I'm more and more impressed with the BGS Geology of Britain Viewer:
http://mapapps.bgs.ac.uk/geologyofbritain/home.html
which you can use to zoom in on anywhere in the country so as to pick up on the details of solid or superficial geology or a combination or the two. The best scale to look at is 1:50,000, since on smaller scales there are some rather strange amalgamations of categories which might leave you confused. Indeed, some of the classifications are confusing anyway -- but that's not surprising, given the difficulty of classifying either sold rocks of or superficial materials when you are wandering around in the field.
If we look at the screenshot above, we can see the vast spread of till in the lowland between the Preseli ridge and the Brynberian - Crosswell area, confirming my suspicions that the Devensian Irish Sea Glacier did push across this area. I think the BGS surveyors have got things pretty well right.
And look at the Craig Rhosyfelin area -- we should not be at all surprised to see, exactly at the site of the archaeological dig, a series of different deposits including till, rockfall materials, torrential fluvioglacial materials, and alluvial and colluvial deposits incorporating frost-shattered slope detritus. Exactly as I have described them.......
By the way, on the map the pink areas are spreads of fluvio-glacial materials. The light brown areas with relief shading on them show territory where there are no thick sediments but thinnish soils resting directly on bedrock.
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