Glencoyne is the most wonderful hanging valley above Ullswater. It clearly had a mining past. This dam is in the valley floor. It must have had an earth filling. I've never seen a drystone wall dam before. The height of the dam must have been about 2 metres in the middle. Pressure in a liquid is directly proportional to depth (or height h of water above me). Pressure = h.rho.g where rho is the density of water - 1000 kg per cubic metre. g is the gravitational field strength, 10 newtons per kig on Earth. So water pressure = 2 x 1000 x 10 = 20000 newtons per square metre. That's like 2 tonnes on 1 square metre or a car balanced on a square metre. A car wouldn't go through a double wall filled with earth.
