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Greywacke is referred to as an immature sandstone, consisting of poorly sorted, angular, sand to gravel sized particles. Greywacke formed during the Silurian (440 million years ago) and has long been used as a building stone, for rubble walling, and occasionally split for roofing. It is also widely used as road stone and aggregate.

It is characterised by its grey colour and hardness. It can have quartz veins and can be high in iron (limonitised) to give an orange-red colour. Greywacke is very durable and weathers little over time. It can be prone to fault-line fractures.

It is still quarried extensively throughout County Down and the larger material used for rubble walling.