Some quick notes on the drainage tunnel:
Main drainage tunnel length given as 1,837 yds (Quarry's own figure) -
Driven by George Twigge & Co of Bangor in 1848. Begun in 1843, Took 5 years to dig, including one bad stretch in wet mud which took 18 months. Adit bears keystone dated 1849. Water balance adits were hand dug and took over a year to dig 200 yds.
There was a 100ft deep shaft near CoedyParc from the foot of which two lengths were driven, one towards the River Ogwen below TanyGastell and the other towards the quarry. The adit was at the level of what became the Sebastopol gallery.
From the quarry plan of the 1950’s, it would appear that the adit was not driven all the way into the Twll, but eventually terminated slightly closer to the twll than the later position of the final Sebastopol balance. It may be that what became the final Sebastopol balance tunnel was originally driven from the twll to connect with the drainage adit. (I say final Sebastopol balance, because there is some debate that the George and Douglas balances were originally named Sebastopol and Fitzroy, the names subsequently being transferred to the current pair.)
Regarding the dust suspension in the drainage, as there is only one drainage adit, I would imagine that at times water can drain into the sinc faster than it can flow out through the adit, so the water level in the sinc rises. This would significantly increase the flow through the adit for a time, and disturb the silt in the adit - with the effects observed. The report I had a few years ago was that there was an awful lot of silt in the tunnel - for the flow only increases when the water level rises rapidly!
Graham
The map is the territory - especially in chain scale.