"Boardwalk" is so named because it goes from a fairly standard 'flagstone and ankle breaking boulder' floor to a lovely wooden .... boardwalk? construction about half way through, using big railway-sleeper type slabs of wood to line the floor of the culvert.
The culvert was built at the same time as the Causey Arch and the Tanfield Railway (1723 according to this website) and is of fine sandstone construction, with only a small amount of later reinforcement in the form of a concrete portal at one end, presumably due to bank erosion. The culvert was built to carry the new-fangled railway, which shipped coals from the Durham pits to the staithes on the Tyne, where they would be loaded onto barges and sailed up and down the country (as far as London and Kent) to feed the furnaces of the burgeoning Industrial Revolution.
Visited on a warm spring day with Kona and Sandman, this was a really idyllic spot, with dippers catching insects, flying in and out of the culvert. Sandman made us all ninja staffs and we attracted some very odd looks from the local horse riders. Of course we were only interested in the historic mediation of bank slippage and slope stability of man-made earthworks...
Pics:
Both ends of the culvert - note the modern concrete structure contrasted with the original stone structure:
A section of the boardwalk - the riffles are caused by the water passing over each wooden section. Also note the wooden ceiling, holding up the several hundred thousand tonnes of rock and earth they used to build the bank the railway runs over
:
Arched stone section
Arty shot:
Another nice little culvert I'd like to go back to at some point