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Thursday, 14 April 2011

Fishguard Volcanics terrain

These two maps give a picture of the geomorphology of the area where the Fishguard Volcanics outcrop.  You can use Newport Bay and the coastline as a reference in reading the lower map.  Broadly, the volcanics outcrop on the north-facing slope of the Carningli upland,  in the broad depression which is the catchment area of the Nevern River, and then in the upland area at the eastern end of the Preseli ridge.  These rocks, including rhyolites and ashes, are softer and therefore more easily eroded than the dolerites and other intruded rocks shown as black on the geology map.  So just as the volcanics are referred to as "soft rocks" or "rubbish rocks" at Stonehenge, because they have not persisted as well as the dolerites in the standing stone settings, they are also associated with a softer and gentler type of landscape because fluvial and glacial processes have managed to erode them away -- leaving the dolerite areas as hill masses and ridges. 

2 comments:

Constantinos Ragazas said...

Brian,

Thanks for your post. Studying these two maps closely, I can understand why you are intrigued and interested in determining more precisely the entrainment location. Would you risk a guess? Where do you believe this may be? It does seem rather odd that stones from the north-facing slope of the Carningli upland should be found at Stonehenge.

Kostas

BRIAN JOHN said...

It wouldn't be odd at all for stones to be picked up (entrained) by overriding glacier ice moving in across a rising land surface from NW towards SE. As I explained in a number of my posts many moons ago, ice moving uphill often shears, and the take-off points of the shear planes are natural places where big blocks of stone can be dragged off the bed and carried up into the body of the glacier.

What intrigues me thus far is that the entrained stones found at Stonehenge appear to have come from a very narrow band of territory -- only about 3 km wide. That was the basis of the article I wrote with Lionel Jackson -- relating to an erratic train between two parallel streams of ice.