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Friday, 16 May 2014

New erratics exposed at Traeth Mawr, Newport


There are some amazing exposures of cobbles and boulders along the foreshore at Traeth Mawr at the moment, following the winter storms and the removal of the front edge of the sand dunes.  The dunes had previously covered these exposures, which are all within 100m or so of the Surf Lifesaving Clubhouse.

What we can see at the moment is not a raised beach or storm beach exposure, since the cobbles are not rounded enough for that.  My instinct is that this is a "lag deposit" derived from Devensian till, with all the fine material washed away by wave action around the time when the sea reached more or less its present position on the coastline.  This "wave washing" process may not have lasted very long (maybe less than a century) before the sand dunes advanced seawards and covered the deposit over.

There is plenty of Devensian till in the area -- sometimes exposed on the beach itself, when the sand level is rather low (as it has been this winter) and frequently exposed within the estuary, on the north side near the Golf Course.  There are some very strange boulders embedded in the till between the tide marks -- I have previously posted pics of these.

So where have these erratics come from?  Not from Pembrokeshire, that's for sure.  The biggest stone in the top photo is Carboniferous Limestone, full of fossils.  Some of the others appear to be volcanics with interesting colours and textures.  There are some buff and grey banded sandstones and gritstones.  And in the bottom photo we see red marls and purple sandstones -- some appear very similar to the Cambrian sediments of the St David's area, and some look like the red sandstones of the ORS series in South Pembrokeshire.  But they cannot have come from either S Pembs or the St David's area, because all the evidence of ice directions is against this -- so my guess is that these rocks have come from Anglesey, the Lleyn Peninsula and the Lake District.  We should not be surprised by that -- there are pebbles of Ailsa Craig microgranite all over the place on North Pembrokeshire beaches. 

15 comments:

Dave Maynard said...

In some places, I have heard mention of ballast from empty ships. Is there any potential for this as a means of transport for rocks to Newport?

I'm not sure how ballast would be identified, whether it would be carefully unloaded and reused, or simply chucked over the side.

Dave

BRIAN JOHN said...

Yes, ballast is always something to be aware of -- but these stones are at and above HWM. I have seen what I think is ballast further out on the beach, around the mouth of the estuary, where cargo vessels used to unload their cargoes on the beach in the sailing ship days. The main unloading / loading areas were several hundred metres from where these stones were exposed. The pebbles and cobbles I photographed do seem to be associated with a distinct layer under the sand -- not very well exposed, unfortunately. I'll stick with the theory that they are from till which has been largely eroded away in this area.....

TonyH said...

Please could you give a Grid Reference for Traeth Mawr, Brian?

BRIAN JOHN said...

The pebbles are exposed around SN 053406

Myris of Alexandria said...

I think that Brian is correct in saying that these are natural cobbles and "local". I mean non-anthropogenically moved.
Ballast is often quite large and mono-lithic.
M

Dave Maynard said...

Has anyone got details of any ship ballast,especially if it is large and mono-lithic?

I'd always imaged it would be not too big so one man could lift. Brian's pepbbles do look too small for that and the topographic location fits more with his interpretation.

Dave

BRIAN JOHN said...

This is interesting:
http://www.ncdcr.gov/qar/Conservation/Artifacts/Ballast.aspx

Apparently sand and beach pebbles were used sometimes -- not to mention lumps of concrete and scrap metal -- so one can't be absolutely certain about all this. But I have certainly seen patches of stones and pebbles out near the mouth of the river and near the bar. Sometimes these patches are exposed, and sometimes not -- depends what the currents and tides have been up to. I think those patches may well be old dumps of ballast, since sailing ships used to be loaded and unloaded on the sand, at low tide.

Dave Maynard said...

Soil seems to be common.

The Island of Mull apparently, had no moles until a 150 years ago, when they came in soil as ship's ballast.

Baku in Azerbaijan, required ships arriving to collect cargoes of oil, to bring fertile soil as ballast. This was used to create the gardens and parks of which there are quite a few.

Dave

Myris of Alexandria said...

I have looking at St Bees Sst as ballast in Tazzy.
That is a trip.
There are well known examples of ballast being described as in situ rocks on coast lines and causing red faces.
Mono-lithic of course here means a single rock type rather than monolithic. Large means cobble or above.
It could not be that anyone thought I was suggesting the transport of big stones by boat. I take Dr Ixer' s views on that see 'Waiting by the River'as the only True Gospel.

Soil is news to me but makes sense.
M

BRIAN JOHN said...

Well, this is a rather interesting topic! These days, of course, the whole idea of travelling one leg of a voyage without a full cargo would be anathema -- but in the good old days it happened a lot..... in which case ballast would be picked up at the port of departure, maybe a pile of old stones left on the beach by the last ship to pass that way, which in turn she had picked up from almost anywhere. Vast scope for endless confusion.....

TonyH said...

Dr Rob Ixer's "Waiting by the River" as mentioned by Myris above, has as its full title [see Dr Ixer's website of publications under "Published Abstracts etc":-

Waiting by the River: Stonehenge and the Severn Estuary. Abstract. Stone artefacts as material and symbolic markers in cultural landscapes. An international perspective. Implement Petrology Group Meeting. York. September 2007.

Myris says he takes this as the only True Gospel on the issue of moving big stones by boat.Wonder if we could persuade Mris to gossip the Gospel a little more? Intriguing, tantalising stuff thus far....

BRIAN JOHN said...

Sometimes Myris reminds me of a great hero of mine, Inspector Cluseau: "I see everything, and I see nothing. I know everything, and I know nothing....."

TonyH said...

"Waiting by the River", is also, Google tells me, the title of a song by ageing long hair Neil Young, so I'm wondering if perchance Rob has been, in one of his past incarnations, a devotee of hippie pursuits such as crystal studies? And I believe crystals and other 20th century offerings occasionally turn up at Stonehenge.

Dave Maynard said...

Is it possible that ballast of any type, might be got rid of while the ship was waiting off-shore for the tide to allow it to come in? This means most of it is properly underwater.

Where the ships berth or beach might not be a good place for stuff to be dumped as it might be in the way.

I guess some of it might be re-used and carried away elsewhere, except for harbours where goods were predominately exporting. Somewhere like Newport probably had a mixture of import and export.

Dave

TonyH said...

My brother may go and take a look at the Traeth Mawr erratics soon, next time he and his wife take one of their countryside/ nature strolls around the River Nevern area at Newport, from their Trefin home.