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Monday, 14 December 2015

Pembrokeshire Devensian deposits: particle size distributions

Recent discussions have prompted me to look up some of my old particle size analyses for Pembrokeshire Devensian deposits, and to put them in the public domain.  They might be of use to somebody -- either teacher or student.

In 1962-65 I took scores of samples from across North Pembrokeshire Quaternary exposures (mostly on the coast) and spent many weeks in the laboratory laboriously sieving and recording percentages.  These are just some of the results.

These graphs show cumulative percentages on the left and particle sizes on the bottom.  Click to enlarge.  This top graph, for samples of calcareous Irish Sea Till such as that in the photo, shows a set of deposits that are well sorted, with very little material larger than .5 mm diameter and between 30% and 40% silt and clay.  This is typical sea floor material, dredged up and incorporated into glacial deposits.  There are some clasts included, of course, but for the purpose of matrix examination these were rejected.  In the real world, of course, this sort of analysis is useful to a degree in diagnosis, but you use many other factors as well in deciding what you are looking at -- colour, striated clasts, stone orientation, microfabric, provenance of erratics, fossil types, reaction with dilute HCl and so forth.  This till is exposed mostly on the north coast of Pembrokeshire and around Druidston on St Bride's Bay, where the ice has come directly onshore after passing across old sea floor deposits.  This type of till is often referred to as a "lodgement till" because of the manner in which it is effectively plastered onto the bed under high pressure.

This second graph is for other samples of till including what I call the "land facies" of the Irish Sea Till, deposited from glacier ice that has passed across the inland landscape, picking up local broken rock materials and pre-existing sediments including -- of course -- a great deal of material from old slope deposits and valley fills.  Here there are larger proportions of coarse and very coarse sands -- in fact in many samples I had to exclude not just larger clasts but also gravels as well, with particles larger than 2 mm in diameter.  Some sample have quite high silt and clay contents, but the curves on the graph are not so sharply concave, indicating a much lower degree of sorting than in the Irish Sea Till proper.  The till at Rhosyfelin is in this category, and from examinations of the textures across the site it is quite variable -- sometimes clay-rich, and in other places sandy and gravelly.  Again that is typical of a depositional environment where till is melting out and sometimes flowing.  The photo above, by the way, is of meltout or ablation till at Aber Mawr.

This third graph is for samples of fluvioglacial sands and gravels deposited across North Pembrokeshire.  The deposits at Trefigin are typical.  Very often in exposures there are clear signs of bedding structures.  Some of these curves are actually convex, showing a high concentration of particles in just one or two size categories.  For example, the Ford sample had over 30% gravel and about 60% coarse and very coarse sand, with very little finer sand, silt and clay.  That indicates turbulent meltwater flow and the flushing out of finer materials -- in other words, there is good sorting.  Many other samples, however, had poor sorting, except for the washing out of fine sand, silt and clay.  As with other deposits, many other diagnostic tools also have to be used in characterising and interpreting meltwater materials such as these.  The fluvioglacial gravels described for Rhosyfelin would fit neatly into this category -- again with considerable internal variation.

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