Looking towards Porthmadog
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Archer's Dam is a relic of the earliest days of the Festiniog Railway, and a construction site on the Deviation. It is now a location on the official route, known as Archer Dam.
The original Act provided for the FR to have a continuous down gradient from Dinas to Portmadoc. For some reason, probably cost or maybe time, the Moelwyn tunnel was not built at first and some alternative arrangement had to be made.
The matter is not definite, but there seems to have been a difference of opinion between James Spooner the surveyor and Henry Archer who was acting as Chairman. Spooner held that the construction of the tunnel from the start would have been more economical, but Archer thought the inclines would be cheaper. Archer had his way, but Spooner, having had his resignation refused, would have nothing to do with the inclines which duly ran into trouble. There is reputed to be a receipted bill from Robt Stephenson for 20 guineas (=£21) in 1834 for redesigning the inclines [(this bill is not available to check at present; this section may need revision when it is)] and when the line was opened in 1836 the wagons would not go under the winding drum at the north incline. There was one of the famous Ffestiniog Railway rows and Archer seem to have left in a huff, after which Spooner set about building the tunnel in 1839-42. It was a horrid tunnel, 8ft 6ins high and barely 8ft wide. It was blocked by the Central Electricity Generating Board in 1958-59; we are well rid of it. Since that time, it has been dammed near the lower end to act as a water supply for Dduallt Mess in Deviation days and to Dduallt Station Water Tank [(not the one at Dduallt Tank Curve)] but the water is very acid and is only used for locos in emergency. A new tunnel, 10ft 6in high and 9ft wide, was built in 1975-77 by the Three Miners.
This was powered by a water wheel which was driven by damming some of the rainfall. This dam only remained in use until construction of the tunnel, but it was not finally breached until over a hundred years later when the Deviation was being constructed, and the new line cut through it. This ceremonial breaching took place on 16th October 1971, i.e. soon after the agreement for the West Side route had been announced, and nearly 4 years before tunnelling on the new Moelwyn Tunnel started. All the stone removed from the dam went into constructing the new culvert and the adjacent embankment.
FRM 55 (Winter 1971) further reported "The size and weight of the blocks of stone have quickly encouraged a healthy regard for early 19th century dam builders. Incidentally, it is clear from the smooth faces found inside the dam that it was enlarged at least twice in its history, but excavations to date have not thrown any light on the actual operation of the water wheel, if indeed there was one." However in FRM 57 Dr M. J. T. Lewis replied to the effect that the waterwheel defintely did exist. According to the Company accounts the drumhouse was erected May-June 1836, the waterwheel in Sept - Oct, and the brakes in November. The wheel was at the foot of the north incline, at right angles to the track, and the foundations of the wheelhouse can be seen when the water level in Llyn Ystradau is low. He was also unconvinced that the dam had ever been extended.