Previous SiteNext Site

MilltownArmagh, Tyrone
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Armagh District Council, Dungannon District Council
Grid Reference: H804516
Rock Age: Triassic, Carboniferous (Dinantian, Middle Triassic, Scythian)
Rock Name: Derrycreevy Sandstone Formation, Milltown Conglomerate Formation, Sherwood Sandstone Group, Tyrone Group
Rock Type: Conglomerate, Sand, Sandstone
Fossil Groups: Trace fossil, Wood
Other interest: cross-bedding, ripple marks, Aeolian sediment, Fluvial sediments, Lacustrine sediments, desert

Summary of site:

When a rock formation is given a name it has to be described and its important characteristics noted and this should always be done with reference to a selected outcrop where all the typical features can be seen. Such sites are called type localities and this account describes the type localities of the Milltown Conglomerate Formation and the Derrycreevy Formation. Both are considered to be part of the Sherwood Sandstone Group of the Triassic period.

The type locality of the Milltown Conglomerate forms the north bank and river bed of the Blackwater opposite the mill factory at Milltown. Here 12 m of conglomerate containing small pebbles can be seen in beds up to 50 cm thick separated by partings of a coarse-grained red and purple sandstone. The dominant rock in the pebbles is white vein quartz but fragments of Carboniferous, Ordovician and Silurian rocks have also been recognised. Between the mill factory and the weir on the north bank of the Blackwater, 800 m to the north west, is the type locality of the Derrycreevy Sandstone Formation. It sits on a foundation of Milltown Conglomerate that rapidly gives way to 150 m of brick-red, fine-grained sandstone which forms the base of the Derrycreevy Sandstone. The top of the formation is not seen. These rocks were formed in a hot desert environment at a time when the British Isles area was within the northern tropics. They were part of a huge supercontinent creeping imperceptibly northwards. Periodic rains flooded the desert floor and winds passing over these temporary lakes generated ripple marks in the sands, in turn worked over by some sediment-feeding animals that left trails. In the long dry periods sand dunes also formed. The redness is due to oxidation, probably of iron but rare green and white laminations betray short intervals reducing conditions in the lakes. The age of these rocks presents a problem because no fossils have so far been found. In the 19th century the conglomerates were considered to be of Permian age but the most recent appraisal places them in the Triassic period as part of the Sherwood Sandstone Group which would make them around 245 million years old. Type localities are the essential units from which the geological history of an area is compiled and are therefore of national importance. For this reason alone the site should be conserved but there is additional interest in the form of hydrogeological (groundwater reservoir) and hydrocarbon (oil and gas) potential which further fortify this view.

Previous SiteNext Site