Leaving behind the Rookhope Inn, where you have doubtless well imbibed, head along the Allenheads road for 50 yards and then turn left on a track which runs alongside Bolt’s Burn. Cross the bridge over the Rookhope Burn and then turn left on to the footpath besides the Burn, with Boltsburn Garage above on your right. 100 yards further on is the small concrete plinth with a metal capping which seals the Rookhope borehole. The borehole, which is over 800m deep and reached granite at 390 metres, was drilled in 1960 and 1961 by Durham University and has made Rookhope famous to geologists around the world. Temperatures around 40deg C were encountered at the bottom of the hole.
In 1934 Kingsley Dunham, who later became Professor of Geology at Durham University and Director of the British Geological Survey, had suggested that the zoned distribution of mineral deposits in the North Pennines indicated that there was a large granite body below the ore bearing rocks, exerting upwards pressure due to its relative buoyancy.
Granite is lighter than most rocks in the earth’s crust which means that gravity is slightly weaker in areas with granite basement rocks. Gravity measurements in the 1950s showed that the force of gravity in the North Pennines was low and that Rookhope had the lowest readings of all. This geophysical work supported Dunham’s theories and so the government made available funds for the drilling of the Rookhope borehole. The finding of the granite body led to a reassessment of the origins of similar mineral deposits around the world.
The cores from the borehole, weighing 25 tonnes, were stored and available for study in the Rookhope miners’ shop next to Burnside Cottages in Rookhope until 2005 when Durham University sold the building.
Sadly, geologists can no longer access the borehole for further investigations because a group of Rookhope youngsters (now allegedly model middle aged citizens) put metal bars down the hole and experts have been unable to remove them.
A borehole drilled in Eastgate in 2004 in an attempt to prove up a geothermal resource encountered granite at 273 metres. This was part of the ill fated Eastgate Renewable Energy Village project (also known as Eco-Disney!) which was to showcase five different forms of renewable energy on the old Eastgate Cement Works.
Dunham’s observations on the zoning of minerals reflects those of the Weardale miners themselves who were the true pioneering geologists. Operating in small partnerships the miners bargained with the mine’s agent to exploit a vein of ore for the year ahead, setting a unit price to be paid for their partnership’s ore extraction. It was therefore vital to the miners both in their negotiations and in their daily work for them to understand the underlying geology of the rocks in which they laboured.
Dunham spent WW2 scouring the Northern England for minerals to help in the war effort. Based on this work in 1949 he published his famous book “The geology of the Northern Pennine Orefield”. Volume 1 called “Tyne to Stainmore” covers Weardale and is sometimes available second hand on Amazon. A second edition was printed in 1967.