture in the Cambrian of North Wales?  I don't know that area well enough.   Ireland, maybe?

Yet another London erratic

Here's another interesting occurrence, now that we are looking at strange boulders in the London area.  the pic above (taken around 1960?) was kindly sent by Rob Ixer -- it shows Broomfield House, Southgate, North London back in the good old days.  The local museum there  had a 1.5m x 2m high erratic outside its entrance.  It's marked with a circle on the photo. Rob thinks it was Millstone Grit, found even further south of the assumed glacial limit than Romford.

Rob reports that this grade 1or 2 listed building was vandalised and burned, but there seem to be plans for it to be restored. I wonder if the stone is still there?

A Millstone Grit erratic in this location is rather interesting -- certainly far more interesting than a sarsen erratic that may or may not have been moved a long way........

You can see the erratic in this picture too, if you look carefully:

Another London erratic

Found this on Tim Daw's interesting web site:

This stone was found in 1899 in a gravel pit on the site of an ancient river bed and moved to its current site just inside the gate to Elthorne Park in Hanwell, west London. It is roughly 18" deep and 5' square but was formerly much larger as large chunks have been knocked off 3 of its sides. It was glacially deposited and belongs to the middle division of the London Lower Tertiary Sandstones. It is of similar age, origin and composition to the Sarsens of Stonehenge.

There was also this comment from Dave Coleman:

I spent most of my life in Harrow and this Sarsen was the middle of three sites that I knew.
Just over a mile north, there are still two stones, today, outside The Hare public house - crossroads on the edge of Harrow Weald Common.
Just over 1+1/2 miles south was the other - outside the Havelock public house - central Harrow. Both stone and pub are now lost.
(I was always told that these stones marked the funeral route to Stongehenge for the burials of ancient Saxon Kings).

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This is generally referred to as a "glacial erratic" but I'm not so sure -- it is similar to sarsen, and there are a lot of residual sarsen stones littering the landscape of SE England -- most of them having been emplaced by non-glacial processes.......

The boulder is currently located north of the Thames and north of the M4 motorway in west London, so this is well to the south of the glacial  limit in this area.  Not sure where the original "gravel pit" find might have been.....

Glacial landforms -- Ramsey Island

I thought these might be of interest.  They are 3D computer-generated images of the St David's Head - Ramsey Island area in Pembrokeshire, based upon mapped topographic information for the land areas and on soundings for the offshore area.  Courtesy Dr Chris Wooldridge of Cardiff University.

Click to enlarge.  As we can see, the vertical scale is exaggerated in order to emphasise topographic differences.

These modern techniques are quite amazing, to an old fogey like me.  When I did my research in this area in 1962-655, all I had was a shovel, a geological hammer, and a few maps............

What can we see in these images?  On the land (green) we can see undulating erosion surfaces -- probably between 10 and 20 million years old, with ancient islands (monadnocks) standing above them both on the mainland and on Ramsey Island.  One of these old erosion surfaces is beneath sea level, and stretches out to sea beyond Ramsey Island -- it reminds me of the skerries of western Norway and eastern Sweden, and the landscape of rocky shoals and islets is a result of glacial scouring and also marine action.  These skerries are top left on the top photo.

The most spectacular feature to be seen is the incredible closed trench that runs along Ramsey Sound -- seen particularly well in the bottom photo.  I'm pretty certain that this was originally a subglacial meltwater channel, like those of Porthclais and Solva on the south coast of the peninsula -- but clearly tidal scour has played a great part in keeping this trench free of thick sediments.  the tidal stream that goes through the sound on both a falling and a rising tide is quite something -- which is why the Bitches reef is a magnet for whitewater kayaking buffs.

A chunk of Whin Sill in London

Thanks to Dan for the following:
On the topic of erratics from Whin Sill, we have just had delivered to Bedfords Park Visitor Centre near Romford, Essex, an erratic traced to Whin Sill found in a quarry in Havering, just by the Thames, so quite some way further south than Stevenage. The boulder was found at the Brett Lafarge Marks Warren quarry, approximate grid ref. TQ488896. It's approximately 0.9 tonnes. I believe it was south of the glacial limit, probably carried further by the Thames whe it flowed further north of its current course.

It's now at the Essex Wildlife Trust's Bedfords Park Visitor Centre at TQ520922. I'm awaiting further information from GeoEssex to finish the interpretation, and I don't have a photo of it yet, but I'll forward relevant info to you when I have it, a blog post about it would be great.

The Quarry site is shown on the top map -- it's just on the fringes of London, about 8 km from the River Thames.  As shown on the bottom map, this site is south of the assumed limit of glaciation in the London area.  There are plenty of other erratics in Essex, but most of them are further to the north, in the area which was demonstrably glaciated.

 

The find of a chunk of Whin Sill weighing almost a tonne here on the fringes of London is interesting, to say the least.  We'll look into this in more when further info is forthcoming relating to the precise location of the boulder in the gravel pit.  Was it in the terrace gravels?  Lying on the ground surface?  Watch this space.....

The Stonehenge Sandstone Mystery (1)

 Two images from the Heddle Collection -- acknowledgements to Glasgow University and the Open University, on whose web pages these are published.  They are both thin sections taken from an Altar Stone sample, interpreted as "arkosic sandstone containing rounded and angular clasts of quartz, plagioclase feldspar and iron oxides".  The upper image is under plane polarised light, and the lower one (coloured) is under crossed polars -- the orange and blue colours of some crystals might be anomalous or confusing, because of the thickness of the cut section of rock.

The Stonehenge Sandstone Mystery (1) -- The Altar Stone

Since my previous posts about the Heddle Collection of Stonehenge slides at the Hunterian Museum in Glasgow, I can report that there is no skulduggery going on!  Dr John Faithfull has been in touch, and he tells me that  the "missing" slides to which I referred are not missing at all, but are in the possession of the Open University and / or the National Museum of Wales,  where geologists are presumably examining them carefully as we speak.

He also referred me to this Open University site, where more information may be found:

http://www.open.ac.uk/earth-research/tindle/AGT/AGT_Home_2010/Stonehenge-HeddleMenu.html

Leaving the matter of the strange "Altar Stone" sample that appears to be made of igneous rather than sedimentary material, let's home in on the sandstone samples. We are lucky to have a keynote paper to guide us here -- "A detailed re-examination of the petrography of the Altar Stone and other non-sarsen sandstones from Stonehenge as a guide to their provenance," by R.A. Ixer and P. Turner, published in Wiltshire Archaeological and Natural History Magazine, 99 (2006), pp 1-9. The authors describe the Altar Stone as "a purplish-green micaceous sandstone" which appears to have originated on the ORS Senni Beds of South Wales. (They argue very convincingly that the place of origin was NOT the Cosheston Beds on the shores of Milford Haven, as assumed by archaeologists for many years.) Here is a part of their description of the rock examined (from a slide held in Salisbury Museum): " Microscopically the rock is a very fine- to fine-grained, well-sorted sandstone with a mean grain size of 0.13mm and a maximum grain size of approximately 0.32mm. The clastic grains are angular to subrounded and the modal group is subangular..............The rock has a homogeneous fabric but bedding is observed by thin, less than 0.5mm wide, opaque mineral-dominated, heavy mineral laminae.......There is very little detrital, interstitial material. Authigenic kaolinite is locally present but the main matrix is a pervasive carbonate cement........" There is much more detail in the paper.

Here is one of the slides examined by the authors:

 

Compare this with the monochrome slide at the top of this post.  Here is another of their slides:

Compare this with the coloured slide above.  It's not easy to draw conclusions, because I do not have a note of the magnifications involved in all of these slides, or of other factors that might make the samples look more different than they really are!  I need some geological advice here........... since when I was in Oxford I was the world's worst mineralogy student.  But I have done a little trick and have put the coloured Heddle slide together with the Ixer / Turner slide, and this is what we come up with:

The larger image is the one from the Heddle collection, and the smaller one (reduced in dimensions so that the grain sizes are more or less the same) is the Ixer / Turner slide.

Could these samples have come from the same fallen monolith (namely the Altar Stone numbered as stonehenge 80)?  To me, they look different, but what I do not know is the range of variation in grain sizes, mineral types and structures that we might expect in a large boulder weighing about 6 tonnes.

If there are any geologists out there, your comments will be very welcome......