|Site Type: ||Stream section|
|Site Status: ||ASSI|
|District: ||Cookstown District Council|
|Grid Reference: ||H69407395|
|Rock Age: ||Quaternary, Silurian (Llandovery, Midlandian Undifferentiated, Pleistocene)|
|Rock Name: ||Limehill Beds, Little River Group|
|Rock Type: ||Mudstone, Shale, Siltstone|
|Fossil Groups: ||Bivalve, Brachiopod, Graptolite, Nautiloid|
|Other interest: ||No data, No Data, till|
Summary of site:
This is a nationally and internationally important fossil site. The rock exposure is small, forming the bed and banks of a small stream. The total thickness of the rocks is around 6 m, almost all of it shales and mudstones. The oldest form the base of a small waterfall extending into the steep bank above. They consist of less than 2 m of black to dark blue shales and mica-specked mudstones containing infrequent stipes (linear colonies) of graptolites, an extinct group of animals living in tiny linked tubes resembling pencil marks on the rock. Above them are about 1.5 m of dark grey, shaley mudstones with a richer fauna of graptolites and, unusually, an associated shelly fauna of straight-coned nautiloids (early relatives of the modern Nautilus), lamp shells (brachiopods) and the tiny, enigmatic cones of Nowakia, thought to be an extinct mollusc of some kind. The fossils are richest near the base. Above these beds are contorted mudstones terminating abruptly at a fault. Further upstream rocks reappear in the stream bed, firstly hard silty beds devoid of fossils followed by softer, black mudstones with glints of mica. It was in these beds that Captain Joseph Ellison Portlock of the Royal Engineers, a famous early 19th century geologist and cartographer, found rich fossil pickings. Some 12 species of graptolites were described from here including 3 new to science, Monograptus sedgwickii, Monograptus sedgwickii variety distans and Lagarograptus tenuis. The first specimens described of any new species have a special status and are called type specimens, placing international obligations to preserve them on the museums that house them. If they are lost, damaged or destroyed it is vital to know the type locality, the place they were collected, so that further specimens can be collected. This is the type locality for these three species as well as for Nowakia brevis, one of the perplexing tiny cones. This is also the earliest known occurrence of Nowakia on the planet. Three bivalve molluscs, a brachiopod and fragments of crustacean outer armour were also found here.
Monograptus sedgwickii is a graptolite species of international importance since it occurs widely and it is a key species for dating and it proves these are Silurian rocks of the Llandovery series and its subdivision the Fronian stage. This makes them around 432 million years old, a time when the area, then part of the supercontinent of Laurentia, was drifting northwards towards the equator.
Graptolite colonies were attached to floats that drifted in the surface ocean currents. When they died and the floats decayed they sank to the mud-cloaked deep ocean beds so the black shales are normally considered to be deep water oceanic or continental shelf deposits. The presence of a shelly fauna here complicates this interpretation. The shelly fossils have been explained in two ways; as a genuine deep water fauna capable of living in dark, oxygen-poor conditions or as species adapted to living in floating masses of seaweed, falling to the sea bed when they or the seaweed died. Either way, the Lime Hill section is the best example of this association.
The site is a designated Area of Special Scientific Interest (ASSI) and consequently recognised as of supreme importance. In Northern Ireland terms, its distinction rests on its extremely rare Silurian shelly fauna but internationally it is the status imparted as the type locality of Monograptus sedgwickii, one of the most widely distributed and useful of all Silurian graptolites, that establishes its prestige and interest.