Preseli -- glacial deposits

Copy of a map by WD Evans (Plate 4 of his QJGS article) -- designed to show where the areas of exposed rock are to be found, but if you concentrate instead on the "boulder clay" or till areas, you can see just how extensive they are, both to north and south of the upland ridge.

I have been digging up some ancient papers and doctorate theses from the 1940's -- and have turned up some quite useful information.  Two researchers had interesting things to say-- WD Evans and JC Griffiths -- both of whom submitted doctorate theses to the University of London.

Sadly, it is not always easy to get at old papers -- the QJGS articles are still trapped behind paywalls, including this one:

The Geology of the Prescelly Hills, North Pembrokeshire
William David Evans
Quarterly Journal of the Geological Society, 101, 89-110, 1 October 1945, https://doi.org/10.1144/GSL.JGS.1945.101.01-04.04

Evans did not have much to say about glaciation, but Griffiths's thesis was titled "The Glacial Deposits west of the Taff, South Wales" (1940), and although in many ways it is a rather poor piece of research, he was convinced that there was at one time a Preseli ice cap, and that it may well have existed before the mountains were "overwhelmed" by Irish Sea ice.

This is an interesting map, from a 1984 NERC report on drainage in the Preseli Hills:

The stippled area covers all the land above 200m -- so this might give an indication of the extent of the intermittent ice-cap, which we have referred to on many past occasions.

Griffiths also mentioned blue clay till at Llangolman -- again I have described this before:

https://brian-mountainman.blogspot.com/2014/10/pre-devensian-glacial-deposits-south-of.html

In 1904 TJ Jehu, one of my great heroes, described a clay pit at Llyn (Grid ref SN112274) or Fagwyr Owen, to the NW of Llangolman.  This was in open country, just above the 600 ft contour.  Jehu described the clay pit as containing "boulder clay" and of being 20 ft deep -- so deep that ladders had to be used to get in and out.  The clay is bluish and very tough, and was referred to back in 1904 as "india-rubber clay."  From the map and written evidence, there may have been more than one clay pit in the vicinity.

One thing that is very confusing, on the Geology of Britain viewer, is that on a small scale, the deposits both to the north and the south of the Preseli upland are coloured blue and labelled as "till", whereas when you zoom in the deposits to the south change from blue to a muddy sort of colour, with a different label.

The blue area is described thus:
Till (Irish Sea Ice) - Diamicton. Superficial Deposits formed up to 3 million years ago in the Quaternary Period. Local environment previously dominated by ice age conditions (U).

On the other hand, the area to the south has these words attached:

Head - Diamicton, Gravel, Sand And Silt. Superficial Deposits formed up to 3 million years ago in the Quaternary Period. Local environment previously dominated by subaerial slopes (U).

So on the one hand the till is deemed to be fresh, and on the other it is deemed to be mixed up with brecciated slope deposits and other materials.  I don't know that this distinction is based on any hard evidence -- I suspect that there is simply an assumption (based on the presumptions of Dai Bowen and others) that the till to the north of Preseli is Devensian and the till to the south is Anglian -- belonging to the mythical "Penfro Till Formation."

In other words, nobody knows what is going on, and somebody needs to sort it out.

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Where did this till on the south side of Preseli come from?  How old is it?  I have assumed in the past that it is very old -- and therefore Pre-Devensian.  Now I am not so sure.