An extensive section through rocks of the Gilnahirk Group (Upper Ordovician) is visible in Cregagh Glen where almost 244m of strata are exposed. Entering the glen from the Knockbreda Road, the first exposure, 174m south of the road, is 5.5m of purple greywackes, slightly shaly in places and dipping NNW at 80°. South of a gap in the section of about 0.75m, 0.6m of faulted shales are visible, then 15m of greywackes, thick-bedded towards the bottom and flaggy and shaly in the top 4.6m. The dip of these rocks varies from NNW at 70° to vertical. Southwards for about 73m, greywackes and shales alternate in almost equal proportions. At 375m south of the Knockbreda Road, hardened shales are exposed. These rocks are siliceous and slightly gritty in places and at least 19m thick. Despite a prolonged search, no fossils have been found.
The next important group of beds consists of thick-bedded fine- to medium-grained greywackes with shaly partings, totalling 36m in thickness, which form the waterfall 410m south of the road. Another thick group of beds, 20m of hardened shales, is visible 520m upstream from the road. These shales, which are bluish grey and purple with some purple and green mottling, are twisted in places and are separated by a gap in exposure of 1.2m from 4m of closely similar shales. These in turn are separated by 1.5m of bluish grey and purple greywackes from another 1.2m of grey and purple shales. For another 183m southwards there are several exposures of greywackes and shales, mainly purple in colour and dipping 10° to 20° either side of vertical.
