The oldest rocks in the North Pennines are slates and volcanic rocks, which form the deep roots of the landscape.
Ancient ocean and colliding continents
The slates and volcanic rocks formed between 500 and 420 million years ago, in the Ordovician and Silurian periods of Earth history. They were once mud and volcanic ash at the edge of a wide ocean, known to geologists as the Iapetus Ocean. The ocean closed about 420 million years ago as the continents on either side collided. The mud and ash were squashed and altered to form hard slaty rocks. These rocks are mostly buried deep underground, but can be seen at the surface along the North Pennine escarpment and in part of Upper Teesdale.
The Weardale Granite
About 400 million years ago, a huge mass of molten rock rose up from deep within the Earth. It cooled and solidified to form the Weardale Granite. Although hidden deep underground, it is a fundamental geological feature of the North Pennines. Heat from the granite was important in the formation of the area’s mineral veins, and its presence is one of the reasons that this is an upland area today. The presence of the granite was proved in 1961 when a deep borehole was drilled at Rookhope.
The Alston Block
Granite is less dense than most other rocks in the Earth’s crust and is relatively buoyant. Because of this, the area above the Weardale Granite – much of the North Pennines – has remained higher than surrounding areas, and is known by geologists as the ‘Alston Block’. Faults – cracks in the Earth’s crust along which there has been movement – developed around the edges of the Alston Block.