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Wednesday, 23 April 2014

Devensian survivals and "landscape zones" in Pembrokeshire


This photo shows Maiden Castle and Lion Rock, near the Trefgarn Gorge (which carries the A40 road) between Haverfordwest and Fishguard.  The rocks are very fragile and very precarious -- and it is doubtful, in this geographical situation, that they could have survived over-riding by the Irish Sea Glacier during the Devensian.  So the suggestion is that central Pembrokeshire remained ice-free around 20,000 years ago, as shown on my latest ice edge map. 


In contrast,  the monadnocks or upstanding rocky hill masses of the St David's Peninsula are much more heavily eroded, as we see here (the two photos are of Carn Llidi, near St David's):



Up on the summit of Carn Llidi, and on nearby Penbiri, and on St David's Head itself, there are extensive ice-smoothed rock surfaces, as we see in the lower photo.  So the ice streaming across this area must have been thick and quite active.

The northern tip of the Pencaer peninsula was also moulded by streaming ice, as we see in this photo from near Carregwastad (where the French landed during the "last invasion" of 1797).  Notice the generally scoured appearance of the cliffs and the landscape inland, and the rounded off or smoothed profiles of the hills:


Closer to the ice edge, as for example in the Dinas - Newport - Carningli area, things are much more complex, with some ice-smoothed surfaces and some quite delicate features that look too fragile to have survived very active glaciation.  Here is one of the glaciated slabs on Garn Fawr, on Dinas Mountain -- as fine a feature as you will find anywhere in the world:


And here is another, from Carningli above Newport:


And here is the area of glaciated slabs at Carn Meini, adjacent to what used to be called "the bluestone quarry":


Are these slabs in these widely spaced locations all of the same age?  Cosmogenic dating is seriously needed........

Another fascinating difference between Carn Llidi and Carningli is the lack of a big scree bank at the former and a very substantial scree bank on the lee (south) side of the latter:


So the explanation must be that there was a prolonged period of ice erosion at the former site, but just a short one at the latter site, maybe with marginal ice melting more or less balancing the rate of ice advance.  Maybe there was a windscoop here, within which periglacial slope processes were able to operate at or near the peak of the glacial episode.

So we have three Devensian landscape types in Pembrokeshire:

1.  An area beyond the ice edge where fragile landforms were able to survive -- but where there will have been extensive snowfields and where periglacial processes operated for many thousands of years. (For example, the Trefgarn Gorge area)

2.  A zone maybe 5 kms wide across which the Devensian ice edge oscillated around 20,000 years ago, with some fragile features surviving, some glacial erosional features created or freshened up, and banks of scree able to accumulate in places. (For example, the area around Carningli and Carnedd Meibion Owen)

3.  An area well inside the maximum position of the ice edge, where erosional processes were able to operate, where fresh scree was not able to accumulate, and where the landscape was rounded off or smoothed. (For example, the hill masses at the NW corner of the St David's Peninsula)

Gradually it is all coming together.  Work in progress.....






4 comments:

TonyH said...

You say the conclusion is that there are 3 Devensian landform types and this is a "work in progress" and it is "gradually all coming together". You also mention cosmogenic dating being seriously needed? These days, where is most on - going research and fieldwork (aside from yourself) being done? Aberystwyth University?

BRIAN JOHN said...

There is good work going on in Aberystwyth and Swansea, but much of it these days seems to be concerned with modelling and with the effects of climnate change in the Arctic and Antarctic. That's great for glaciology, and essential that this work is done, for the sake of mankind...... and as reported on the BBC, BAS is getting a new icebreaker / research ship for use in polar waters.

So field geomorphology is getting a bit of a raw deal -- I feel rather sad about that, just as Myris bewails the lack of decent petrography being taught these days in geology departments.

TonyH said...

I can see how rather essential it is that the "eager bright young things" are taught correctly in the field, that way their initial enthusiasm for their Earth Science subject may properly be ignited......plenty of time for them to ALSO get involved in the imperative issues to do with Climate Change.

Anyway, all these "bright young things", for the sake of their future health and sanity, should have healthy outdoors interests in the environment around them, as well as the "Big Issues". Then they'll all have weekend and post - retirement hobbies!

BRIAN JOHN said...

Quite so. I can thoroughy recommend the practice of wandering about in the countryside looking at landforms, digging holes and looking at things, as a counterbalance to all those heavy strains of modern life. My two summers of fieldwork in N Pembs in 1963 and 1964 were quite idyllic...... as have been many other prolonged spells of fieldwork in the polar regions.