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Thursday, 3 March 2011

The site of Lake Brynberian?

The photo shows the great expanse of gently-sloping moorland beneath the north face of Preseli.  On the skyline are Foel Drigarn (left) and Carn Meini (centre right).  This is where Etienne et al (2006) assume that a great lake was impounded as the ice came in from the left.  See the previous post for the exact location on the map.  I'm not sure about this -- this is not a flat-floored depression, but one that has a rather consistent gentle slope.  I have wandered about on it quite a bit, and I have never seen anything resembling lake deposits.  But sediment exposures are few and far between.  A programme of drilling is probably needed, to see if there are any varved sediments present beneath the peat..

4 comments:

Constantinos Ragazas said...

Brian,

If there was a great Lake Brynberian, as Etienne et al (2006) claim, is it all that far fetched to also consider a great Salisbury Lake? (I know Robert is loving this!).

And if there were such great lakes in W. Wales and South UK, can't we also consider that these lakes would have frozen over during 1000 years of freezing? (this part I love!)

Sooner or latter, Brian, you'll come around to acknowledging my theory!

Kostas

BRIAN JOHN said...

The literature is full of references to proglacial lakes at various stages of various glaciations. Some of those lakes existed, and others probably didn't. The key thing with proglacial lakes is that they can only form in enclosed hollows or depressions in the landscape where there is no escape for meltwater or impounded river water. As with any lake, there must be an effective dam -- and with proglacial lakes the impounding material is glacier ice. Sure, glacial lakes can freeze every winter -- and in some circumstances (eg with an advancing glacier and a deteriorating climate) a lake might freeze to its bed and become incorporated into the glacier as the glacier slowly advances. Glaciers can also bulldoze lake ice if they are advancing.

But the idea of lakes freezing over and extending over vast distances and acting as convenient sliding beds for erratics is -- as I have repeatedly said -- not one that I can sign up to.

Robert will no doubt have his own views -- but as far as I can see there is no mechanism for the impounding and retention of the vast expanses of water that he describes.

Constantinos Ragazas said...

Brian,

Thank you for your honest reply. Understand that my comments are compelled by argument and reason. I am painfully aware of your complete and categorical rejection of my 'ice theory'. That, of course, makes it that much more difficult for you to acknowledge it in the future, as the evidence builds up. But be that as it may.

I ask.

In your post http://brian-mountainman.blogspot.com/2010/12/glacier-retreat-and-pressure-release.html you have a photo showing a fragile hill barrier that is eroding away and is separating the sea from the low lands on the other side. It's hard to see if the low lands are at or below the sea level. Reminds me a little of the dikes in Holland. Could this hill have impounded a lake at one time? And if the hill completely erodes away, would the lake once there seize from ever being there!

Kostas

BRIAN JOHN said...

The evidence mounts up, Kostas? I don´t think so.....

I'm afraid you misunderstand the photo. The fields at the bottom R of the photo are over 150m above sea level -- the landslides are occurring on a very high cliff face. The environment is completely different from that of the Netherlands...