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

Friday, 15 May 2015

Rosemergy niche glacier, Cornwall


 The Rosemergy hollow or niche, about 10 km from Land's End.  From Harrison et al 2015.  The red line shows the catchment and the area within which the small glacier is thought to have been located.


Another interesting paper from Stephan Harrison and colleagues, providing evidence for a very small niche glacier tucked away at Rosemergy, in a NW-facing hollow near the outer tip of Cornwall.

This is so far to the south and west that most people until now have discounted the idea of Devensian glacier ice forming on the mainland and have assumed that the basic processes of landscape evolution during the Devensian (and earlier glacial episodes too) must have been periglacial rather than glacial.  That "traditional" view is not that sensible, given that periglacial environments in Devon and Cornwall during the Late Quaternary must have been both very cold and very snowy -- with extensive perennial snowpatches in favourable locations -- just as we see today, for example, in places like Arctic Canada and the South Shetland Islands in Antarctica.  Where there are perennial snowpatches there is a good chance that some of them will be thick enough for the formation of firn or glacier ice, and for them to be classified as "niche glaciers".  (Cirque glaciers tend to be larger, and need greater relief than we find in the South-west of England.)  So ice-related processes could -- in theory -- have operated in some places, and moving ice on the bed of some of these niche glacier features might even have resulted in the creation of striations on bedrock and on transported pebbles and boulders. The processes responsible for these subtle landscape alterations are referred to as "nivation" processes.


Landscapes in the South Shetland Islands, Antarctica.  These are extreme periglacial landscapes with permafrost and frost-related processes dominating.  In the spring we see a mosaic of snow-patches as the winter snow cover melts off.  Some of these snow-patches are seasonal, and others are perennial.  Some, in the most favourable locations, are more than 50m thick, and contain snow, firn and glacier ice which moves very slowly, at a rate of maybe a metre or two each year.  This is a reasonable "model" for the Devensian situation across much of Devon and Cornwall.

Another reason why we should tend towards acceptance of the evidence presented in this paper is that in the late Devensian the ice of the Irish Sea Glacier affected parts of the Scilly Islands, even further towards the SW; and at the same time there was a sizeable Dartmoor Ice cap which was thick enough and long-lasting enough to have left substantial features behind when it melted.  See here:

http://brian-mountainman.blogspot.co.uk/2012/06/dartmoor-ice-cap.html

Then we have the evidence of Devensian glaciation on Lundy as well, which I have covered in a number of posts on this blog.  Here is one of them:

http://brian-mountainman.blogspot.co.uk/2014/07/lundy-island-and-lgm-dilemma.html

And as for Exmoor, there are yet more posts.  Find them with the search box.  Here is one:

http://brian-mountainman.blogspot.co.uk/2011/01/glaciation-of-devon-and-cornwall.html

If there were perennial snow-patches, niche glaciers and even local ice caps in the far South-West during the Devensian, there must have been a pretty extensive snow cover on Salisbury Plain as well.  In the Anglian Glaciation, which was by common consent much more prolonged and severe, the idea of glacier ice pressing far inland, maybe as far as Wiltshire, does not seem half as "extreme" as it might have done twenty years ago.

===============================

"The southernmost Quaternary niche glacier system in Great Britain"
STEPHAN HARRISON, JASPER KNIGHT and ANN V. ROWAN
 3 February 2015
JOURNAL OF QUATERNARY SCIENCE (2015)    ISSN 0267-8179. DOI: 10.1002/jqs.2772

ABSTRACT
Until recently, the scientific consensus has been that the uplands of south-west Britain remained unglaciated throughout the Quaternary, with glacial ice sheet limits lying to the north of the south-west peninsula. However, recent work has shown that small glaciers and ice caps existed in the uplands of Exmoor and Dartmoor during the late Quaternary, demonstrating that the consensus of an unglaciated south-west Britain requires considerable revision. Here we report geomorphological and sedimentary evidence supported by glacier- climate modelling for a Quaternary niche glacier from west Cornwall, south-west England. This niche glacier represents the southernmost such system from mainland Great Britain, and provides evidence for the presence of extra-glacial niche glaciers probably during the Last Glacial Maximum of the Devensian glaciation, and well outside the limits of the main British–Irish Ice Sheet.

2 comments:

Myris of Alexandria said...

Ah the mid 20th cent paradigm gets slowly chipped away.
M

AG said...

On the geology-petrology side, Apart from Olwyn Williams-Thorpe.

The writings of MPP's and his collaborators suggest that it's still a paradigm in the 21st century.