The circular walls of Hagdale Horse Mill lie next to the abandoned Hagdale Quarry, to the east of a small industrial estate and 1¼ miles (2 km) northeast of Baltasound on the island of Unst in Shetland. This horse mill was built c.1850 and was used to crush chromite extracted from the quarry between 1839 and 1873, then the principal source of the mineral in the UK. Chromite was used in the manufacture of steel and aluminium, as a source of chromium, as a dye and in the chemical industry. Its occurrence in this area was due to the unusual geology, where sections of ocean floor and the earth's mantle - known as ophiolites - are pinched up to the surface through the process of collision between tectonic plates.
A vertical millstone, which remains in place, was used to crush the ore and then water was used to separate the heavier chromite from the host rock.
The quarry re-opened briefly in the years before the First World War and thereafter modern machinery was installed to recover chromite from the waste from previous operations. This became uneconomic and the plant closed in 1927. Chromite quarrying resumed once again between 1938 and 1944, combined with the extraction of serpentine which was used in the manufacture of refractory bricks. Serpentine extraction continued until 1969.
Representing the only example of a mineral-crushing horse mill remaining in Britain, Shetland Amenity Trust purchased the structure in 2001 and it was restored c.2003, including the installation of interpretation panels.
The Keen of Hamar National Nature Reserve lies immediately to the south.
