|TYpe locality of Cloggan Hill Limestone Member (Dartry Limestone Formation), west side of Cloggan Hill, Marlbank Townland, Co. Fermanagh.|
|Site Type: ||Inland exposure|
|Site Status: ||PASSI|
|District: ||Fermanagh District Council|
|Grid Reference: ||H090360|
|Rock Age: ||Carboniferous (Asbian, Visean)|
|Rock Name: ||Cloghan Hill Limestone Member, Dartry Limestone Formation|
|Rock Type: ||Limestone, Shale, Siltstone|
|Fossil Groups: ||Brachiopod, Coral, Crinoid, Trilobite|
|Other interest: ||No data, Marine sediments|
Summary of site:
When a rock formation is recognisably different from its surrounding and regional geology, it is normally described (and its base defined where possible) and named, most often from a landscape feature in the vicinity. That locality then becomes the type area or stratotype of the rock concerned and is an essential reference point for rocks of similar age. This locality is the stratotype of the Cloghan Hill Limestone Member, part of the Dartry Limestone Formation of the Carboniferous period.
A long exposure on the west side of the hill shows the member to be 60m thick, typically with a variety of limestones, lime-rich shales, shaley limestones with nodules (looking like slightly flattened spheres a few centimetres across) and lime siltstones packed with plates of fossil sea lilies. These beds vary considerably in thickness when traced laterally, entirely due to the growth of small mud mounds on the sea floor of the time. Between the mounds the sediments are at their thickest but across their tops they thin considerably. These mounds are around 5m high, significantly smaller than similar structures in the Knockmore Member immediately beneath. The base of the Cloghan Hill Limestone Member may not have a single age but could vary, depending on when individual mud mounds commenced growth.
The shaley sediments on the summits of the mounds show signs of disruption in a higher energy environment, either at or near the wave base. Except for the cores of the mounds, which are the next best thing to barren, the rocks are highly fossiliferous although the stone lily plates, colonial and solitary corals, lamp shells and trilobites are fragmented and abraded. Freshly weathered shales in this sequence yield superb fossils.
The higher beds of the member, best seen on Cullentragh Hill immediately west of Cloghan Hill, are fine, pure carbonate muds containing occasional mounds of fossils up to 1m thick and 9m in diameter, mostly composed of the common, free branching colonial coral of the genus Siphonodendron.
In the shallow tropical seas of the late Asbian, around 335 million years ago, small mud mounds began to build up on the sea bed by a process that is still not explained. The sea floor around them teemed with life; groves of stone lilies (actually crinoids, animals related to starfish and sea urchins) swept the currents for food, lamp shells (brachiopods) nested with solitary corals in the niches below and between the colonies of branching corals, and trilobites scurried around in their constant search for food. Although shallow, the sea beds were below the reach of wave movement and only the tops of the mounds show evidence of fragmentation by wave action.
The member has a very limited distribution on the upper slopes of three hills crossing the western Marlbank Road adjacent to the border with the Republic of Ireland.
There are no obvious threats but the site should be designated to recognise the significance of a rare environment in its stratotype at the close of the Asbian stage.