Summary of site:
The main resurgence of the three rivers entering the Marble Arch cave system pours from beneath a cliff face into the chaotic limestone collapse fields immediately above the Marble Arch, a natural limestone bridge. This merged river, the Cladagh, cascades under the Arch and turns abruptly north into the head of Cladagh River gorge. The course of the river to this point is entirely bounded by the mud mound facies of the Dartry Limestone Formation, here called the Knockmore Member.
About 100m downstream from the Arch, the river cuts into lower beds and an immediate change can be seen at the base of the cliff forming the west bank. Below an overhang formed by the thickly bedded Knockmore Member a thickness of 8m of thinly bedded rocks (the top of the Glencar Limestone Formation) forms a striking contrast.
River action has cleared and cleaned these upper beds to reveal both sections through the rocks and extensive bedding plane surfaces. The top 4m are mostly lime-rich sandstones of a medium grey colour with thin partings of brown, shaley siltstones. There are also irregular beds of coarse grained fossil fragments and dark grey muddy limestones composed of crinoid debris. As well as the crinoids, bryozoa, brachiopods and small solitary corals are also found.
The next 4m below these beds are composed of pale lime muds and rocks composed of angular fragments (breccias) of hardened muds containing many brachiopods. Below them are rocks essentially similar to the top 4m. It is almost as if the unbedded lime muds are precursors of the mud mounds of the Knockmore Member only 10m above. The Glencar mounds are not continuous and other outcrops of the same junction 4.5km to the east show sections both with and without them.
There is no difference in the dip (angle of the bedding planes) of the Glencar Limestone below the junction and the Dartry Limestone above, suggesting no break in sedimentation, and both belong to the Asbian stage of around 337 million years ago.
It would appear that there was a substantial change in the sediment supply to the extensive sea bed of the north-west basin around the middle of the Asbian, with a declining influx of mud leading to a rise in biologically derived carbonate sediments. The change appears to have been patchy, creating thicker developments of limestone in some areas. The small, scattered mud mounds near the top of the Glencar look like products of a process not fully established until the start of the Dartry, when they dominated for a considerable time and formed the Knockmore Member.
This section, near the southern end of the Cladagh gorge, shows one of the best exposures of the Glencar Limestone/Dartry Limestone contact (dominant formations in the region) which exhibits many interesting and some unusual features in the top 8m of the Glencar Limestone. It is important regionally and is clearly exposed, while public ownership offers good footpath access and protection.