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Saturday, 22 January 2022

Igneous erratics in the Bristol Channel

Gabbro (?) erratic boulder on the beach at Lydstep, S Pembs

Loveston Farm igneous erratic, probably dolerite, South Pembs.

Following up on some points made by Peter Kokelaar in his new book "All our own Water" (unfortunate title!), I have been checking out the references to igneous erratics on the Bristol Channel coasts.  There are quite a lot of them.  Here are some of the localities:

Flimston Churchyard:  seven erratic boulders in churchyard, and others nearby. A 'brecciated spherulite, albite, trachyte or rhyolite.'  Other volcanic ashes, dolerite and gabbro, from NW Pembs. (Adrian James)

Lydstep, Pembs: gabbro (?) erratic on beach near fresh till outcrop.  BSJ observation.

Loveston Farm: dolerite (?) erratic in farmyard, recorded by Adrian James.

Broughton Bay, Gower: green granophyre boulder from Pembs, and possible Leinster granite. (Campbell and Shakesby, QRA 2015)

Butterslade, Gower:  non-granite igneous erratics from Pembrokeshire, contained in the Patella Beach deposits. (Jenkins et al, 1985)

Ram Grove Beach, Gower: volcanic and dolerite erratics.  (Kokelaar, 2021.)

Hunts Bay, Gower: erratics of quartz dolerite, pink granite and fine grained andesite or basalt of "Irish Sea provenance". (George, 1933)

Reddenhill Farm, nr Pennard, Gower:  a coarse-grained diorite erratic (from northern Britain?).  Grid ref: SS 536 894.  (Kokelaar, 2021)

Western Slade, Gower: several large igneous erratics including a volcanic conglomerate from Skomer Island (?).  Grid ref: SS 484 855.  (Kokelaar, 2021)

Eastern Slade, Gower:  boulders of gabbro and volcanics, probably from Pembrokeshire. (Kokelaar, 2021)

Caswell Bay, Gower:  a dolerite erratic 61m above sea-level, probably from Pembrokeshire.  Grid ref: SS 595 878.  (Kokelaar, 2021)

Pencoed and Ewenny, near Bridgend: igneous erratics in stiff clay.  Storrie collection: more than 20 erratics from the west, including ash flow tuff (Fishguard Volcanics?), basalt from the Skomer Volcanics, rhyolite, Precambrian rhyolitic tuff, gabbro from near St David's Head, ignimbrite from Skomer, volcanic debris flow deposit from Ramsey Island.  Many are striated. (Bevins and Donnelly, 1992) These are sometimes referred to as "boulders" but full size information is missing.

Cardiff, Ely valley: pyroxenic keratophyre possibly from New Inn, Pembs. (Griffiths, 1940)

Cowbridge, Vale of Glamorgan:  3 boulders of ignimbrite, possibly from Ramsey Island (Bevins and Donnelly, 1992).  South of supposed Devensian ice limit. (Are these the same as the 3 blocks of rhyolitic tuff -- up to 30 cms in diameter- found by Briggs in a wall near the cattle market toilets?)

Pentre, near Llantrithyd: tuff from north Pembrokeshire. Vale of Glamorgan - south of Devensian limit (Strahan and Cantrill, 1904)

St Athan:  quartz felsite -- north Pembrokeshire origin. Vale of Glamorgan - south of Devensian limit (Strahan and Cantrill, 1904)

Flat Holm:  large pink granite erratic boulder and many smaller igneous erratics, including  feldspar porphyry, feldspar-rich lava and silicified welded crystal lithic tuff. West Pembrokeshire sources? (John, 2015)

Steepholm: three glacial erratics identified as metamorphic amphibolites (Ixer, letter to Brit Arch 1999)

Lundy Island:  erratics on island plateau, including rhyolite and "miscellaneous igneous erratics" (Rolfe et al, 2012).

Kenn, Court Hill and other sites as far east as Bath: basalt, other igneous and metamorphic rocks from the west. (Campbell et al, 1990)

Some of the locations mentioned

The erratics (giant and otherwise) on the coasts of Devon and Cornwall have not been properly provenanced.  Some of the dolerites and rhyolites may have come from Pembrokeshire,  but they may also come from further afield -- and I am quite attracted to Kellaway's idea that to the south of the dolerite-bearing stream of the Irish Sea Glacier in Bristol Channel, there was another stream on its southern flank, carrying erratics from as far afield as Scotland.

Giant erratics on the coast of Devon and Cornwall.  The Giant's Quoit at Porthleven (above) and the Freshwater Gut boulder near Croyde (below).  Origins unknown...........

The famous Saunton pink granite erratic, now thought to have come from from western Scotland.  According to Kidson (1971) it is cemented onto the rock platform and sealed beneath cemented raised beach sands which are assumed to be of Ipswichian age.  This means that the boulder (weight around 10 tonnes) must have been emplaced during a pre-Ipswichian glacial episode.  There are at least 20 known large erratics along the Devon coast.


Tony Hinchliffe said...

Your last paragraph makes me think of the large erratic weighing one tonne in the North Devon village of Shebbear.I didn't see it, but have been to Shebbear. You describe it ( page 200) as possibly made of pink granite. So you think it may have arrived courtesy of the stream bringing erratic from, say Scotland?

BRIAN JOHN said...

Quite possible. There are some strange occurrences of erratics high up, on clifftops and a way inland. Of course, aliens in spaceships might have dumped them just to confuse the enemy.......

Steve Hooker said...

Of course, there would be a breadcrumb trail. Just one of these to match up to the Preseli Mountains will trump the human transport theory.

And surely, of the collection of different 'Bluestones' at Stonehenge will be brothers and sisters in this trail of breadcrumbs.

Happily, English Heritage will not prohibit samples being taken. It would be fascinating to trace the path. Did some of those Bluestones come from Ireland, after all?

BRIAN JOHN said...

Very true, Steve. We are working on it....... The new book by Peter Kokelaar about the Gower has provided interesting new info.