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Scrabo Hill - TriassicDown
Sherwood Sandstone Group intruded by Tertiary dolerite sill complex at South Quarry, Scrabo, Newtownards, Co.Down.
Summary Full report
Site Type: Quarry (disused)
Site Status: ASSI
District: Ards Borough Council
Grid Reference: J479725
Rocks
Rock Age: Tertiary, Triassic (Middle Triassic, Palaeogene, Scythian)
Rock Name: Scrabo Sill, Sherwood Sandstone Group
Rock Type: Agglomerate, Dolerite, Sand, Sandstone
Interest
Minerals: Apatite, Hematite, Limonite, Magnetite, Rutile, Sphalerite, Sphene, Staurolite, Tourmaline, Zircon
Fossil Groups: Trace fossil
Other interest: breccia, cross-bedding, dessication cracks, dyke, ripple marks, sill, Aeolian sediment, Intrusion, Shallow water sediments

Summary of site:

The South Quarry at Scrabo Hill is internationally famous for its dolerite sills seen on the north west face, stepping upward through the sandstone sequence, and the massive dyke at the east end. An early photograph of these intrusions, taken by Robert Welch, was widely used in geological primers from the early twentieth century and particularly by Arthur Holmes in all editions of his Principles of Physical Geology. More recently, some fine watercolour drawings by George V. du Noyer of the same structures have been rediscovered.

The sandstones were recognised as Triassic in age from the earliest mapping in the nineteenth century but, because the outcrop is so isolated from the nearest British equivalents, there was some difficulty in identifying which part of the period. It is now known that they are part of the Sherwood Sandstone Group, formed at the very start of Triassic times. The succession consists, for the most part, of beds of sandstone around 1m thick, interbedded with laminations of siltstone and chocolate brown mudstone. There are many internal features, called sedimentary structures, in these beds that betray the conditions in which they were formed. Cross bedding shows two styles, one clearly formed by water-washed sands migrating into standing water, the other indicating blown sand, sometimes called dune bedding, formed by wind action. There are abundant ripple marks, both linear and linguoid, indicating shallow water and many mud horizons with mud cracks characteristic of dried-out pools in modern deserts. The sand grains are typically well rounded, also a desert feature, and up to 40% of them are feldspar, a mineral that quickly breaks down in wet conditions, suggesting rapid burial in a generally dry environment. All these clues, together with the red-brown staining of the sands, indicate a lowland desert setting with periodic flash floods carrying in sands and muds. Consistent with this desert interpretation, there are few fossils and those found are entirely trace fossils (which record the activities of animals) - such things as burrows of a type suggesting shrimps, several other invertebrate burrows and the footprints of a crustacean and of a large reptile. The reptile prints (called Chirotherium) are the only ones found in Ireland and they are preserved, with their original photographs and other trace fossils from Scrabo Hill, in the Ulster Museum. Scrabo Hill is capped by a large laccolith (a flat-based intrusion of molten rock with a convex roof), of the same age as the sills and dykes in the South Quarry. The quarry also exposes a wide fracture filled with a chaotic jumble of angular sandstone blocks in a matrix of volcanic ash. This is interpreted as an explosive vent, formed by a sudden and violent release of volcanic gases, ripping out blocks of sandstone and blasting them over a wide area. As explosive activity died down, blocks eventually choked the fracture, leaving the structure seen in the quarry today. This site is important because it is a rare and fine exposure of the sandstones of the Sherwood Sandstone Group, showing many environmental features and a suite of trace fossils that includes a fragment of a large reptilian track. It also shows classic intrusions which record its Tertiary volcanic history, including evidence of explosive volcanism. The site is internationally famous because it was the subject of early classic imagery, drawn and photographed, of transgressive intrusions.


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