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Thursday, 14 July 2016

Felin y Gigfran and Castell Mawr LIDAR


Thanks to Dave for drawing our attention to Lle -- a Welsh Government web site on which we can see LIDAR imagery for many parts of Wales.  Above is one composite image extracted by Dave -- showing part of the Afron Nyfer valley.  Click to enlarge.  The letters are inserted by Dave.  His notes:

The meltwater channels show up very well (A), but are there other features more like pre-glacial cut-off valleys at the points I’ve labelled B.  What about what looks like a terrace at C, on the Afon Bannon upstream of Pantyglasier.  It has always looked like it when driving, but not so on the lidar as there are other incised cuttings at D.  How did the flat valley floor develop in places on the Bannon and the Nanhyfer?   Is this some form of glacial meltwater down cutting followed by a period of alluvium buildup?

http://lle.wales.gov.uk/GridProducts#data=LidarCompositeDataset

Yes, the meltwater channels do show up very well, and they are highly complex.  The course of the Nyfer has changed many times as a consequence of glacial diversions and valley blockages by morainic and other materials.  Much work has been done (not by me) on recognising the pre-Devensian course of the Nyfer and comparing it with the post-Devensian course.  The biggest dry valley section is near Nevern itself, where the modern river runs in a great loop to the north of the old valley.  I agree with Dave that the sites lettered A are all glacial meltwater channel sections; and yes, the bits labelled B may be interglacial (not pre-glacial) sections -- but they are more likely to be dry channels used subglacially during the Devensian.  I would need to take a careful look at them to be sure.  The features marked C and D are interesting -- will try to take a look at them some time .......

Unfortunately, there is no LIDAR coverage for Rhosyfelin and Pensarn -- both sites lie just off the bottom of the image shown above.  But take a look at the fantastic detail on the images shown below -- all from the Felin y Gigfran - Castell Mawr area.


The uppermost of these images has OS map data superimposed, including contours and place names.  Note that in this one small area there are three prehistoric features -- Castell Mawr is the big circular earthwork.  Castell Llwyd (the westernmost settlement) is a fortified site on a spur above the river, and we can see another simpler fortified site (called Cwm Pen-y-Benglog Camp) on another spur south of Castell Mawr and close to the Felin y Gigfran nature reserve.  Both of these could be labelled as "promontory forts" and they are most likely to be from the Iron Age.  But who knows?  These days many "Iron Age" forts are being found to have much older origins........

In the Nyfer Valley itself, the big flat terrace to the SW of Castell Mawr must be an old valley floor into which the modern valley has been incised.  There are several features here suggesting subglacial meltwater flow;  in the middle image, look at the elongated rocky spurs inside the main valley.  Do they remind you of anything?  Just think Craig Rhosyfelin...........

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