Summary of site:
The metamorphic rocks of the Dalradian are amongst the oldest seen in Northern Ireland and predate fossils that are visible to the naked eye. They are divided into two groups, the Argyll Group at the base and the Southern Highland Group above. This second group is further subdivided into four formations, from the base upwards: the Dart, Glenelly, Glengawna and Mullaghcarn. The Dart Formation consists of two members, the Glenga Amphibolite Member below and the Henry’s Bridge Member above. This site is important because it is the stratotype of the Henry’s Bridge Member, the place where it is best developed and from which it is characterised.
The outcrop is in the Glensass Burn, immediately north of Henry’s Bridge where there is good exposure upstream for about 150m. All the rocks are inclined to the north west which, in normal circumstances, would put the oldest rocks nearest the road; but here they are part of a massive, overturned fold, more than 40km wide, and forming the southern, overturned limb - so they are upside down. The rocks are described from the oldest, about 200m upstream, to the youngest, near the road.
The member is about 50m thick and rests on the dark green, chloritic schists of the Glenga Amphibolite Member. It consists of massive, dark, greenish-brown schists with scattered large crystals of albite called porphyroblasts, formed by recrystallisation of the rock during metamorphism. Between the wavy foliation planes in the schists there are thin films rich in quartz and feldspar with evident white mica and deep red-brown garnets enclosed by the albite porphyroblasts. There are also a few thin, gritty beds.
Originally the green schists of the Henry’s Bridge Member were sediments of volcanic origin, either volcanic dusts or weathered volcanic rocks. Small amounts of quartz sand also became incorporated. They were formed around 600 million years ago in the vicinity of the South Pole when the continent of Laurentia was separating from the giant continent of Gondwana under the influence of diverging convection currents deep below the crust. These eventually so stretched and thinned the crust that molten igneous rocks broke through on a massive scale. The Henry’s Bridge Member was created in the final phase of this activity.
All these rocks were regionally metamorphosed (altered by heat and pressure) during the Grampian continental collision about 465 million years ago. On the scale of metamorphic change this was a relatively mild event, creating a suite of rocks described as the greenschist facies. This collision also created the massive Sperrin overfold that inverted the rocks in this entire area.
Stratotypes are the fundamental divisions of geological history and all are important and should be preserved. The Henry’s Bridge Member has the additional advantage of being conspicuously different from its enclosing rocks, making it easy to recognise and trace across country - so assisting in unravelling the structure of this complex area.
The site has no obvious immediate threats although overgrowth is possible unless it is well managed. Exposure is sufficient to allow judicious hammering for scientific purposes.