Extensive remains of the slate quarrying industry on the island of Belnahua: its excavations, waste material, associated industrial and domestic buildings, machinery, infrastructure and other archaeological evidence, dating mainly from the middle of the 18th to the early 20th century.
The remains are visible as two substantial quarries (now flooded), widespread loose material (the by-product of slate extraction and processing), and the buildings, machinery and infrastructure used to excavate, process and transport the slate, and accommodate and service the quarry workers and their families. The main quarry measures up to 180m across and occupies a significant part of the island.