The coal seams under the Forest of Dean have been worked for centuries. One of the most successful collieries was the Cannop Colliery which was started in 1906 and commenced full operation in 1909, nationalised in 1947 and closed in 1961 when the boundaries or limits of its gale (the area it was permitted to mine) were reached. At its peak in the early 20th C it produced 1,500 tons of coal a day and employed 1,050 workers.
The Cannop Brook runs through the forest above the colliery workings. It roughly follows the line of geological fault. In most of the forest the brook runs in an impervious layer of clay overlying the limestone and coal measures. But in the upper reaches of the brook this clay has been stripped off by natural weathering. The course of the Cannop Brook within the Forest follows a section of highly faulted strata. This zone of fractures is five miles long and is made up of 25 small faults collectively known as the Cannop fault belt.
Because of the highly fractured and disturbed nature of the limestone over which the brook runs there was a considerable problem of leakge from the brook into the workings of Cannop Colliery below.
To try and prevent this leakage approximately 600 yards of the stream has been put into concrete channels or conduits such as the one shown in the photo which were built by the colliery . There are seven separate lengths made of cast concrete and two built of stone. This is the view downstream to where the brook disappears into the hillside. There is reputedly an inscription in the concrete saying "Sept. 1913" but I could not find it. See
SO6113 : Cannop Brook in Concrete Trough