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Tuesday, 30 September 2014

Flat Holm Erratic Hunt (4): previous records

Erratic pebbles have been known on Flat Holm for almost 200 years, and the earliest record seems to be that of Buckland and Conybeare in 1824.  They recorded pebbles of "chalk-flint, red sandstone, quartz, flinty slate and porphyry....." on the limestone bedrock and on the beaches which effectively double the size of the island when the tide is low.

The island is in the middle of the Bristol Channel, 4.7 km from Lavernock Point near Penarth on the Welsh side and almost 10 km from Weston-super-Mare on the English side

In 1971 Geoffrey Kellaway collected 100 pebbles from East Beach.  This will not have been a totally random collection -- presumably he just picked up the things he thought were interesting.  So any statistics and percentages cited are meaningless.  Nonetheless, the list of pebbles from this collection, as identified by RK Harrison, is interesting.

Laminated cherty dolomitic siltstone (E40285)
hornfelsed siltstone (E40286)
spicular chert (E40288)
dolomite pseudo-microbreccia (E402910
graphic microgranite (E40289, E40293, and E40294)
feldspar porphyry (E40287)
fluxioned feldspar-rich lava (E40290)
silicified welded crystal lithic tuff (E40292)

Presumably thin sections were made from all these pebbles.  The remaining -- maybe less interesting -- rocks were:

jaspers and other fine-grained quartzose rocks, dark grey to purple (24%)
purple-brown quartzitic sandstone (Old Red Sandstone?)  15%
Cretaceous flints (13%)
fine-grained and indurated quartzites (? Precamrian)  11%
Carboniferous limestone and dolomite 10%
coarse and medium grits 10%
purple and green andesite (?) 5%
 grey quartz-feldspar-porphyry - less than 5%
?obsidian - trace
vein quartz and other igneous rocks - trace

Harrison considered that there were two groups of pebbles -- relatively local and far-travelled.    The former were readily distinguished -- eg limestones, flints, dolomites, purple-brown quartzitic sandstones.  The latter were "highly variable in lithology, and provenances could not be given with any certainty.  Some -- eg vein quartz and Precambrian (?) quartzites -- might have been secondarily derived from ORS.  The andesites and welded tuffs might have come from North Wales or the Lake District.  The graphic microgranite might have come from Northern England -- for example, Carrock Fell.

The non-local material, thought Harrison, must have come from a glacial deposit, probably on the bed of the estuary.  This is reasonable, given that remnants of glacial deposits exist onshore in the area to the east of Flat Holm  (Gilbertson and Hawkins, 1977).  Also, the gravel spit that forms a spectacular low-tide feature at the eastern end of Steep Holm, although consisting mostly of local rocks, includes flints and greensand cherts. 


Source for this information:  Ch 6 in:  Whittaker A and Green GW 1983., Geology of the country around Weston Super Mare. Mem. Geol. Surv GB Sheet 279 with parts of 263 and 295.  Thanks to Chris Lee for drawing this to our attention.

Flat Holm Erratic Hunt (3): the pink granite erratic

More on the famous pink erratic boulder on the limestone rocks of Flat Holm's West Beach.  In 1936 a sample was collected from this boulder -- E17444 -- and a thin section was made.  It was examined by Dr AJ McGregor and identified as a pink leucocratic soda-rich granite; but he was unable to offer any possible provenance.

Another analysis (of the same thin section?) by Dr Stuart Baskerville of the University of Glamorgan was communicated to Chris Lee in 2011.  The petrology was described as "igneous quartz, with some slightly sutured boundaries; microperthite (quite fresh); minor hornblende; possible biotite."

Dr Baskerville said the sample was possibly from an alkali granite, granidiorite or adamellite, but he was concerned about a lack of significant amounts of mica.  All of the feldspar was thought to be microperthite -- and this might indicate equal amounts of plagioclase and potash feldspar, pointing towards adamellite (?).  The texture was described as holocrystalline, phanerocrystalline.

As far as a possible origin was concerned, Dr Baskerville suggested that the specific granite which best fitted the bill was the (Late Pre-Cambrian?) Coedana Granite of Anglesey.  But he emphasised that this identification was highly speculative.

All other suggestions gratefully received........

Flat Holm Erratic Hunt (2): the big erratics

These are the big erratics found on the island.  The pink granite erratic is of course well known, but the strange conglomerate found by Chris on the coast between Castle Rock and North-west Point is a very unusual one which had us all flummoxed.  More info will follow when we get it.  In the meantime, are there any opinions?

The famous pink granite erratic resting on a broken limestone surface at the north end of West Beach --directly to the west of the farmhouse.

Sid and Chris examining the mysterious conglomerate boulder.

The conglomerate found on the foreshore in the NW part of the island.  It does not look like the Cambrian Basal conglomerate or the Ridgeway Conglomerate of Devonian age -- one guess is that it might come from the Millstone Grit series.

And a false alarm!  I was quite excited when I saw this boulder on West Beach -- almost black in colour, very hard and heavy, and with a dense rectangular fracture pattern....  I thought it was basalt, but then realised that there was an outcrop not far away of incredibly hard (silicified) chert within the limestone series.  Great care is needed in erratic hunting......

Flat Holm erratic hunt -- a grand day out (1)

 Danger -- geologist at work.  Sid hunting for clues on Coal Beach.

Yesterday, I had a very jolly day on the island of Flat Holm, in the company of geologists Sid Howells and Chris Lee, hunting for erratics.  Grateful thanks to Linda Burnell and her colleagues from the Flat Holm Society for facilitating the visit and looking after us so well.    And thanks too to Sid and Chris, who were able to identify many things with which I was unfamiliar.  We were blessed with a calm sea for both crossings from Cardiff Bay in a RIB powered by almost 500 HP of motor power -- and the crossings took about 10 mins each way.  The return journey was in the dark -- a slightly spooky experience when travelling at 30 knots or so.  Very exciting......

On the island, we were  blessed by bright calm conditions and by a lack of sea gulls.  There is a very dense gull colony here, but luckily the breeding season is long gone, and the birds had departed.  In the early summer protective clothing is required. 

Lots of interesting things were discovered on the island.  Sid and Chris will be getting second and third opinions on many tentative sample identifications, but already we have some preliminary conclusions about where the main erratic sources may be.  More of that anon......

Sunday, 28 September 2014

Rhosyfelin footpaths

Following my recent chat with Dave W on this blog, I thought I'd better check out the footpath situation -- and I have discovered that there are TWO footpaths affected by this dig site, not one.

This is the map that pops up on "Where's the Path" when one looks at this area:

What we see are two paths / public rights of way, leading from the cottage near the ford towards the S and SW.  The path is shown from the edge of the garden.  One path runs very close to the edge of the rocky spur, just where the 2014 digging has been going on, and the other runs up the middle of the little channel between the spur and the road.  The path is not shown leaving the road on the hairpin bend.  That path, if it is indeed a designated right of way, appears to have been completely obliterated by the 2011-2014 digs.

So if the path as shown is correct there has either been a deliberate blocking of the path or an attempt to prevent public access, which would either be a criminal offence, or else, with the assistance of the Highways Authority (presumably the National Park in this case) a temporary diversion of some sort.  That presumably would involve some notification to the public -- and I'm not aware that there has been any such notification.

Then things get even more messy.  If you look at the 1:25,000 map for the area it looks as if this second path leaves the road right on the hairpin bend, more or less as we see on this old OS 6" map:

That must be an old trackway, running beneath the word "felin" -- and you can also pick it out on the satellite image.  So the path running past the tip of the spur is shown by the dashed lines, but the other path isn't.

All very confusing.  But is is important, because I and lots of other people will want to know whether, if we go and wander around the dig site and take photos, we are actually on a designated public footpath.  Alternatively, are we technically trespassing?

I will try to find out from the National Park what the truth of the matter might be.........

Friday, 26 September 2014

Weathered and abraded rock surfaces at Rhosyfelin

These are some of the rock surfaces currently exposed at Rhosyfelin within the area of the 2011-2014 dig.  They are all quite heavily weathered, and some are so deeply weathered that you can scrape the surface away with a sharp tool and get a soft grey powder from a deep gouge in the surface.  That speaks to me of long-continued exposure to cosmic bombardment, sunshine, wind and rain over many thousands of years.

More to the point, these surfaces (some on exposed bedrock outcrops, and some on detached slabs) are heavily abraded.  By which processes of erosion?  Well, the only ones that make sense here are glacial action and abrasion by torrents of meltwater during the deglaciation phase. This of course is supported by the abundance of large rounded boulders and stones exposed in the lowest part of the dig, under the present-day grassy valley floor.  There is no way that you could find these abraded surfaces in these positions if this was a quarry.  If the rock face was a man-made feature, you would expect all of this material to be jagged, angular and sharp-edged, like the material you find upslope on some of the more recent rock-falls:

......... or maybe the quarry owners employed apprentice quarrymen to go around polishing rock surfaces and rounding off sharp edges in case anybody might get hurt by them?

Rhyolite inclusion in a quartz block

Something for Rob and Richard.  I hadn't noticed this before, but that big detached block of quartz resting on the bedrock floor just upslope of the "proto-othostat" is very interesting indeed.  It's very brittle and sharp-edged, so it hasn't been subjected to much erosion -- and that suggests strongly to me that it is more or less in situ.  But if you click to enlarge these pics you'll see that there is a large bluish rhyolite inclusion within it.  There are traces of fine fractures within the rhyolite, but it doesn't seem to be strongly foliated.  I don't suppose there is any great significance in this, but it's a nice geological curiosity. 

Precisely what might have been the mechanism of formation?  I thought quartz was often created as a "secondary" mineral in pre-existing volcanic or sedimentary deposits?  But here it looks more as if the rhyolite is the thing that is secondary.....