These two prominent structures are unexpected in such a rural setting, but they are explained by the famous Armagh Observatory nearby. A stone arch with two pinnacles was built in the early 1790s, and a cast-iron obelisk on a stone base was added in 1864. These Northern Markers, together with a Southern Marker in the form of a pillar in Ballyheridan townland (a scheduled monument), provided stable reference points on the horizon for checking instruments in the Observatory, and various small features – a copper disc with a hole in the pinnacles, and a pointer in the top of the obelisk – allowed for fine adjustments and measurements. The arch is probably the work of Francis Johnston and the obelisk was made by the Gardner foundry in Armagh. These, together with the Southern Marker, are the only known surviving examples of this type of structure in Ireland. Castledillon Obelisk, another monument in State Care, is clearly visible on Cannon Hill to the east-north-east.
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