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Tuesday, 27 September 2011

Erratics and deposits in the Vale of Glamorgan

 Source:  Bevins and Donnelly 1992

The lowland Vale of Glamorgan -- the southernmost part of Wales -- is rather like South Pembrokeshire in that it does not have widespread deposits of till, either from the Irish Sea Glacier or from the glaciers which came down, several times, from the valleys of the South Wales Coalfield.  Indeed, some authors have suggested that there are so few traces of glaciation that the Vale was unglaciated.  But there are SOME glacial deposits, and SOME erratics, and geomorphologists do now accept that at least once the Irish Sea Glacier flowed across this area, travelling from west to east.  One of the key indicators is an erratic of pyroxenic keratophyre from the New Inn (north of Haverfordwest) found at Cardiff.  Other erratics have been found from Scotland and North Wales.  Heavy minerals in the lower soil horizons, shell fragments in the till, and "Irish Sea pebbles" from Cardigan Bay and west Wales also show that the ice came from the west and extended some way to the east of Cardiff.  This of course matches up with the evidence that ice crossed the Bristol Channel and extended eastwards as far as Bathampton near Bath -- and maybe even further.

The Storrie Collection

At Pencoed, near Bridgend, there are thick deposits of clay, sands and gravels and coarser deposits containing erratics.  Some of the clays -- at Ewenny and elsewhere -- have been used in the local pottery industry. The deposits are spread across an area at least 10 km x 10 km.  There are lake deposits and tills here -- and it's been known for more than a century that the erratic suite sampled by John Storrie contained at least eight different types of rock traced back to outcrops in west Pembrokeshire (Strahan and Cantrill, 1904).  Some of the erratics appear to be in their "original" positions -- others appear to have come from the reworking of older glacial deposits.  In a detailed reexamination of the erratic boulders from the Storrie Collection, Bevins and Donnelly (1992) described more than 20 which had come 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, various sandstones and siltstones from South and West Wales, Carboniferous Limestone, ORS sandstones etc.  No less than 8 of the examined clasts had striations on them, confirming a glacial origin.

The interesting thing about this work by Bevins and Donnelly is that it shows a considerable mixing of erratics from the west and erratics from the north, which have come down from the valleys of the South Wales Coalfield.  (This is important when we consider how the Altar Stone, thought to have come from the Senni Beds, might have found its way into a suite of erratics being transported by the Irish Sea Glacier.)

Even more interesting is the incorporation of erratic material from the Lake District or Snowdonia.  And even more interesting still is the apparent exclusion -- in this area at the edge of the Vale of Glamorgan) -- of erratics from the eastern end of the Preseli upland ridge.  The Pembrokeshire erratics here are all from western Pembrokeshire -- and this means that if we reconstruct a simple ice flow pattern, it comes out as suggested in a 1965 paper by CB Crampton:

In fact, since there are erratics in the collection which have apparently come from Skomer and Ramsey Islands, we could push the arrow ever further towards the west in Pembrokeshire.

So if this ice stream was entraining erratics from the eastern end of Mynydd Preseli at the same time, where are the spotted dolerite and rhyolite erratics which we all know and love?   Was this part of the ice stream somehow "blocked off" by ice coming down from the uplands of Mid and South Wales?

I think the answer to this apparent dilemma is that there must have been many different episodes of erratic entrainment and ice flow in the West Wales - South Wales arena.  This is what is coming out of the recent work on the Devensian British and Irish Ice Sheet -- asynchronous behaviour around the fringes, with one part advancing while another part is retreating, and with swings in ice movement directions too.  As I have said before, individual erratics -- or even assemblages of erratics -- may well have followed zig-zag courses, over very long periods of time. 

I'm sure that in this area of conflict between ice from the west and ice from the north, we are only just beginning to unravel the history of what happened maybe 450,000 years ago, given that most of the evidence has been obliterated by what happened in the Devensian......

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