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Devonian - Cross Slieve Group; Cushendun to Port ObeAntrim
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Summary Full report
Site Type: Coastal section
Site Status:
District: Moyle District Council
Grid Reference: D252326, D247289
Rock Age: Devonian (Lower Devonian)
Rock Name: Ballyagan Formation, Cross Slieve Group, Cushendall Formation, Cushendun Conglomerate
Rock Type: Conglomerate, Mudstone, Sand, Sandstone
Other interest: cross-bedding, dessication cracks, No Data, alluvial fan, braided stream

Summary of site:

The magnificent coastline between Cushendun and Port Obe is second only to the Causeway Coast cliffs in Northern Ireland for its splendour and geological spectacle. This coastal section, around 4km long, exhibits the entire thickness of the Cross Slieve Group and forms the eastern flank of Cross Slieve itself. Along this shore, north to south, the three major formations that make up the group can be seen in sequence. They are the Cushendun Formation, the Ballyagan Formation and the Cushendall Formation. Despite the absence of fossils and any other dating information, they are widely believed to be Devonian desert deposits. About their age, all that can be said with certainty is that they are younger than the Dalradians (Precambrian) and older than the middle Triassic.

The Cushendun Formation can be divided into three parts, although they are not easily defined and have not been individually named. The base of the formation (the first part) sits unconformably on the Dalradian rocks beneath, and the first 5-10m are composed of a basal breccia with large angular fragments (clasts) of Dalradian schists and milky white vein quartz in a matrix of sandstone. This is followed by the second part, marked by a sudden and dramatic change to a cobble conglomerate with a sandstone matrix including bedded sandstones and infrequent mudstones. This classic conglomerate is a striking rock with well rounded clasts, up to 25cm and more across, consisting entirely of dense, grey quartzite. Together, these two units, tilted to the south east, form the lower Cushendun Formation. It is beautifully exposed along the shore and behind the new apartments in Cushendun, south along the raised beach to the caves carrying the road through the headland to Cave House. It is estimated to be over 300m thick. After a short transition (see site Key Site 308 ‘Devonian – Cave House (south of)’) the upper division follows, mostly coarse, cross bedded sandstones with thin strings of cobble conglomerate and thin mudstones. The thickness of the upper division is not quite 300m. The Ballyagan Formation follows and is marked by a colour change and a reduction in grainsize. The sandstones here are finer and pinkish brown with cross bedding and the occasional thin mudstone. In total, the formation is around 160m thick. Further south still, continuing to Port Obe, there is a further change to pebbly sandstones and conglomerates, at first interfingering with the Ballyagan sandstones before eventually becoming dominant. This is the base of the Cushendall Formation, differing from the earlier Cushendun Formation in that all its clasts are volcanic debris of intermediate composition (dacite). The source rock for these clasts is something of a mystery and cannot be found in the area now. The formation is around 600m thick. All these deposits declare their desert origins. The Cushendun Formation was part of a series of desert fans spreading from mountain valleys into neighbouring lowlands. Paradoxically, although water is scarce in hot desert environments such as these, it is the major agent forming the sediments and shaping the landforms. Short but intense bursts of heavy rain in desert mountains run off in roaring torrents, washing the valleys clean of loose debris which is swept along to their mouths. Here, where the flood is no longer confined, it loses velocity and its ability to carry a coarse load, which immediately settles into an alluvial fan (a kind of cobble and sand scree) banked against the high ground. Fans from neighbouring valleys tend to merge along mountain fronts, with their finer sediments reaching far out into the lowland basins below. Landscapes of this kind can remain undisturbed for years and will only be substantially changed by the next rainstorm. The conglomerates are middle fan deposits and the sands are washed further out into braided channels as the waters drain away to playa lakes or simply evaporate. All the Cross Slieve Group can be explained by this related series of processes and the associated faulting in the desert basin. The source for the quartzite cobbles is as enigmatic as that for the dacite pebbles, although a number of suggestions have been made. These deposits by their nature have very little lateral or vertical consistency and so no stratotypes have been defined but each formation is described in more detail (see Key Sites 307, 308 and 309). The Bay Hotel mentioned in the full account has since been demolished to make way for the more recent holiday apartments. Modern development has degraded the aesthetic of the northern tip of this coastline but almost all the rest is still a pristine coastal section. The only serious threats would arise from further development and natural coastal erosion.

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