Summary of site:
The Barnes Gap cuts through the east-west crest line of the southern Sperrin Mountains ridge, between the hills of Mullaghbane and Mullaghbolig. It is a deeply incised, north-south, glacial overflow channel between the Glenelly River to the north and the Owenkillew River to the south; its walls and the small quarry on the west side expose long sections of Dalradian metamorphic (altered) rocks of the Glenelly Formation.
The rocks have a motley and speculative history, having been first dated by J.E. Portlock in 1843 as ‘mica schist’ with an implied age equivalent to the Primitive Rocks (a group including everything before the Triassic period); then in 1884 Nolan and his colleagues in the Geological Survey of Ireland referred to them as “micaceous schist” and gave an age of Lower Silurian (what we would now call Cambrian). In 1938 Hartley published a detailed geological map of the area but without a definite age. Finally in 1976 Arthurs allocated the rocks to the Glenelly Formation within the Dalradian Supergroup of the Precambrian, an interpretation now firmly established and employed by the Geological Survey of Northern Ireland on their maps and in their memoirs.
The crags in the overflow channel are composed of greenish-grey, quartz-mica schists and shaley flags with prominent, well-formed crystals of plagioclase feldspar developed during the metamorphic alteration of the rocks. The various phases of metamorphism aligned minerals and structures within the rocks, giving them a fabric (a kind of ‘grain’), expressed as schistocity, foliation, fold orientation, etc. which can be fully appreciated in the Gap. Boudinage structures, caused by the stretching of relatively strong rocks such as quartz beds and veins during the course of metamorphism, can be seen here as sausage shaped masses. As at the Barnes Burn site just to the east (see site description) all the rocks are overturned in the inverted limb of the gigantic Sperrins overfold.
The rocks were originally deposited in sedimentary basins on the margin of the supercontinent of Laurentia during the Precambrian period (around 580 million years ago) but they were metamorphosed and folded during a time of continental collision and mountain building, called the Caledonian orogeny (around 400 million years ago).
This is one of the best long sections of the Glenelly Formation, which is commonly obscured by vegetation, and allows its diversity of rock types, mineral composition and complex structures to be appreciated in detail.
It is easily accessed from the roads through the channel and there are no threats to the area.