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Friday, 29 May 2015

Meltwater channels at Rhosyfelin



I thought I should record these for posterity, in a separate post.  On the image above we can see the edges of the Afon Brynberian Channel and (in the upper part of the image) its confluence with the gorge of the Afon Nyfer.  On the top edge of the photo you can see the edge of Castell Mawr -- another of the sites investigated by Prof MPP and his colleagues.  The gorge of the Afon Nyfer is quite complex -- it seems to have been cut in several phases, which are not well understood. Within it, at Felin y Gigfran, there are crags reminiscent of those at Rhosyfelin.




In the image above we see the "micro-morphology" associated with the crag of Craig Rhosyfelin.  There are undoubtedly two minor channels.  The western one has a "blind" intake.  The eastern one is actually a humped channel, and in this photo, with the shadows just right, we can see the intake channel which carried water uphill on the western side of the Brynberian Gorge, over a little col and then after swinging to the right, down along the NW flank of the spur.  At the NE tip of the spur, where we see much evidence of bedrock moulding and block abrasion by torrential sediment-rich meltwater, the flowing stream joined the much more substantial torrent flowing within the main valley.

These are classic features of sub-glacial meltwater erosion,  and although in their main outlines they may date from the Anglian Glaciation, I suspect that there has been considerable modification and "freshening-up" during the Devensian glaciation around 20,000 years ago.

Just a reminder:  these were the directions of meltwater flow:




7 comments:

Robert John Langdon said...

Post Glacial Flooding!!

Well done at last you have found evidence for my theory at Rhosyfelin - keep up the good work and you will see this happened all over Northern Europe after the last ice age.

RJL

BRIAN JOHN said...

Don't get over-excited, Robert. When ice melts, it creates water, which then flows away according to the laws of physics.

Robert John Langdon said...

Actually when ice melts it creates water that soaks into the ground creating groundwater which creates the rivers according to the laws of hydrology.

Each ice age replenishes this resource and as you point out the last ice age "freshening-up" the groundwater increasing the size of the rivers as you have noted correctly. But the process of movement from groundwater to rivers takes many thousands of years once the surface waters are gone - otherwise the Thames would have dried up many centuries ago!

BRIAN JOHN said...

Permafrost, Robert.

Myris of Alexandria said...

You can't make it up!

Bring back Kostas, for a sense of reality.

M

BRIAN JOHN said...

Kostas has been banished to outer darkness, and so has our friend RJL -- life is too short to start rewriting the hydrology text books and the laws of physics. Those who want to do such things can do it elsewhere.

AG said...

To me RJL's avatar had always brought to mind the popular nick name of Jay Kay of Jamiroquai!

Clue: It was about something in a hat!