Cow Green Geological Trail, Teesdale, August, 2018

Base of the Whin Sill

Upper surface of Great Whin Sill

Upper surface of Great Whin Sill

Cauldron Snout

The top of the Whin Sill

Five members and friends met at the Wheelhead Sike car park by Cow Green Reservoir to visit the Great Whin Sill and the waterfall that is Cauldron Snout.

The Whin Sill

“About 295 million years ago , molten rock, or magma , at over a 1000 degrees C, rose up from deep within the Earth. The magma spread out between the layers of limestone, sandstone and shale which lie beneath much of the North Pennines. It cooled and solidified underground to form a huge sheet of rock up to 90 m thick- the Whin Sill.  This is made of a hard dark rock called dolerite or, as it is known locally, whinstone. Dolerite is a type of igneous rock – one which is formed when molten rock solidifies. After millions of years of erosion the Whin Sill is now exposed at the surface in several places.” (North Pennines AONB Partnership handout “The Whin Sill”)

Lead by Josephine Drake the group walked down from the car park to an exposure of the dolerite which is the top of the Sill. Various old mine workings were passed on the way, and some interesting plants examined. The River Tees flows through Cow Green Reservoir and just a couple of hundred metres below the dam, itself an interesting construction, the River flows over the Sill in the very impressive waterfall that is Cauldron Snout. The group descended the path by the side of the falls, part of the Pennine Way, to the foot of the falls where lunch was taken  before continuing the short walk round to the base of the Sill. There at Falcon Clints white marble , part of the Melmerby Scar Limestone can be seen beneath the crags of the Sill. This is the bottom contact.

Sugar limestone

We had already seen at the top of the Sill the effect that the heat had on the limestone above it producing a rock that is white and crystalline and showing a distinctly crumbly weathering. This is also a marble and and is known locally as ‘sugar limestone’. This unusual rock supports  the unique ‘Teesdale Assemblage’ of arctic-alpine plants, including the beautiful spring gentian.

Between the top and bottom contacts the rock cooled much more slowly giving it a columnar appearance which whilst not quite ‘Giant Causeway’ was still quite distinctive. Also observed from the foot of the falls was the old channel of the River Tees dating back to before the last glacial period and now plugged by boulder clay. Finally we spent some time searching for a conglomerate dating back to early Carboniferous times about 330 million years ago after which we made our way back to the cars. We then retired to the Langdon Beck Hotel for refreshments and to view the excellent collection of geological specimens on display in the back room of the pub!

Jh; 05/09/18

Text and images by J Hutchinson

2019-01-06T13:59:17+00:00

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