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Geology: Mineral Veins
The mineral veins that were so vital to the industrial exploitation of the Rookhope Valley were formed from hot
salt solution percolating through fissures in the carboniferous limestones, sandstones and shale, which form the
Alston Block.
The water was heated by a large mass of granite that lies under the rocks of the North Pennines. Granite conducts
heat much better than most other rocks so the heat of the molten magma under the earth's crust is carried up nearer
the surface by the granite The salty solutions partly dissolved the rocks they passed through. As this hot mineral
solution rose through the rocks above the granite it cooled and redeposited a wide variety of minerals in the cracks
and fissures of the carboniferous rocks.
The faults through which the mineralising solutions passed were wider in hard rocks, such as sandstone and especially
limestone. Limestone also dissolved in the hot salty water leaving cavities that were filled by other mineral
deposits. These horizontal mineral filled cavities next to the veins are called flatts. They often contain the
richest deposits of ore.
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