tag:blogger.com,1999:blog-1228690739485734684.post2150966087169800541..comments2024-03-28T14:00:12.372+00:00Comments on Stonehenge and the Ice Age: Devensian Welsh Ice in North PembrokeshireBRIAN JOHNhttp://www.blogger.com/profile/00413447032454568083noreply@blogger.comBlogger19125tag:blogger.com,1999:blog-1228690739485734684.post-34039934161681309742012-12-06T15:45:33.402+00:002012-12-06T15:45:33.402+00:00Brian,
Thanks for explaining your “enigmatic excr...Brian,<br /><br />Thanks for explaining your “enigmatic excrescences”. You have in the past sensitized me to the perils of ideas based on scant knowledge and information. So consider the following brainstorming as tentative intellectual musings. Based on just what I see in your posted image and my “sensible reasoning” on this. For whatever its worth …<br /><br />From your posted image of these “excrescences”, I see the following. Correct me if I am wrong.<br /><br />1) A generally flat area where these “excrescences” are located somewhere in the middle. <br />2)To the right (of the image) of the “excrescences” there is a flat circular lobe extruding with drain gullies along much of its perimeter.<br />3) The shape of the “excrescences” area appears to be crescent, with the greatest buildup of sand/gravel along the very well defined and steeper inner side of the crescent.<br /><br />Question: What geological process can account for these facts on the ground?<br /><br />My (tentative) answer:<br /><br />Possibly meltwater streams, with ice embankments, flowing through a narrow opening into a wider open flat area (the “circular lobe” in my description above) with egress drainage (the drain gullies in my description) along the perimeter of this circular lobe.<br /><br />The rate of water flowing into the “lobe opening” through the narrow passage will be much greater than the rate of flow at and over the perimeter of the “lobe”. In fact, (the “rate in”) = (the perimeter of the lobe) x (the “rate out”). Put in simple clear terms, the water velocity at the “lobe opening” will be far greater than the water velocity at the “lobe” and over its perimeter. Thus, any coarse material (like gravel and sand) will be deposited near the “lobe opening” and not further into the “lobe” where the water velocity will be much less.<br /><br />This explains the position and crescent shape of the “excrescences”. Want to coauthor a paper with me?!<br /><br />KostasConstantinos Ragazasnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-40002122199528899002012-12-06T10:11:08.332+00:002012-12-06T10:11:08.332+00:00Kostas -- enigmatic excrescences! they are excres...Kostas -- enigmatic excrescences! they are excrescences because they look like nasty boils or bubbles on the skin that have burst -- without getting too graphic about it. They are enigmatic because I can't quite work out why they are just there rather than anywhere else -- in the middle of a large area which has all been glaciated relatively recently, fairly high up, and not in an area where you might expect some topographic control over the position of an ice margin. They have all the appearances of kames, formed through the accumulation of sands and gravels in temporary lakes bounded at least in part by walls of ice at a time of deglaciation. There are signs that some of them might be eskers -- formed in subglacial tunnels. I am still working on it.....BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-83375076869226376162012-12-06T10:04:53.310+00:002012-12-06T10:04:53.310+00:00I was about to point that out. Olwyn and Olwen --...I was about to point that out. Olwyn and Olwen -- both excellent contributors to our little discussions. At least it's easier with names than with multiple people called "Anonymous" -- when you have no idea who they are and what they are up to....BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-29368374941337044712012-12-06T08:37:14.938+00:002012-12-06T08:37:14.938+00:00WOOOOOOOOOOPS
Wrong Olwyn.
People should not use s...WOOOOOOOOOOPS<br />Wrong Olwyn.<br />People should not use single names to hide behind it is all very confusing.<br />I am sure Myris would not call a complete stranger mad, he would wait a few minutes, assessing quite how mad and then modify the noun using an extravagant adjective.<br />I expect he feels rather crushed and may try to be benign for ten minutes.<br />Thomas Rhymer.Myris of Alexandrianoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-33407231957787262042012-12-06T08:11:52.921+00:002012-12-06T08:11:52.921+00:00Oh Dr Ixer is a petrographer -not bright enough fo...Oh Dr Ixer is a petrographer -not bright enough for a petrologist- Dr Bevins is a petrologist.<br />Secretly Dr Ixer feels petrology to be dull all that modelling.<br />MMyris of Alexandrianoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-70477214785724959492012-12-06T08:09:57.852+00:002012-12-06T08:09:57.852+00:00MMMM I am channelling
"I agree with experienc... MMMM I am channelling<br />"I agree with experience much can be done, I can now tell SH48 knock-offs in hand specimen (If I have a bit beside me to compare but I only discovered in July that one of Stone's stones aka the shoebox assemblage was a 48 knock-off and I must have handled that stone 30/40 times). IF you think that macro id is better than 70-80% you are fooling yourself.<br />My comments really were restricted to f.g rhyolitic rocks. Plus we are dealing with precise provenancing there even with lmst you would need TS petrography to give a location.<br />In approx. 150 years it has been recognised that for id'ing of rock thin section (I would of course say thin section petrography in both transmitted and reflected light my so-called 'total petrography')is the supreme queen of techniques (a bit like leeks and veg).<br />To recognise SH rhyolitic or volcanic bluestones in the field ONLY TS petrography (better alongside geochem- we have been here before Olwen- did we not all co-write something along these lines? (Note MR Johnson an example of hidden readers being addressed))will show us the one true path.<br />Do not fret I noted your little pile of stones and it will not be forgotten). Allow me to get Brian's done first- these days favours in academia are as rare as<br />SH provenance sites.<br />I did try to tap the owner of the NW England fracking (I always want to say fragging!!!) company for TS money but no luck mind you he tried to tap BP's Tony Hayward years ago in the same vein (WE are all highly and long time associated) but with the same result.<br />I shall write to Mr Gates. Him I do not know but what is a few hundred pounds to Micronesia.<br /><br />Ah the mists are clearing and Sublime Apollo as Ra greets us once again.<br />Myris<br /><br />Myris of Alexandrianoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-68592126042164647802012-12-06T00:53:16.936+00:002012-12-06T00:53:16.936+00:00Brian, you write
“The enigmatic accumulations of ...Brian, you write<br /><i><br />“The enigmatic accumulations of fluvioglacial sands and gravels in the Monington area “</i><br /><br />What makes these “excrescences” enigmatic? <br /><br />KostasAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-20650735353041761932012-12-05T22:33:45.196+00:002012-12-05T22:33:45.196+00:00That info on sites is very interesting, Olwyn, and...That info on sites is very interesting, Olwyn, and many thanks for it. I will check out some of those locations......BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-74384506151244956312012-12-05T22:32:15.415+00:002012-12-05T22:32:15.415+00:00I understand where Rob is coming from -- he is aft...I understand where Rob is coming from -- he is after all a petrologist, and he would say that we must all have thin sections, wouldn't he? OK -- I respect that. But I wouldn't be so dismissive of identifications from hand samples. When I was a research student I knew the rocks of the St David's area so well that I could pretty well pick up any chunk of rock from a beach or from a till exposure and tell you where it had come from. I didn't need thin sections to do that. For identifying major groups of rocks (red Cambrian sandstone, grey Carboniferous Limestone, bright red ORS, and even spotted dolerite, for example) it would be a ludicrous waste of time even to think about thin sections as an aid to identification. You bring in that technique if you want extremely accurate provenancing -- and even then you need a lot of luck as well as judgment!BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-50218212959066027522012-12-05T21:32:37.325+00:002012-12-05T21:32:37.325+00:00Rob - you may well be right but I have been mad fo...Rob - you may well be right but I have been mad for years so possibly wouldn't be able to recognise it..if you have the where with all to do thin sections, I have a variety of samples..<br /><br />Olwynnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-64948102089677913662012-12-05T18:49:26.246+00:002012-12-05T18:49:26.246+00:00Brian,
The “excrescences” at Monington raise an i...Brian,<br /><br />The “excrescences” at Monington raise an interesting curiosity. Could similar “excrescences” of sand and gravel formed at drained lake bottoms at places with gullies surrounding such plateau? Where the water flow is much less and so less able to carry these coarse material further downstream?<br /><br />If you look at your posted photo, the plateau where the Monington “excrescences” are located is surrounded by drain gullies. Thus the raised plateau acts like a “filter” catching the coarser matter in the flow. A little like a more “elevated” discussion!<br /><br />Just some intellectual musing … you can disregard if you like!<br /><br />KostasConstantinos Ragazasnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-55410370376603269582012-12-05T08:16:11.117+00:002012-12-05T08:16:11.117+00:00Olwen macroscopical description and id of these ro...Olwen macroscopical description and id of these rocks is a sure way to madness. Sadly they have to be sectioned.<br />Trefael Stone (about which I can no longer say much without permission) is not Cryf and I am afraid does not really look much like it macroscopically-it has visible feld.microphenocrysts; at one time I thought it looked more like SH48.<br />It is not that either or trust me we would all have known about that.<br />Matching by eye or by PXRF can only go so far and a good thin section, better polished thin section is the only certain way.<br />The point about Cryf is that it is petrographically NOT NOT NOT like most of the rhyolites/rhyolitic tuffs.<br />These rocks, as the 1990s OU work and the I and B work this century has shown, are not for amateurs. It has taken us years to recognise the subtle differences and to refine the descriptions and classifications and of course remains on-going.<br />RobMyris of Alexandrianoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-63397610292343541582012-12-04T20:39:46.056+00:002012-12-04T20:39:46.056+00:00Sorry all -- I have been away in Brussels for 3 da...Sorry all -- I have been away in Brussels for 3 days, making presentations to the Parliamentary Petitions Committee.. back home now, feeling exhausted......BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-76170981840162794152012-12-04T20:26:32.550+00:002012-12-04T20:26:32.550+00:00Hi Brian, did you get the grid refs and stuff ?
A...Hi Brian, did you get the grid refs and stuff ?<br /><br />All the best,<br />OlwynOlwynnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-34497064860494098372012-12-03T09:50:51.122+00:002012-12-03T09:50:51.122+00:00Hi Brian - further to yesterdays message, the grid...Hi Brian - further to yesterdays message, the grid ref for the Pant-y-Butler mound is SN21454675. At the time the stone was identified by someone from DAT as 'Fishguard Volcanic series'. I have wondered whether there could be further volcanic outcrops out in Cardigan Bay, which have been scraped by the Irish Sea ice and delivered onto the coastal area in N.Pembs/S.Ceredigion. If I survive long enough, I'd like to take some of my samples round the on-land outcrops and compare them..Olwynnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-90540374726312266262012-12-02T20:39:06.116+00:002012-12-02T20:39:06.116+00:00OK - further info - accepting that I'm not a s...OK - further info - accepting that I'm not a skilled geologist, so my rhyolite might include ashes and tuffs but I can distinguish between these laminated bedded fine grained volcanics and the more massive and crystalline dolerite - at the end of the lane to Llech-y-Dribedd dolmen near Moylegrove I found a pile of rocks raked off the ploughed field, have some samples, they are blue grey, laminated, finegrained, with little cream inclusions ?accretionary lapilli? the grid ref for these is SN10124320. Bayvil - collected samples from the spoil heap of George Nash's recent excavation,(trefael) and peered closely at the chipped side of the cup marked rock - again, blue, fine grained, laminated, much like Craig-rhos-y-felin and nearby outcrops. The Trefael stone is worn and smoothed though. grid ref SN10254025. The dump near Crymych is in the vicinity of Craig-y-Fran SN18553125. This site is marked on the OS map as an area of rocky scrub, it is full of the most amazing rocks. Some years ago there was a bit of archaeological interest when some 'white rhyolite' neolithic stone axe working debitage and polishing stones were found in the fields downslope of the scrub. There is a paper on this somewhere but I don't have the reference immediately to hand (if you're interested though, I could get a copy for you). There does not seem to be an outcrop - the raw material is derived from the glacial deposits. A similar axe working area was also identified near Glandy cross, also using similar material, also in the topsoil. This is thought to be one of the sources of the 'group VIII' axes.<br /><br />I know what you mean about the sandstones from up in Ceredigion ! Again, I have a few samples,and am fairly convinced that what I found at Llech-y-dribedd is not the same.<br /><br />I was also working on a dig on a hill top southwest of penparc a couple of years ago (pant-y-butler)and saw a blue finegrained lemon-pip shaped erratic boulder there which seemed to have been incorporated in a bronze age round mound. I wasn't so skilled at distinguishing rhyolite from dolerite then, but it was chipped by the digger and quite a bright blue inside. Can't find the right map just now for grid ref, sorry. But there are 'bluestone' gateposts in that vicinity, too.<br /> <br />It IS all very confusing ! Hope this is of interest anyway :)Olwynnoreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-91853862038408704092012-12-02T15:29:20.681+00:002012-12-02T15:29:20.681+00:00I am surprised you think the exit for the putative...I am surprised you think the exit for the putative lake is to the West.<br /><br />The North would seem more likely, perhaps even Hermon. Take a look at the streams feeding the Teifi. Even today Prescelli drains in that direction and with surprising force - Cenarth is a good place for lunch!<br /><br />chris johnsonhttps://www.blogger.com/profile/16210890033354730381noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-79025734821000762782012-12-02T12:43:47.010+00:002012-12-02T12:43:47.010+00:00Thanks Olwyn -- interesting! Can you give me some...Thanks Olwyn -- interesting! Can you give me some grid refs and field notes on all of this? It will be interesting to see what the Trefael Stone turns out to be made of. The trouble is that NE Pembs generally hasn't had any serious Quaternary field work in recent decades, apart from the work connected with the Teifi Lake story. The problem with glacial deposits that have come from the NE (with Welsh Ice) is that the grits, sandstones and shales of mid Wales are rather nondescript, and I certainly would not feel confident on any provenancing from that great tract of sedimentary rocks between the Teifi and Plynlimon. the North Wales rocks may well hold the key -- but again I'm not familiar enough with them. Maybe Richard and the other guys from Cardiff can help?<br /><br />Yes, I'm aware that there are rhyolites and even spotted dolerites in the area north of the Nevern valley -- OT Jones drew attention to them many years ago. I am increasingly coming to the view that there have been many different ice movements across this area, in various phases of both the Anglian and Devensian Glaciations. Local ice from Preseli, Welsh ice from the NE and Irish Sea Ice from the NW all waxing and waning at different times. It's a really confusing scenario.....BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-1594181797573983042012-12-02T12:26:04.500+00:002012-12-02T12:26:04.500+00:00Thanks for this interesting post Brian - does the ...Thanks for this interesting post Brian - does the presence of rhyolitic field stones around Llech-y-dribedd, and at Bayvil, similar to the rocks at Craig-rhos-y-felin fit with your new thinking ? There are also morrainic accumulations near Hermon, outside of Crymych, including blue and white rhyolites, and large wedge shaped erratics (both dolerite and rhyolite) with striations. I continue to scratch my head...olwynnoreply@blogger.com