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Friday, 4 January 2013

Rhosyfelin Dig Site


As a service to mankind in general, and Kostas in particular, I have marked the approximate dig site at Rhosyfelin with a greenish blob on this satellite image.  The river flows along the shady area in the wood from the south (bottom of photo) towards the north, curving gently along the bend in the valley.

The topography of the valley is actually quite complicated here, with many relict features which I attribute to meltwater erosion on a substantial scale, maybe partly during the Devensian glacial episode but probably mostly in earlier glaciations including the Anglian.  The meltwater erosion might have been subglacial.  The zig-zag in the road has messed things up a great deal, but if we eliminate the activities of the road-builders we can see that there is a strange double channel cut on the north side of the infamous Rhosyfelin rocky ridge -- water must have been carried north-eastwards into the main channel.

As far as the slope deposits are concerned, as exposed in the dig, they have come from several different directions, moving NE (up against the rock face in the southernmost part of the dig site) -- then E, and then SE along much of the edge of the dig as shown in the photos. 


In this photo the direction of debris movement has been more or less from behind my highly reputable friends towards the bottom of the photo (NW towards SE).

5 comments:

Constantinos Ragazas said...

Brian,

Thanks for the post. You have confirmed what I have been assuming all along regarding the location and direction of the river off Crag Rhosyfelin.

While I speak of a wider bigger river in the past, you speak of meltwater channels. While I speak of river deposits, you speak of periglacial slope deposits.

A river is a river! Contained within well defined embankments and flowing over long distances from 'source' to 'sea'. Does it really matter if the 'source' at one time was glacial meltwater or latter seasonal snow melt and rain overflow?

Where we really, I think, disagree is in the dating of these deposits. And whether or not Crag Rhosyfelin was engulfed by a wider river in the not too distant past (say 4000BP). We both rely of 'instinct' and 'sensible reason' consistent with the facts on the ground.

So how do we determine what the truth is? RC dating done right may help here. We both agree. But only MPP has the 'proprietary data'. And he is not saying!

Kostas

BRIAN JOHN said...

Please, Kostas, enough. I am a geomorphologist, and I have seen these deposits on the ground. Please give me the benefit of knowing the difference between a slope deposit and a river deposit, and please stop your long-distance speculations over what is what. Armchair geomorphology is bad enough, but when combined with armchair sedimentology, it gets a bit wearing.

Constantinos Ragazas said...

“Armchair geomorphology is bad enough”

True! Walking the fields is better. Sorry for being wearisome.

Kostas

Geocur said...

Wonderful " reputable friends " pic .

The Duke of Wellington said...

Is walking barefoot on rocks the latest geophysical technique I wonder?