The Great County Adit was an enormous industrial undertaking in its time and was essential for the continuing operation of many of the tin and copper mines in Cornwall.
What is an Adit - its a level tunnel (usually driven into a hillside) in order to give access to a mine, and used for drainage or the hauling of broken ore. Deeper adits did not necessarily connect to surface, and were used to carry water back from distant workings to a pumping shaft.
The County Adit was started in 1748 by John Williams of Scorrier, then manager of Poldice, with the aim of draining the mine and removing the water to a river, and hence to the sea. From its exit point near Point Mills in the Carnon Valley, the Great County Adit reached Poldice in the late 1760s, by which time the mine was over 100 fathoms deep in places.
The Great County Adit is a major engineering feat, perhaps not recognised as it is hidden underground. The main tributary of the Carnon River which empties into the Fal at Devoran was used to contains the flow of water from the County Adit which is the longest man-made water tunnel in the world.
This remarkable drainage system was extended to the other mines, by 1778 having been driven through Wheal Busy to North Downs and on into Wheal Peevor, another branch from Poldice had been cut by 1792 into Wheal Unity and Gorland, whilst Consolidated and United Mines also discharged their water into it. It eventually reached the outskirts of Redruth and was some 38 miles long. Deep adit construction played a vital role in getting rid of the water from Cornish mines and at its peak, the County Adit drained an 13 million gallons/day from over 40 mines around Redruth and St Day through 38 miles of adits.
The disastrous winter of 1876 led to heavy flooding and the County Adit, which had been neglected by the mines, broke through to release thousands of tons of water, with attendant rubbish and silt. This was carried down the river and most was deposited at Devoran. Navigation was never restored and the uppermost quays fell into disuse.
'The Great County Adit', a slim volume by J. A. Buckley is a well researched book on the subject. 144 pages, well illustrated with B & W photos, plus maps, documents etc. Paperback. Penhellick Publications
Mine and Mining in Cornwall