Glacial deposits on the Redstone-Penblewyn-Llanddewi Velfrey A40 bypass

From BGS Viewer -- the suerficial geology of the same area.  Blue areas = till.  Pink areas = sands and gravels.  Note that this mapping is VERY inaccurate.......

Iron-stained stony clay and sands and gravels (probably glacial and fluvioglacial) thrown out from one of the trial pits west of Penblewyn.

A40 PENBLEWIN TO REDSTONE CROSS IMPROVEMENTS

GROUND INVESTIGATION Factual Report

Report No. Q0083/FR June 2020

https://brian-mountainman.blogspot.com/2022/09/llanddewi-velfrey-glacial-deposits.html

Further to my earlier post about the glacial and fluvioglacial deposits of this broad ridge in central Pembrokeshire, Dave M has now kindly sent me the civil engineering reports (2020 and 2021) relating to the field investigations undertaken prior to all the consents being given for the project.  Many thanks to Griffiths Brothers for allowing access to this material.  There are two vast reports, filled with details of open pit investigations and boreholes.  There are also many particle size analyses of the sediments encountered, reminding me of the scores of analyses I did when working on my doctorate thesis in 1962-65............  In those days I did everything by hand -- nowadays I suppose it is all mechanised and digitised.......

There is far too much detail in these reports for us to reproduce here.  But the main findings confirm what I reported in my earlier post:

1.  Most of the ground surface is covered with deposits of till and fluvioglacial sands and gravels -- to depths sometimes exceeding 8m.

2. The sands and gravels are not all above the till deposits.  There is much interbedding, suggestive of deposition in an ice wastage environment.

3.  There are some patches where sticky blue clay till is exposed at the surface. 

4.  In some localities weathered bedrock "rockhead" or "rottenstone" material overlies till -- this is interpreted as slope breccia, and occurs only where adequate slopes exist to encourage the mobility of unconsolidated sediments.

5. In the materials classified as "glacial till" there is a lot of variety, with the diamictons incorporating stiff orange brown clay, sandy and gravelly material incorporating angular and sub-angular mudstone fragments, grey and brown clay, and sticky blue clay.

6.  There are some exposures of black stained materials, but they are not commented on in the reports. So the manganese oxide stained layer observed in my visit does not seem to be particularly widespread.  The orange and reddish colouring associated with iron oxide staining is much more widespread.

7.  The extent and scale of bedrock rotting and weathering (mostly in shales, mudstones and sandstones) is frequently commented on.

8. There are few comments about boulders or erratic pebbles, apart from brief mentions.  They were probably mostly ignored because they were a nuisance, or brought the drilling operations to a halt!

9.  In the western part of the area investigated (Penblewyn to Redstone) the deposits are thinner, with weathered bedrock struck less than 2m beneath the ground surface.  Nonetheless,  from the descriptions, fluvioglacial sands and gravels and variable glacial till are still encountered, but because the work was done in that area by a different contractor, those terms are not used at all........

10.  The distribution of glacial and related deposits on the land surface as shown on the BGS maps is wildly inaccurate!

Conclusions

This whole area has been affected by glaciation, with till, ephemeral lake deposits and sands and gravels laid down by meltwater quite abundant. I suggest that the ice that affected this area was not capable of powerful bedrock erosion -- because considerable thicknesses of weathered and rotted bedrock have turned up in many of the pits and boreholes.  If there had been intensive glacial erosion much of this material might have been removed, even if it was frozen by permafrost.  So maybe the glacial episode we are talking about was relatively short-lived? 

I see no evidence here of more than one glacial episode.

When was this glaciation?  Well, there do not appear to be any extensive concreted sediments, although there is widespread gleying and staining by iron and manganese oxides.  I suggest that the glacial and related deposits are of Late Devensian (LGN) age.  If they had been older, I think there might have been a clearer and thicker cap of later deposits including windblown loess and colluvium.  But we need a dating programme (OSL or cosmogenic) in order to sort this out.......