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Friday, 12 April 2013

New evidence of glaciation on Salisbury Plain?

Prof Colin Pillinger and the Lake House Meteorite

New evidence of glaciation on Salisbury Plain? Er, no, actually.  But the latest info on the BBC's "The Sky at Night" (and on the BBC web site) refers to the glaciation of Salisbury Plain as if such a thing is perfectly well accepted, which of course it isn't.  Well, not at the time Colin Pillinger wants it, anyway......

The meteorite is apparently 4.5 billion years old, and Prof Pillinger thinks it has been on planet Earth for about 32,000 years.  Presumably on Salisbury Plain, where it was later incorporated into a Bronze Age barrow at some unknown location, possibly near Barbury Castle in Wiltshire.  The reason for that piece of reasoning?  Well, the traces of chalk on the meteorite surface, suggesting that it was embedded in chalk or covered with chalk at the time when it was found and taken away by Rev Edward Duke prior to ending up on the doorstep of lake House.  There is a lot of doubt about the provenance and the story..... but that is for others to discuss.

So where did the glaciation theory come from?  I have written to Colin about this, and await a reply.  But I assume that the thesis is that the meteorite is pretty well preserved, and that it therefore landed on something soft like a snowdrift or a glacier surface (the alternative is presumably that it landed on the ground surface, which would have been hard enough to ensure the shattering of the whole meteorite and the wide dispersal of all the bits.)  The other line of evidence appears to be that there is remarkably little surface weathering of the meteorite -- and this also leads Colin to the assumption that it was covered, buried or protected by some medium or other -- and he has suggested ice.

Much as I would like to go along with this thesis, I'm afraid it doesn't seem to fit with what I know about the Devensian.  Around 30,000 years ago we were in an interstadial -- warmer -- phase prior to the rapid cooling that led to the Devensian Glaciation of around 25,000 - 20,000 BP.  I'm not even sure that there would have been extensive snowfields and permafrost at the time.  Maybe there were cold winters and seasonal snowbanks, but they would certainly have melted away in the summers.

I'm not even sure that a glaciated landscape would have ensured a "soft" landing for the meteorite.  Glacier ice is pretty hard -- often harder than a normal ground surface with a soil layer and bedrock beneath.  Even under a periglacial regime, permafrost is pretty hard too if it is at the ground surface (during the winter) or maybe a metre or two down (during the summer.)

Before going any further with this, I would like to know what the basis for the date of 32,000 BP is, and where the "radiocarbon" date of 10,000 BP came from.  I'm not aware of any publication in which all is explained.......

10 comments:

Constantinos Ragazas said...

Brian,

Judging from the very spherical shape of this meteorite it seems to me it landed not on ice but in WATER! This fits perfectly with what I have been arguing about Salisbury Plain. The dates also fit.

To reiterate: Salisbury Plain was a glacial lake formed by the melting of the glaciers. The Irish Sea Glacier along Bristol Channel formed an ice dam. During the Younger Dryas (some 10,000BP) this glacial lake froze solid to form an ice cover over Salisbury Plain.

This hypothesis seems to be supported by this latest evidence. And all other 'facts on the ground'. It explains everything we know about Stonehenge and all the other prehistoric sites and landscape.

Nature was more responsible for the forming of Stonehenge and all the earthworks than Neolithic people.

Kostas

alex gee said...

Hi Brian
I query your skepticism about the survival of meteorites after impacts with ice surfaces?

The literature suggests that the Arctic and Antarctic appear to be the most abundant sources for the retrieval of largely intact meteorites??

Be most interested in your reasoning?

regards
Alex Gee

Anonymous said...

You will find an explanation of the dates in his lecture 'stones from the sky' given at the royal society in 2012.

http://royalsociety.tv/rsPlayer.aspx?presentationid=1035

His hypothesis is about as accurate as the famous beagle 2 experiment in 2003.

Anonymous said...

Hi Brian
I presume that Prof Pillinger is referring to the fact that the Arctic and Antartic ice sheets are the main source of recovered meteorites.

Which rather puts the mockers on your hypothesis?

Apologies
Alex Gee

BRIAN JOHN said...

Thanks for the link, Anon -- I have checked it out and it looks interesting. I note that he used one of my slides of the computer-generated Irish Sea Ice Sheet in the lecture to the Royal Society, but rather mischievously labels it as >10,000 years BP!! Be that as it may -- I use other people's slides without their permission all the time, so I can't complain!

On the matter of meteorite survivals on the Arctic sea ice (NOT ice sheet, by the way) and on the Antarctic Ice Sheets (two of them, if we are being pedantic) this has nothing at all to do with the frequency of falls or with the survivability of impacts. Meteorite fragments and small "complete" meteorites are found on these snow surfaces simply because they are easier to spot (dark things on white surfaces) when you are wandering about and because on a melting surface you may actually get "concentrations" of meteorite debris accumulated over decades or even centuries -- if you know where to look. This all has no bearing on whether there was an ice surface on Salisbury plain 32,000 years ago.

Just having listened to a snippet or two of Colin's RS lecture, in fairness to him he seems to be perfectly open to the idea of the meteorite being embedded in permafrost rather than in a glacier, but on balance seems to go for the glacier. I'll listen to the rest, and return to the topic...

Anonymous said...

The most interesting aspect of this meteor is the probability that it is only a fragment of the original, another piece is supposed to be found at Danebury.

Therefore, if it landed on Salisbury plain, as suggested, it would leave an impact crater as it would be travelling over 45,000 kmph - this may explain the 30,000 BP radiocarbon date from the chalk strata it finally rested within.

Consequently, the estimated weathering date of 10,000 or 8,000 BCE would be the date of impact and as humans were around (which according to reports on the internet the BBC programme on Stonehenge next Friday is about to report human activity region 5,000 years before its construction date) and so would have probably seen this event and sort out the impact zone of the 'suspended in the air' event, retrieving this object.

So any geologists found an impact crater in the region?

GCU:Intwominds said...

How exactly would you get a C14 date from a meteorite hitting a chalk regolith?
Where is the radiogenic carbon in this meeting?.
M
Is the little meteorite at Danebury the same-on whose authority?? I thought not but may be wrong

BRIAN JOHN said...

I'm still not sure of that. There appear to be 2 dates at least -- awaiting info from Colin. I assume that there was some organic material embedded in crevices in the meteorite, dating from the time during which it was buried? Info awaited.......

TonyH said...

Did you ever actually get the information you requested From colin Pillinger, Brian? I would just remind you he may have significant health problems making a quick reply difficult for him.

Tony

BRIAN JOHN said...

No reply, Tony. But I haven't hassled him -- I know he is not well.