Granite erratics on Gower

This is an interesting old paper arising from research in Butterslade Bay, Gower, bu a team investigating the contents of the Patella Beach.  The beach is assumed to date from the last interglacial (Ipswichian), which means that any contained erratics must have been carried into the vicinity during the Anglian or Wolstonian Glaciation.

D. G. JENKINS, R. D. BECKINSALE, D. Q. BOWEN, J. A. EVANS, G. T. GEORGE, N. B. W. HARRIS & I. G. MEIGHAN.  The origin of granite erratics in the Pleistocene Patella beach, Gower, South Wales

Geol. Mag. 122 (3), 1985, pp. 297-302.  

Abstract

Rare pebbles of granite have been discovered in the raised Patella beach at Butterslade, Gower, South Wales. Their petrography, trace element geochemistry and the Rb/Sr whole rock ageof 55 ± 5 Ma confirm that they are derived from the Lundy granite which is about 49 km to the southwest of Gower. Amino acid analyses of fossil gastropods in the Patella beach have provided an age of 210,000 years. Various hypotheses of transportation of pebbles from Lundy and Pembrokeshire to Butterslade are considered. Erratics from Pembrokeshire were probably transported by Pleistocene ice into the area while clasts of Lundy granite were moved by progradation of beach deposits northeastwards towards Gower during glacio-eustatic marine transgressions in thePleistocene.

https://www.researchgate.net/publication/231996145_The_origin_of_granite_erratics_in_the_Pleistocene_Patella_beach_Gower_South_Wales

Note:  Butterslade is a small bay in the far west, difficult of access, just to the east of Mewslade. 

Quote:  Of 100 pebbles examined there was a size range of 1.7-8.8 cm and an average of 4.6 cm, and composed of sandstone (50 %), Carboniferous Limestone (23 %),grit (18%), volcanic and igneous rocks (other than granite) (6%), vein quartz (2%) and conglomerate (1 %); the granite pebbles are very rare and less than 1 %. The pebbles of sandstones, grit and conglomerate are mainly derived from local outcrops of Devonian strata, probably from Rhossili  Downs only 1.5 km to the north and the Carboniferous pebbles are from the rocks on which the beach deposit lies. Igneous rocks identified include an ignimbrite (Ordovician: Pem-brokeshire or Powys), feldspar porphyry (Ordovician: West Wales), porphyritic rhyolite (Ordovician: Fish-guard), quartz enstatite gabbro (Ordovician: St David's Head).

This is interesting.  As expected, most erratics are local, and many of the others are from Pembrokeshire, confirming the overall direction of ice travel during glaciation by the Irish Sea Glacier.

The six granite pebbles that were investigated proved to have come from the Lundy Island granite.  Their presence is a bit of a puzzle.  The authors' preferred explanation is that the pebbles come from an old storm beach in the vicinity of Lundy Island which was pushed north-eastwards in the direction of prevailing winds, over many thousands of years at a time of rising sea level following deglaciation.  Hmmm -- possible.  Let's just say that the jury is still out.......

West Gower Geol Soc memoir 246 (part IX) by Strahan (1907):

https://pubs.bgs.ac.uk/publications.html?pubID=B01826

Strahan and his colleagues failed to notice any erratics in the raised beach, and found nothing that they could identify as "Older Drift".  However, they did confirm that most of Gower was affected by "Coalfield ice" from the north, except for the extreme west (around Rhossili) and the extreme east (around Caswell Bay).  the glacial deposits they were looking at were those now classified as Late Devensian (LGM) in age.  Here is an extract from page 40:

TN George and various others followed with much more sophisticated analyses of the glacial features of Gower.  But there has been great confusion over the "Older Drift" (Anglian till)  and the "Newer Drift" (Late Devensian till) of Gower.   Everybody assumes that the Older Drift exists somewhere, since erratics from it appear in the Ipswichian raised beach, but where is it?

There must be remnants on Gower, as there are in Pembrokeshire,  but the attempts to determine what is what on the Gower were thrown into complete confusion by Prof David Bowen's obsession with putting "lithostratigraphic" labels onto everything, and by his "aminostratigraphy" which produced what are clearly false dates on all sorts of sediments in all sorts of stratigraphic positions.  The Geological Conservation Review for Wales (1989) was edited and heavily influenced by David Bowen, and that made matters even more convoluted.  He contended that the deposits in southern Gower were for the most part "Older Drift" and those in the northern part were for the most part "Newer  Drift"  -- and a further over-simplification contended that any ice coming from the west was associated with an ancient glaciation while ice from the north (local Welsh ice) was associated with the Devensian LGM.  Geomorphologists from Swansea University have been trying to sort out the mess for 30 years..........

Anyway, with regard to erratics, there is a new book by Peter Kokelaar (2021) called "All our Own Waters" which deals with landscape evolution on Gower and includes sections on the Patella beach and on the process of glaciation.  In Chapter 3 he refers to many erratics and often refers to them as "Anglian erratics" simply because they have come from the west.  He also purports to show a Late Devensian till overlying an Anglian till at Llethrid (SS 533 915) -- but provides no evidence to support the contention.  i am not convinced.   He also reports on a coarse-grained diorite erratic (possibly from northern Britain) at Reddenhill (SS 535 893) and a coarse-grained dolerite erratic found at 61m above sea level at Caswell Bay (SS 595 878), assumed to have come from north Pembrokeshire. He mentions "Anglian" erratics at Western Slade on the shore platform that are purported to include volcanic conglomerate from Skomer Island.

As I have mentioned earlier on this blog,  the assumption that most of the Gower south coast glacial deposits are Anglian is very likely to be incorrect.  Most of the recent work by Swansea geomorphologists points to a complete ice cover of Gower during the LGM.  While it does seem that most of the "western" erratics are found in the far west and far east of the peninsula, it's best to assume for the moment that during both the Anglian and Devensian glaciations Irish Sea ice and Welsh ice lobes were confluent, with an ice margin that oscillated in a fashion still to be worked out!