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Saturday, 20 December 2014

Rosebush Sand Pit

One of the nice things about going back to the original ancient sources (as Myris constantly exhorts us all to do) is that one rediscovers treasures.  I had forgotten about this description by Jehu (1906) on the GLACIAL DEPOSITS OF NORTHERN PEMBROKESHIRE.  On page 72 he refers to a sand pit a few yards south of Rosebush Station, on the western side of the railway (which no longer exists). showing about 10 ft of sediments, the most interesting of which is a fine yellow sand, "very ferruginous in places" and apparently layered, with patches and "imperfect layers" of blackish hard pan-like material "which is probably organic in nature."  This is overlain by 3-4 ft of rubbly material, full of fragments of slaty and other local rocks.

I doubt that the black material is organic -- but it would be interesting to check, if only we could find this deposit again........  We can speculate that the sand layer has discontinuous bands of manganese oxide concretions in it -- just like the black bands in the gravels at Llangolman.  We may assume that the two gravel deposits are therefore the same age -- ie probably Anglian. 

The overlying bed of "rubble" is perhaps more interesting --  is it a slope deposit?  I would doubt that, given the location well away from any steep mountainside or valley side.  But just a couple of km away is Foel Cwmcerwyn, the highest summit of Preseli.  If there was a Devensian ice cap here for a few centuries, maybe it spread far enough downslope on its western side to have affected the Rosebush area........... in which case the uppermost deposit at Rosebush could be Devensian till.

This is one of the computer-generated images from Aberystwyth suggesting the maximum extent of the Preseli ice cap in the Devensian.   Note from the blue square that the ice in the Rosebush area MIGHT have been warm-based for a short time.  This would have allowed sliding on the bed and till deposition.

Watch this space.......

Here are three of the several posts I have put up about the Preseli ice cap:

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