The metasedimentary strata on the eastern side of Torr Head represent the stratigraphic transition between the Argyll Group of the Middle Dalradian and the Southern Highland Group of the Upper Dalradian. Two lithologically contrasting formations are present. The Torr Head Formation (Argyll Group) consists of massive very coarse-grained often graphitic limestone, whereas the Altmore Formation (Southern Highland Group) consists of coarse-grained and gritty psammite. The transition between these two formations is marked by a 2m interval of thinly interbedded finer grained metasediments.
This site is structurally significant because it contains abundant sedimentological evidence indicating that the succession is inverted. This is a key factor in understanding the wider structural picture in the Antrim Inlier.
Across the eastern side of Torr Head, thick masses of coarse-grained metabasite (epidiorite) intrude the limestones forming a number of concordant sills and lenticular masses. Similar upsurges of basic magmatism have been observed at the Middle-Upper Dalradian boundary elsewhere in Ireland and across Scotland (Tayvallich). The intrusion of these basic igneous rocks is the result of regional scale crustal thinning which marked the onset of the predominantly extensional tectonic regime, which prevailed throughout the Upper Dalradian.
The rocks at Torr Head have experienced low-grade regional metamorphism. The mineral assemblages present in the limestone include black graphitic calcite, white calcite, quartz and phlogopite and are consistent with metamorphism in the quartz-albite-epidote-biotite subfacies of the Greenschist Facies.
