The Geology of Cow Green

Recently, NHSN members set out to explore the rich and varied geology of Cow Green in Upper Teesdale. Take a closer look at what they discovered.

West Cowgreen Mines consist of two main veins, the Winter Hush Vein and the Green Hush Vein, running N-S and NE-SW respectively, and a number of smaller veins including the Angle Vein. These veins are in faults which are offshoots to the south of the major Teesdale Fault (NW-SE). They were worked in the late 18 and early 19 centuries, primarily for lead ore (galena) which tended to occupy the centre of the vein, surrounded by barytes. Evidence for this early working can be seen in the form of hushes, dams and old shafts, prominent in the landscape.

The Winter Hush Vein consists of a series of lenses of predominantly barytes, with galena more abundant in the narrower sections. The lenses are around 100m long and 3m wide on average with the intermediate sections of the vein about 1.2m wide. A bore 80m deep was sunk through the vein towards its southern end that penetrated the Whin Sill, the vein within the Sill consisting entirely of witherite (BaCO₃). The fault dowthrows to the east by up to 30m but varies along its length with hardly any displacement in some places.

After lunch, we headed for a shallow quarry. The grey rock exposed here is the Smiddy Limestone. It is quite fossiliferous with numerous fragments of crinoids and small brachiopods. The slightly raised mound away from the road is a bioherm, formed by corals around 330 Ma.

The fossiliferous Smiddy Limestone

We then continued along the North Pennines AONB Geo Trail to Rod’s Vein where the old spoil heaps contain fragments of the barytes that was mined here, and there are also traces of galena. The vein was worked opencast in the Melmerby Scar Limestone where the main vein was 1.1-1.8m wide. The adjacent shaft, sunk in 1949 entered the Whin Sill at 20m depth where the vein became narrower.

Further down the track the outcrops of rock, most noticeable to the left of the road, are of the Melmerby Scar Limestone. These rocks are white and crystalline and show a distinctive crumbly weathering, earning them the local name ‘sugar limestone’. Despite its name, it is a type of marble, having been altered and recrystallised by intense heat.

Outcrop of the famous sugar limestone

The rusty brown rocks in the stream at Red Sike are part of the Whin Sill. Here in Upper Teesdale, this layer is almost 75 metres thick. The molten rock that cooled to become the dolerite of the Whin Sill baked the surrounding rocks, creating the sugar limestone we saw earlier.

Top of the Whin Sill at Red Sike

The final stop of the day was the waterfall at Cauldron Snout (England’s longest at 120m) which is formed as the River Tees flows across a very hard outcrop of the Whin Sill dolerite. We noticed the vertical cracks in the rock, which are known as columnar joints which are formed as the magma contracts during the final stages of cooling.

Cauldron Snout and columnar jointing in the Whin Sill

The next trip is to Seahouses on Saturday 31 July. All are welcome and knowledge of geology is not necessary. Notes, similar in pitch to this write up, will be available beforehand to assist understanding of what will be seen during the outing.

Karl Egeland-Eriksen
Earth Sciences Section Coordinator

Born (1961) and educated in Newcastle, I took early retirement from a career in NHS dentistry in 2015 (with a few years working in Australia and New Zealand as well). I became interested in geology whilst studying for an Open University degree in the early 1990s and, on completing that degree, went on to take a Master’s degree in Environmental Sciences at Lancaster University. I have been an active member of the Open University Geological Society since 2006 and have organised around 200 geological field trips in Northern England and the Borders for the local Northumbria branch (and other groups) over the last 10 years.

Brenda Turnbull
Earth Sciences Section Coordinator

Born and educated in Newcastle including A Level Geology, studied Geography at Durham before doing a Humanities PGCE at Nottingham. Taught Geography in Kenya and Botswana for 20 years and completed an MA, Geography in Education with UCL. Currently, Course Leader of A-Level Geography in an FE College, Assistant Examiner for A-Level Geography, plus writing and reviewing geographical material. Volunteer Chair of the Geographical Association, Tyne and Wear Branch and run their Twitter feed, @GA_Tyne&Wear, to promote Geography, Geology and Earth Sciences.