Yesterday I was over at Ceibwr, near Moylgrove on the N Pembs coast, during a pleasant walk along the clifftop. I was struck once again by the rather interesting landforms and sediments there. There are two big meltwater channels separated by an elongated ridge. One of the channels is now occupied by an arm of the sea (Ceibwr Bay) and the other is dry and occupied by the roadway and car parking area. This dry channel is fascinating, because it contains a smaller channel which is cut into its floor at the seawards end -- as seen above. Really this small channel is a series of connected pot-holes, excavated out at a time of very turbulent meltwater flow, with large erratic boulders rolling about in enclosed hollows and excavating out very soft Ordovician shales. Similar boulders can be seen in the foreground in the photo above.
It's tempting to think that the main channels here are of Anglian age and that they have been modified during the Devensian at a time of more limited meltwater flow. But that would be too simple. There are solidly cemented tills, frost-shattered periglacial accumulations and fluvioglacial deposits plastered on the inside of the "newer" small channel, and then fresh till sitting on top of these red-stained cemented deposits. The fresh till has to be of Devensian age. So how old are the cemented deposits? Might they date from the Anglian glaciation? If they do, that means the small channel might also date from the Anglian, and it might be even older.
Solidly-cemented deposits are common in the Moylgrove-Ceibwr area, maybe because groundwater here is very rich in iron and manganese. So the reddish and black cement is made essentially of iron oxide and manganese oxides. How long does it take for a deposit to be turned into concrete?
I'm increasingly convinced that these cemented, stained and rotten deposits (for example at Llangolman and Ceibwr) are very old, but they do need to be dated through the use of one of the modern techniques. Must work on that........
There is a lot of discussion of the channels and the cemented sediments in this paper by Glasser et al (2004):
Glasser, N. F., Etienne, J. L., Hambrey, M. J., Davies, J. R., Waters, R. A. & Wilby, P. R. 2004 (August): Gla- cial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, Vol. 33, pp. 224–237. Oslo. ISSN 0300-9483.
I don't agree with all that the authors say -- they don't sufficiently differentiate the uncemented and cemented deposits, and they refer simply to a "pre-late Devensian" episode of sedimentation and cementation. If I understand them correctly, they seem to think that the cemented materials are mostly fluvial in origin, and relate to a period of very ancient fluvial valley erosion and sedimentation. I think the cemented deposits are much more varied than that, with ancient till included.
(Note: thanks to Myris for correcting me! I have changed the text to refer to manganese oxides....)