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Friday, 20 April 2012

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.....


11 comments:

Tony Hinchliffe said...

Coincidentally, this same Quarry Company, Lafarge, has planning permission to quarry an area right adjacent to the Westbury White Horse near Bratton (see very recent Post on that location and the possibility that a bluestone glacial erratic train might have been deposited there or thereabouts). You can view an English Heritage YouTube commentary on this.

Anonymous said...

ones is another thing" Perhaps
Whin Sill erratics are quite widespread in central England.
OWT published at length on them a few years ago with regard to axe-heads.
It is the odd mafic rocks(ignoring any Niedermendig material) found in archaeological contexts in Kent and Sussex away from the coast that really are a puzzle. No one (??)invokes glacial movement this far south of the Thames.I trust.
"The work of drop-stones is another thing"
Has anyone done serious work on small drop-stones in southern Britain.
Jude the Obscure-there's Wessex tag for you.

BRIAN JOHN said...

Axes and flakes are one thing -- a boulder weighing almost a tonne is quite another. RThat;s why I'm rather intrigued by the precise position of the boulder -- assuming that it is genuinely a piece of Whin Sill basalt. Was it on the ground surface? Or embedded deep within the gravel sequence, and exposed during the quarrying operations? I think we can forget about drop-stones carried in floating ice at sea; the sea wasn't there at the time. So we have two other dropstone possibilities --
1. boulders carried in glacier ice fragments floaring in a pro-glacial lake, and
2. boulders carried in torrential meltwater streams beyond the ice edge.

I hope the good people who are investigating this have a good geomorphologist on board......

Anonymous said...

Don't get too excited, the site lays on (or around) the original path of the Thames during the pre-Holocene period - so it could be tidal.

Annie O.

BRIAN JOHN said...

Annie -- I fear that at a time of deglaciation or ice melting in this vicinity, at the end of one or other of our big glaciations, the early route of the Thames was immaterial. Sea level will have been at least 100m lower than it is today, so there is not the slightest prospect of having tidal water anywhere near the site where this discovery was made.

Anonymous said...

Brian

What a wonderful simplistic mind you have.

River estuaries from the melting ice would have pushed your little boulders all over the place, as the volume of the melting ice would have been to great to absorb into the ground creating rivers like the old river Thames that exited to the sea at Ipswich, passing the area in question.

That's why you don't see your displaced rocks in neat little lines shown on your map (you see them only in isolated pockets) if you did we would not be debating about where and when glaciations ended.

The Thames Estuary study in 2000 by MOL found erratics from Wales (brought down by the Holocene Thames). I'm sure your not suggesting that any past glacier stretched all the way from the Irish Sea to London via the M4 corridor.

Or are you?

Annie O.

BRIAN JOHN said...

Annie -- what on earth are you on about? "River estuaries from the melting ice" -- a fanciful idea of yours that bears no relation to evidence on the ground or the laws of physics. When ice melts on land -- as it was doing here -- you get meltwater torrents flowing on sandar or outwash plains -- with complex braided patterns. Sure, big boulders can be moved along if the gradients are high enough. But estuaries? Forget it.

The Thames terrace gravels do contain erratic materials -- mostly in small pebbles -- but there is a big debate about whether these pebbles came from loose sediments in the upper reaches of the rivers, following glacial episodes, or from the erosion of consolidated conglomerates which were themselves made partly of Welsh material.

My simplistic mind can cope with all of that, and a lot else besides.

Anonymous said...

"River estuaries from the melting ice" -- a fanciful idea of yours that bears no relation to evidence on the ground or the laws of physics"

See Wikipedia for:

"Approximately 14,000 years ago, following the melting of the glaciers, the Great Bay estuary was formed."

"The Delaware Bay is an estuary formed from a drowned river valley. During glacial times, the river channel flowed along the bottom of the present-day bay. Because the glaciers stored much water in their ice, the ocean level was much lower then. When the glaciers melted, the river valley "drowned" and formed the estuary."

Wessex archaeology

"Underneath the glacier, water remains liquid. This is because the pressure of all the weight of ice lowers the melting point to below 0o Centigrade. This water moves around underneath and beyond the glacier, creating rivers that carve channels into the land."

What do I know, I'm just a mere woman, you the 'qualified' man.

Annie O.

Ps Dartford Heath Gravel, Orsett Heath Gravel and Cobets Tey Gravel show three phases of the original Thames through this area, no doubt bring other erratics.

BRIAN JOHN said...

Annie -- I care not whether you are a woman or a man. this is getting wearing. An estuary is a river mouth where there is a very low gradient and where there is generally tidal influence from the sea. The Thames estuary, the Severn Estuary and all the other estuaries of Britain were estuaries in interglacial times, but not in glacial times. Please do a bit of homework on this and save us all a lot of trouble....

Geography Police said...

Romford is in London, not Essex.

BRIAN JOHN said...

Ah, we stand corrected.