The bulk of northeast Ireland is underlain by extrusive rocks of the Antrim Lava Group (Palaeogene). The Group is cut by several minor dyke and larger plug intrusions. The plug on the southern edge of Carnmoney Hill has an elliptical outline 600m x 300m and is aligned northwest to southeast. The plug cuts the Lower Basalt Formation, the linear outcrop of the Ulster White Limestone Formation (Upper Cretaceous), Waterloo Mudstone Formation (Lower Jurassic), Penarth Group (Upper Triassic) and the underlying Mercia Mudstone Group (Middle to Upper Triassic). The massive dark grey coarse- to medium-grained, vesicular ophitic dolerite is best exposed in the now disused quarry behind Carnmoney Cemetery. The dolerite displays large-scale well-developed radial columnar jointing, a feature characteristic of Palaeogene dolerite plugs in northeast Ireland (Walker, 1959).
Rohleder (1926) strongly contested the view that a 'volcanic neck' was exposed at Carnmoney Hill; rather he suggested that what was seen was merely a massive flow of the Lower Basalt Formation. Charlesworth (1927) refuted this view and subsequent investigations have supported Charlesworth's hypothesis. Evidence that this plug communicated with the surface in Palaeogene times, and thus presumably acted as a local lava feeder, is found in the close association of ashes and agglomerates, representing the pyroclastic debris laid down around the vent. Equally however, the agglomerate may occur as wedges, relics of a tuff filled vent which was subsequently infilled with ponded magma, the vent not acting as a major lava feeder. Pyroclastic rocks are now recognised to occur in a small arc around the eastern end of the dolerite body. Ashes and agglomerates, which carry pebbles of basalt and flint up to 5cm across, and larger basalt blocks, set in a deeply weathered earthy matrix are best exposed in the farm tracks 300m east of Carnmoney Quarry.
