The Battle for Marchlyn River – a little-known Snowdonia war

Marchlyn Mwr Reservoir
Marchlyn Mawr Reservoir

Across the valley from my house in Snowdonia, about 2km away, is the Marchlyn Reservoir which feeds the hydro-electric power station inside Elidir, known locally as the Electric Mountain. What used to be a just a natural lake has been transformed into a reservoir which provides power rapidly on demand to the National Grid in an amazing 15 seconds. The used water ends up in Llyn Peris in Llanberis and is pumped back overnight using off-peak electricity. The difference in electricity prices makes it commercially viable.

Long before hydo-electric power had even been conceived as a notion, the area was the subject of a bitter dispute. The development of the two slate quarries at Dinorwig (in Llanddeiniolen parish, owned by the Faenol Estate) and Bethesda (Llandegai Parish, owned by the Penrhyn Estate) in the late 18th century began two hundred years of what at times became fierce rivalry. The infamous ‘battle for Marchlyn River’ is but one example. The dispute arose because although the river’s source was Marchlyn Lake located in Llanddeiniolen), it at one point flowed briefly in and out of Llandegai Parish.

In the 1830s, the Penrhyn Estate decided that it wanted the water to run a mill a mile or so down the valley in the direction of Bethesda. The Faenol Estate was incensed as the river water already ran a mill near Deiniolen (or Ebenezer as it was called then). The two estates had a difficult relationship at the best of times but as the Penrhyn Estate began to build a substantial leat (a trench or ditch that conveys water to a mill wheel) to take the water down towards Bethesda, things got extremely heated, indeed violent at times.

The dispute became so fierce that the Bishop of Bangor had to intervene. It was eventually agreed (after much litigation and counter litigation) to place a stone known as the ‘Heater Stone’ at the exact spot where the river was split. The name derives from the Welsh ‘Carreg Hetar’. The ‘Heater’ refers to the shape of the stone which resembles an old iron for pressing clothes.
Hearter stoneThe Bishop’s Coat of Arms was carved onto the top of the stone (which you cannot now see as the wall has since been built above it). Now the water had to be shared – a period for one estate then for the other.  The Heater Stone swivelled on its base restricting the supply to Bethesda to power the mill in Deiniolen or visa-versa. The stone even became an early tourist attraction.

Heater stone todayIf you look closely at the stone today you will be able to see rope marks on it where horses were used to swivel it into the appropriate positions. Today a wall has been built over the stone which marks the boundary between the parish of Llanddeiniolen (which Trosgol is located) and the neighbouring parish of Llandegai. An old boundary stone can be seen just above the Heater Stone.

It’s worth a visit as you may also find the cave in which King Arthur’s treasure was left. It is said to be “a source of bedazzlement to the wanderer who sees it, and of disaster to the pilferer who touches it.” You are warned!

(With grateful thanks to Gareth Roberts who told me the story and provided the photographs). Do take a moment to click on his amazing night photography