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Saturday, 28 January 2017

Rotten rock in Pembrokeshire

I have been reading an interesting article about the evolution of the Pembrokeshire landscape, from the French researcher Yvonne Battiau-Queney.  She argues that the land surface is very old indeed, and she does not like at all the "traditional" view that the low-lying, undulating landscape is the result of a gradual and intermittent emergence of Pembrokeshire from the sea, with "peneplains" or fragments of marine-cut erosion surfaces separated by steeper slopes or steps.

This old view was nurtured in the days of "erosion cycles"and peneplains, after WM Davis, Wooldridge and Linton, and many others -- and I remember being brought up on the idea that in Pembrokeshire, if we looked hard enough, we could see a 100 ft raised marine platform, and another at 200 ft, and another at 400ft, and another at 600 ft.  If we were really observant, it was said that we could see traces of dissected or partly destroyed platforms at even higher levels. Built into the scenario were the old "islands" or monadnocks such as Carnllidi and Penbiri near St Davids and Garn Fawr on Pen Caer.  It was all quite convincing -- although I have to admit to having had difficulty, as a young student, in seeing the platforms and the steps in the places where more senior academics claimed them to be present.

Yvonne is having none of that.  She doesn't like the idea of a falling sea-level (or rising landmass) and prefers a scenario of very great stability over many millions of years, stretching back at least to the Cretaceous.  She says that the land surface has evolved very slowly as a "fundamental erosion surface" with gradual lowering by fluvial and other processes more or less in step with the gradual lowering of a "weathering front" as a result of deep rotting processes under a warm and wet climatic regime.  In other words, you strip off 10 m of material from the ground surface, and then the weathering front penetrates deeper, and then you strip off some more, and so the process continues........

The map above shows the contours of this "fundamental erosion surface" -- partly influenced by tectonic tilting and warping,  by the reactivation of ancient fault lines and the initiation of new faults as well, some initiated by unloading.  The map also shows the presence of exposures of rotted regolith on Millstone Grit and Ordovician sedimentary rocks, around the fringes of the Daugleddau Basin.  We can also see the so-called "inselbergs" such as those we see in Africa and Australia and other warm-climate -- and the tors on Preseli and elsewhere which have long been interpreted as the products of deep weathering and rock stripping.

 Carn Llidi near St David's -- monadnock or inselberg?  (Photo: Nathan Davis)

This is all "hard geology" which deserved to be thought about quite seriously.  I do have problems with some of it. For a start, we cannot assume that over tens of millions of years the climate has always been favourable for deep weathering processes to operate.  Secondly,  deep weathering products, deep regoliths and palaeosols occur all over the place, and their presence does not militate against coastal and marine processes operating on many parts of the Pembrokeshire coast beneath an altitude of 100m or so.  Then there is an old cliffline with things that look like stacks and old sea caves near Dinas --  if this looks like the position of an old coastline, maybe it is.  Yvonne also assumes that deep weathering and doline formation (for example) have caused many of the undulations on the ground surface -- which seems to deny the operation of other powerful forces including fluvial processes, periglacial slope processes and glacial processes during several glacial episodes.  They may of couse all be interrelated -- I am very happy to accept that deep rotted material is easier for a glacier or a river to remove than sold rock devoid of "weaknesses."

A very interesting article.

Battiau-Queney, Y. 1984. The pre-glacial evolution of Wales. Earth Surface Processes and Landforms 9, pp 229-252.

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