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Mournes - Spelga DamDown
PHOTO TO BE ADDED Stream section below Spelga Dam, Mournes, Co. Down.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Newry & Mourne District Council
Grid Reference: J265273
Rocks
Rock Age: Tertiary, Silurian (Eocene, Silurian Undifferentiated)
Rock Name: Granite
Rock Type: Granite, Hornfels
Interest
Other interest: stoping, Contact, Intrusion, Private ground secured by the Water Authority

Summary of site:

The five granites of the Mourne Mountains were intruded into their enclosing Silurian sediments (called country rock around major intrusions) between 56 and 51 million years ago by a process called cauldron subsidence. There were two centres of activity, the earlier, in the east, emplaced granites 1 to 3; the later, in the west, granites 4 and 5. Cauldron subsidence is a process in which massive cylindrical blocks, kilometres across, subside relatively quietly into the molten mass below. This locality suggests that the picture may be somewhat more complicated than this explanation suggests and that emplacement in some places could have been more violent.

Downstream of the Spelga Dam, a substantial dyke of the fourth granite can be seen to penetrate the country rock. The granite in the dyke exploits the bedding planes and joints, detaching masses up to several metres long that are now suspended in the granite (xenoliths). The contacts between the xenoliths and the granite are everywhere sharp. Such destruction of wall and roof rocks during injection of large bodies of molten rock, to create space for them to occupy, is called stoping, considered to be a relatively violent process. It would be helpful if more sections of granite/country rock roof contacts could be seen but in the present state of erosion there is little of these areas left to explore. It is, however, possible from the evidence at this locality, that stoping played a more important role in emplacing the granites than is generally believed. The structure of the dam covers part of this exposure and the volume of overflow controls access to the rest.


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