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Thursday 22 December 2022

Sands and gravels in the Trefgarn - Wolfscastle area: Ford gravel pit


In his highly influential 1906 paper Jehu described sands and gravels -- and other deposits -- in many North Pembrokeshire locations where exposures are no longer to be seen.

One of these was at Ford, where the old maps show two small gravel pits in the neighbourhood of the little "chapel of ease" which is on the east side of the A40n road.  The old pits were on the other side of the road.  He describes 12-15 ft of sand and gravel, dark grey in colour -- and a different colour from the yellow or brown sand found further north and at Rosebush which he also sometimes referred to as "ferruginous" material.  This was also described as containing blackish streaks -- we now know that the association between iron oxide staining and cementation and black manganses oxide is commonplace.  But the jury is still out on whether this means that these stained deposits are always older than those that are unstained.......

https://brian-mountainman.blogspot.com/2014/12/rosebush-sand-pit.html

Jehu also refers (on p 72) to a deposit in Trefgarn Hall park which has coarser material with stones that are rounded and sub-angular.  Could this be till?

When I visited the Ford gravel pit in 1963 I recorded fluvioglacial gravels at least 10m thick, made up for the most part of shale fragments but with many erratics including quartz, sandstones of several types, purple and red Cambrian (?) sandstones.  The lower 3m of gravels were fine-grained, with few fragments over 10 cm in diameter, but in the upper part of the section the stones were larger, with some boulders up to 50 cm diameter.  This corsening-upwards sequence is difficult to interpret, but it might be associated with  an advancing ice front coming from the north -- or simply an increase in water volume, sediment transport and turbulence associated with ice wastage.

At Ford no till was recorded, and the sands and gravels were free of shell fragments -- in contrast to those at Manorowen, about 10 km to the north.

https://zenodo.org/record/2243530/#.Y6Q1kC-l2Lg

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