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Wednesday, 13 March 2013

Erratics from the Newport area

Many thanks to Rob Ixer for making sections of these rocks and identifying them in the cause of science.  This helps to build up the database of local and erratic rocks in North Pembrokeshire.  As Rob indicates, some of the rocks are difficult to give locations for, except to say that some of them might have come from SE Ireland.  This would make sense, since we know that the Irish Sea Glacier also affected that area -- and in a number of posts I have suggested a gradient on the Devensian ice surface across St Georges Channel.

Caution required here -- much work still to be done, and many more erratics to be examined and identified!  See the two links below to previous relevant posts.

1.  Bedrock from near the summit of Carningli – dolerite or microtonalite. An even-grained fine-grained igneous rock with dark mafics and feldspar. Not a typical spotted dolerite.


2. Very coarse volcanic agglomerate / ignimbrite (??) from the southern flank of Carningli. Not sure whether the outcrop is bedrock or a very large erratic. Not certain. It is a siliceous rock with simply twinned feldspar megacrysts but also black fine-grained areas (single quartz crystals??)/?slate clasts.  (NB. This cannot be an ignimbrite as there are no fiamme.)

3. The strange rock found on the shore of the estuary, across the river from the Parrog, Newport. Felsite, maybe from North Wales?  Not certain. Siliceous igneous rock with feldspar megacrysts and a sub-spheroidal fabric. Crystal tuff??  

4 - 8. Erratics exposed during land clearance off the Cilgwyn Road, about a mile from Newport.

4. Fine-grained grey igneous rock (NOT a sandstone) -- knocked off a nice rounded boulder. Most interesting Feldspar megacrysts in siliceous matrix with a sub-spheroidal fabric. Quartz-chlorite mainly.   This is definitely a felsite erratic. I suspect that this may be a granophyre; it has plagioclase megacrysts in a sub-spherulitic matrix all within a chlorite ‘matrix’.  Granophyre.

5. Dark reddish marl -- from the Cambrian sandstone series? Flaky -- almost a shale... from a rough sub-angular boulder.    Foliated fine-grained indurated laminated ?metamudstone with pink-orange and grey alternating laminae. Not Permo-Trias but Palaeozoic.  Could be SE Irish.

6. Medium-grained purple sandstone. Cambrian? From a nice rounded boulder. Origin -- SE Ireland? Very indurated red-purple fine-grained sandstone/meta-sandstone. Certainly at least Palaeozoic Exotic!

7. Fine-grained red sandstone. Cambrian? From a bigger rounded boulder. Origin -- SE Ireland? Fine-grained indurated micaceous sandstone with quartz veining along joint plane. Devonian/Lower Palaeozoic? Not Permo-Trias.

8. Rough greyish volcanic ash //agglomerate? Has vesicles -- highly variable crystal structure. From a large sub-angular boulder. Don't think this is local..... Very silicified coarse-grained sandstone (litharenite) with shale/slate clasts and voids after lost shale/fossils?. Trace muscovite and weathered feldspar. Meta-sandstone polylithic with igneous and meta-sedimentary rocks and trace ??bone.  Lower Palaeozoic in age.


Anonymous said...

So what does it all mean? Anything new and exciting?


BRIAN JOHN said...

As far as the local rocks (from Carningli) are concerned, Rob says he has not seen anything similar in the Stonehenge collections. So that probably rules out Carningli as a possible bluestone source -- as others have said before.

As far as the erratics are concerned, they are clearly not local, and have come from some distance away. The tentative idea that some of them may have come from SE Ireland is interesting -- and might back up the idea of ice flowing right across St George's Channel approx as shown on the map. (The conventional wisdom is that the ice flowed through the Channel from NE towards SW.....)