About North Pennines - Geological Overview
The deep roots of the North Pennines formed between 500 and 420 million years ago as mud and volcanic ash at the edge of a wide ocean. When the ocean closed about 420 million years ago they were squashed and altered to form hard slaty rocks which form distinctive conical hills along the North Pennine escarpement.
The history of lead mining is brought to life at the Killhope Museum About 400 million years ago a huge mass of molten rock rose up beneath the slates and cooled to form the Weardale Granite. Being less dense than most other rocks in the Earth’s crust and relatively buoyant, the granite remained higher than the surrounding areas for millions of years, forming the raised ‘Alston block’.
360 to 300 million years ago in the Carboniferous Period the North Pennines lay near the Equator and was covered by shallow tropical seas. Sediments within the seas such as sand and mud formed layers of shale and sandstone while the organic remains of sea creatures accumulated on the seafloor to become limestone.
Sediment raft at High Force Stretching of the Earth’s crust 295 million years ago caused molten rock to rise up and spread out between the layers of Carboniferous rocks. It cooled and solidified to form a vast sheet of hard dark dolerite which has been exposed by erosion to form the Whin Sill.
The North Pennines is famous for its mineral deposits. Veins of lead ore and other minerals formed about 290 million years ago when warm mineral-rich waters flowed through fractures underground. As the fluids cooled their dissolved minerals crystallized within the fractures. These mineral deposits, such as lead, iron ores and fluorite were the foundation of the area’s economy for centuries.