An erratic boulder found on Flatholm
A cobble found on Abermawr beach -- almost certainly washed out of the Irish Sea Till
Another cobble -- this time found on Newport beach
Erratic from Sleek Stone, Broad Haven
There are similar erratics on the beach at Rhossili, Gower, but there the typical ORS reddish colouring is more evident.
I have found another of these mystery erratics, this time in my own garden. It just dropped out of a thin till layer in the bank of a small stream, and I immediately recognised it by its creamy brown colour and smooth surface texture. I say "recognised" -- but I have no idea what rock type it is or where it has come from. Is it a sedimentary rock (a conglomerate of some sort?) or is it an agglomerate of volcanic origin? All suggestions gratefully received.........
The diameter of this cobble is about 8 cms. It's well-rounded, and most of the surface is smooth, with traces of manganese oxide staining -- but in some places the matrix has dropped away, exposing inclusions that are mostly rounded but sometimes angular. Are the inclusions made of creamy or white quartz? It looks like it to me, but there are a few darker coloured inclusions as well -- shale or mudstone fragments?
I'm now inclined to think that these erratics have come from outcrops of Devonian (Old Red Sandstone) conglomerate and that the inclusions are mostly of Precambrian quartz pebbles. But the nearest ORS outctrop, upglacier, is on the NE coast of Anglesey. If the erratics have not come from there, they must have come from Scotland.........
By the way, the conglomerate at the base of the Lower ORS in Pembrokeshire does have a brownish or grey colouring and is distinct from the Ridgeway Conglomerate (with a dark red colour) at the top of the Lower ORS. And it does contain sandstone and mudstone fragments and inclusion derived from igneous rock outcrops. But the Pembrokeshire ORS is in the south of the county, and nobody would suggest that the north Pembrokeshire erratics have come from there......
https://brian-mountainman.blogspot.com/2015/01/two-more-sleek-stone-erratics.html
https://brian-mountainman.blogspot.com/2014/10/another-mystery-erratic-from-abermawr.html
1 comment:
It is/they are conglomerates. Have you noticed how newsreaders now refer to the Arab Emirates as the 'Emeratees', as in Pilates? Maybe your pebbles are Conglomeratees.
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