THE BOOK
Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Friday, 8 August 2014

Glacial Lake Teifi

I came across this map in my box file and thought it worth sharing.  It shows the Teifi catchment, the main rivers of today, the old course of the Teifi prior to glacial diversions (there are lots of bits of old valleys all over the place, making the Teifi Valley landscape very complicated) and the outline of the lake as it might have been about 20,000 years ago.

There is one mistake on the map.  Instead of referring to the "edge of the retreating ice sheet" the authors of the map should have referred to it as an "advancing ice sheet" -- since following the laying down of the lake sediments the glacier edge pushed over them and laid other glacial and fluvioglacial materials on top.

The map was used as part of a display at the Welsh Wildlife Centre near Cilgerran.  Click to enlarge.

6 comments:

Constantinos Ragazas said...

Brian,

You write in this post, ” [.] following the laying down of the lake sediments the glacier edge pushed over them and laid other glacial and fluvioglacial materials on top

Is this an example of a glacial lake freezing over and later overridden by an advancing ice sheet?

Very interesting! This actually does happen …

Kostas

BRIAN JOHN said...

No, Kostas, this is not. Nobody has ever suggested that.

Constantinos Ragazas said...

Brian,

You are of course right if the glacial lake Teifi was no longer a lake.

But if we assume for a moment it was and conditions were freezing, wont the lake freeze if there were advancing ice sheets going over it? And wont an advancing ice sheet going over a dry or wet lake carry and disturb the prior sediments of the lake bed?

Seems reasonable to me. But I could be wrong … just thinking out aloud and clear.

Kostas

Dave Maynard said...

This feature has always interested me. When growing up I lived between Beulah and Blaenporth on a farm with very wet clay land. This was probably underwater in the maximum extent of the lake (on its north side). Part of the land was covered by banks that were described as pingos by a drainage officer. I'm ready to accept that interpretation, but other pingo type formations seem a little different.

As these are supposed to be formed in a peri-glacial environment, would they be later than the lake formation, which is itself a sign of the ice wasteage, or perhaps not, if there are different episodes of ice movement.

Dave

BRIAN JOHN said...

Kostas -- please remember that even in the depths of the Ice Ages there were SEASONS and that lakes became liquid every summer and froze every winter. Only in the most extreme circumstances (eg in parts of Antarctica) do air temperatures remain sub-zero throughout the year.

Constantinos Ragazas said...

Brian,

Fair enough. But wont the advancing ice sheet over a dry or wet lake bed carry or disturb the previous sediments of the lake bed? The scenario I am describing would leave such layers of sediments distinct and undisturbed.

Kostas