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Friday, 18 May 2012

More about those rock samples from the Newport area...

 Eight rock samples from the Newport area, mostly from erratic boulders.  Some of the erratics have not moved very far -- they have quite mysterious origins.....  Click to enlarge.

Here are some preliminary rock sample identifications, by a mysterious geologist who delights in the name of Xipe Totec. Many thanks to him for giving the time to look at the samples. These are identifications made on the basis of hand samples only -- and of course accurate identifications really need to be done on the basis of thin sections and microscopic examination.

But this is all rather interesting, and confirms that there is a great deal still to be discovered about where the last ice came from in this area, and what rocks it carried.

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. Affinities with #8 . A thin section would be useful.

3. The strange rock found on the shore of the estuary, across the river from the Parrog, Newport. Felsite, maybe from North Wales? See my post dated 23 Feb. Not certain. Siliceous igneous rock with feldspar megacrysts. Bit porphyritic.

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

4. Fine-grained grey igneous rock (or could it be sandstone?) -- knocked off a nice rounded boulder. Most interesting Feldspar megacrysts in siliceous matrix with a sub-spheroidal fabric. Quartz-chlorite mainly. Rather like Stonehenge orthostat SH48. A thin section would be interesting.

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. Trace muscovite and weathered feldspar.


chris johnson said...

Interesting. Some further analysis of thin section seems to be needed but so far - no banana.

Are any of these from the lane widening I described recently?

BRIAN JOHN said...

Not looking for bananas or banana skins here -- just increasing the size of the data base. No -- the erratic samples here are from the area where land clearance is going on. Not that that matters -- many of these erratics have been moved about several times since this is a farming landscape. I would never want to claim that any of them are in their "original" positions.