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.
8 comments:
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
No, Kostas, this is not. Nobody has ever suggested that.
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
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
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.
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
I first became interested in this topic when I noticed the land formations and road locations around the Llandysul Teifi area and wondered what could have caused the flatness of the valley base at this point in the course of the river.Also the location of Vortagens fort suggests a time when there was a navigable lake. The fort would at some stage possibly have been a strategic location with large amounts of transport possible.Also the image of the valley seen from Llanluwni when filled with morning mists looks like a giant lake and so thinking about it perhaps the mist is falling and forming into the valley and being held there for a time perhaps in the same way as water was once retained.
Is there a known definite point in time when the sediment damming the river that allowed the lake to form was finally breached?
The lake was there rather a long time ago -- during the ADVANCE phase of the Last Glaciation. Difficult to date, but probably at least 28,000 years ago. Vortigern came along somewhat later......
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