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Saturday, 18 February 2012

North Pembrokeshire Ice Flow Directions

Above:  a synthesis if known ice movement directions across Pembrokeshire (many lines of evidence; many sources).  No particular glaciation portrayed -- but most evidence comes from the Devensian.  Click on this map and the others to see them enlarged.


The classic map of glacial erratic trains in Pembrokeshire -- a synthesis from Griffiths (1940) based upon his own fieldwork and the work of the Geological Survey surveyors (Cantrill, Thomas, Strahan and Dixon in particular).
Key:  
4 Preseli rock types (dolerite, spotted dolerite, rhyolite etc)
5  Roch - Hayscastle erratics
6  St Davids granite
7  Ramsey Island volcanics
8 St David's Head gabbro
9  Clegyr agglomerate
10  Llandeloy porphyrite
11  Cader and Arans felsite
12 Green Harlech grit
13  New Inn pyroxenic keratophyre 
The line enclosing the Preseli Hills marks the approximate boundary between Irish Sea Drift (till) and Welsh Drift.  Outside the line there are many erratics which have come from Irish Sea sources -- inside the line (to the east) the erratics are much more difficult to provenance, comprising mostly shales, mudstones, sandstones etc from the Silurian sedimentary sequences of Mid-Wales.


 
 Glacial striae at coastal locations in Pembrokeshire.  This map has never been published before -- data from my field notes 1962-1964.  Compass deviation just over 7 deg W from grid N in 1964.  Corrected on this map.  Sites:  1 is Ogof Golchfa; 2 is Whitesands; 3 is Porthmelgan; 4 is Pen Deudraeth (Abermawr); 5 is Parrog; 6 is Newport Sands; 7 is Gwbert.  Most common directions are shown; in reality much more variation at each site.  This map confirms that for some of the time ice flow has been almost N-S -- but for most of the time the ice has come in from the NW quadrant.  There is one very intriguing feature -- some old striations indicating an ice flow from NE towards SW.  That means Welsh ice flowing down into St George's Channel, unhindered by Irish Sea ice.  I'll put up another post about this apparent anomaly.

 
JC Griffiths -- synthesis of ice flow directions from erratic trains and other evidence.

5 comments:

Anonymous said...

And the outlook for Friday is the same old arrows going nowhere, proving nothing.

Michael Fish

BRIAN JOHN said...

Michael -- you have obviously become bitter and twisted, as a result of some deep trauma when you were young.

Anonymous said...

BBC weather tonight says we are in for "buckets of ****" on Sunday!
http://www.youtube.com/watch?v=DI1_WfVEo9A

chris johnson said...

Puzzle for me. Seems like the Rhos-y-felin stones would have been moving east most likely. Wonder how the got onto the Bristol Channel route?

BRIAN JOHN said...

Not a problem, Chris. Because of the "zig-zag" factor which I have discussed on previuous posts, all we know is where a stone started off and where it finished up. we have no way of knowing -- as yet -- how many different journeys it might have made between point A and point B. All of the arrows on these maps might be out by 20 degrees or more. but they are broadly right -- and remember that whatever the ice movement direction might have been over Craig Rhosyfelin and the eastern Preseli area, there would have been massive ice pressure from the glaciers coming off the Welsh ice cap and pressing southwards. I did a post on the "South Wales battleground" a short while ago.