H M Prison Dartmoor: Reservoir, with retaining wall, conduit head and nearby tank

H M Prison Dartmoor features a reservoir, constructed around 1806 to 1809 and restored in 2013. The reservoir is a long, slightly irregular elliptical shape, located to the west of the prison and aligned north-south. It is lined with granite ashlar, using blocks of various sizes.

To the east of the reservoir, there is a circular unroofed conduit head with a central doorway facing the prison. This conduit head is made of regularly-sized and coursed granite ashlar, featuring a tall plinth and a bull-nosed stringcourse beneath the plinth.

Supporting the raised ground where the reservoir and conduit head are situated is a retaining wall made of granite rubble. The wall has a capping of rock-faced granite with drafted margins and includes piers topped with pyramidal caps. The dressed granite of the capping and piers likely indicates a later 19th-century phase of work.

North-west of the reservoir, there is a circular tank lined with coursed granite and dressed granite around the rim. This tank, which is fed by a spring, appears on the 1884 Ordnance Survey map, indicating it predates the later covered reservoir that is now adjacent to it.