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Crilly House Quarry, CrillyTyrone
Crilly House Quarry, Crilly, Co. Tyrone, exposes Carrickaness Sandstone Formation (thin mudstone with abundant miospores on bedding surface in left side of photo inclined to the right).
Summary Full report
Site Type: Quarry (disused)
Site Status: PASSI
District: Dungannon District Council
Grid Reference: H694499
Rocks
Rock Age: Carboniferous (Visean)
Rock Name: Carrickaness Sandstone Formation, Tyrone Group
Rock Type: Conglomerate, Sand, Sandstone
Interest
Fossil Groups: Alga, Foraminifera
Other interest: No data, Marine sediments, Unconformity

Summary of site:

This small but enigmatic quarry exposes more than 5m of supposed Carrickaness Sandstone. The rock is Carboniferous in age, formed towards the close of the early part, the Visean, around 335 million years ago.

An angular unconformity (a break in time with an angular discrepancy of bedding across it) crosses the middle of the quarry face, separating 3.3m of orange to pale brown, very coarse sandstones below (dipping at 15 from the horizontal to the east) from a horizontal conglomerate 60-90cm thick above. The pebbles and cobbles in the conglomerate are less than fully rounded fragments of grey limestone and white vein quartz set in a quartz lime sand. They contain microscopic fossils of foraminifera and simple plants. The Carrickaness Sandstone belongs to a fossil stage called the Asbian and the fossils in the conglomerate blocks are no older than the stage immediately preceding it, the Holkerian, and could also be Asbian. These rocks were formed in a high energy environment associated with earth movement. The coarse sandstones below the unconformity were formed in shallow water, close to a recently emerged land surface. Originally horizontal, contemporary deforming stresses tilted this part of the sea bed and raised it above sea level. Erosion then stripped part of the succession, carving a horizontal surface across the dipping rocks. Following this phase of activity the sea again encroached and a beach deposit of limestone cobbles derived from the immediate area was created. The slight rounding of the cobbles indicates that they had not travelled far or been washed on the shore for long before burial. The matrix of limestone and quartz chips in which the cobbles are set again suggests high energy conditions and the winnowing out of fine sediment. Obviously conditions were highly unstable at this time with rapidly fluctuating sea level caused by earth movement, following the release of local stresses. This led to a tearing up of recently solidified sediments and the exposure and erosion of newly emerged land. The importance of this small quarry lies in the story it tells of instability in an area alternating between land and sea during Asbian times, demonstrated in a single outcrop. It deserves protection but at the time the full report was prepared it was rapidly filling with farm refuse and may now be lost.


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