The Great Flat Lode - a lode is a vein of ore, in this case of tin. And it was called 'flat' because the angle of the lode was not as steep as most Cornish lodes.
The Great Flat Lode is an enormous ore bearing body south of Carn Brea. It is tilted at an angle of 10 to 45 degrees, and is at a shallower angle than most lodes which are at angles of 60 degrees to 90 degrees. The Great Flat lode is therefore, in relative terms, a lot flatter in the ground.
Mines could follow the tin or copper ore without having to dig vertically down to great depths, and the associated problems with pumping that this entailed.
Between the late 16th century and about 1870, mining was for the copper deposits overlying the Great Flat Lode. These were the deposits that were nearer the surface.
When these copper deposits became exhausted, the miners went deeper and discovered the high-grade tin ore. Wheal Bassett from 1832 to 1880 produced over 128,000 tons of copper ore before striking the rich tin deposits of the Great Flat Lode.
The mines of the Great Flat Lode helped to provide employment to Men (miners), women and children (Ore dressers) at a time when the rest of the Cornish Mining industry was in decline. As the copper ores became exhausted in about 1870, the mines went deeper to the high quality tin concentrations underlying the copper. This gave the mines of the Great Flat Lode a new lease of life. After some of the companies amalgamated in the late 1890's the mines continued producing until about 1918.
Today you can follow The Great Flat Lode Trail.
Mines and Mining In Cornwall