I have not been able to find reference to perched water tables developing over relict, deep-seated, permafrost which may have persisted for some time into the post glacial thaw, or to any landforms which could be attributed to water issuing from such water tables. But it seems likely (or even obvious) that they would have existed.
The premise here is that the great depth of burial of the permafrost (up to 50m is quoted) would have insulated it from surface temperature changes. This could have prolonged its existence into the early stages of the post-glacial, temperate climate. Precipitation would have percolated downward and frozen when it reached the depth of the relict permafrost or joined with the ice already in the permafrost if it was saturated. Either way, this ice would have formed aquicludes which could have supported perched water tables.
It seems to be accepted that deep seated permafrost can exist for some time after the initiation of ice melt. In their review, Murton & Ballantyne, (2017) describe a situation where persistent melt water, under artesian pressure confined beneath a permafrost layer, can push up a dome of permafrost ground (a Pingo). This is an example of buried frozen ground acting as an aquiclude, albeit one which is preventing upward rather than downward infiltration of water.
So, it is possible to envisage a situation, during the glacial thaw, where temporary perched water tables supplied springs which issued from the sides of valleys, higher up than the springs along the bottom of the valley. The timing and duration of any such fluvial phases would influence the effect on the landscape.
Where these springs occurred at the site of coombes, the effect on the form of the final coombe could depend on the timing. If stream flow was initiated prior to the mass movement phase, the chalk structure could have influenced the orientation of the coombe if the stream followed lines of weakness in the chalk. Intuitively, it would seem more likely that course of a stream would be more easily influenced by discontinuities in the chalk than would a mass movement flow. So, stream flow preceding mass movement could explain the odd right-angle bends shown in several coombes if they are reflecting the orientation of joint sets in the chalk.
If such high-level springs existed, their effect should still be visible as dry gullies some distance above the bottom of the valley, but they are not seen. So, either they did not exist, or their effects have been removed by subsequent mass movement on the sides of the coombes.
Andrew Coleman
12/01/2024
Reference:
Murton, J. B., & Ballantyne, C. K. (2017). Periglacial and permafrost ground models for Great Britain. In Geological Society Engineering Geology Special Publication (Vol. 28, Issue 1, pp. 501–579). Geological Society of London. https://doi.org/10.1144/EGSP28.5
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