Summary of site:
The earliest Carboniferous strata at Ballycastle lie beneath an extensive development of Carboniferous lavas and are rarely seen. The rocks immediately above the lavas contain a fossil fauna (largely brachiopods) that establishes a Brigantian age, certainly for the Main Limestone and possibly up to the Main Coal. The succeeding beds of the coalfield are of Namurian age and equivalent to the ‘Millstone Grit’ of the north of England (see #19).
The age of the beds below the lavas (mainly coarse to fine sandstones with silty mudstones) has never been established. They could be Brigantian or even earlier. A sample of silty mudstone and fine sandstone from the locality was processed for plant spores and found to contain 5 species useful for dating purposes. One of them, Grandispora spinosa, identifies these rocks as definitely Brigantian. Since the rocks samples are very close to the base of the Carboniferous at Ballycastle it seems likely that all the rocks below the lavas are of the same age and that the sedimentary basin in which they were deposited dates from then.
The locality is important since, uniquely, it establishes the age of the earliest rocks in this isolated enclave of the Carboniferous in north-east Ireland.