How much do we know about Stonehenge? Less than we think. And what has Stonehenge got to do with the Ice Age? More than we might think. This blog is mostly devoted to the problems of where the Stonehenge bluestones came from, and how they got from their source areas to the monument. Now and then I will muse on related Stonehenge topics which have an Ice Age dimension...
THE BOOK Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it.... To order, click HERE
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).
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.