This Cache is part of a trilogy (eventually) of Caches in Co Kilkenny which celebrate various rocks that are and have been of cultural and socio-economic importance to Co Kilkenny and its surrounding area. The second type of rock up for examination is Slate.
The slate story starts with a long vanished ocean: The Iapetus Ocean, which opened about 600 million years ago between what are now North and South America. At its peak, about 510 million years ago the ocean may have been as wide as the modern Atlantic. Traces of the Iapetus Ocean are to be found today in rocks exposed along the seaboard of North America, through Eastern Greenland, into Scandinavia, Scotland, Northern England, Wales and Ireland.
Amazingly, the distribution of distinctive fossils and other evidence indicates the axis of the Iapetus ran Through central Ireland, roughly from the present day Shannon Estuary to Clogherhead in Co Louth and across the Irish Sea along the border between Scotland and England. It is difficult to now imagine that during this period North-western and South-eastern Ireland were situated on different sides of an ocean several thousand kilometres wide!
Plate movements during the Ordovician and Silurian periods 400 – 500 million years ago, eventually decreased the size of the ocean to perhaps no more wide than the current Irish Sea. In this remnant of the Iapetus huge amounts of sediment was deposited as the plates came closer together and the gap closed. These sediments accumulated on the sea bed floor perhaps up to thousands of meters thick. As the gap of the ocean decreased the sediment got squeezed and squashed and eventually through a process of ‘low grade metamorphism’ slate formed.
Slate may be black, blue, purple, red, green, or grey. Its colour depends on material that are present in the original sediment. For example, dark slates usually owe their colour to carbonaceous material or to finely divided iron sulphide. Reddish and purple varieties owe their colour to the presence of hematite (iron oxide), and green varieties owe theirs to the presence of much chlorite, a green micaceous clay mineral.
Slates are split from quarried blocks about 3 inches thick. A chisel, placed in position against the edge of the block, is lightly tapped with a mallet; a crack appears in the direction of cleavage, and slight leverage with the chisel serves to split the block into two pieces with smooth and even surfaces. This is repeated until the original block is converted into 16 or 18 pieces, which are afterward trimmed to size either by hand or by means of machine-driven rotating knives. It is then usually used as a roofing or building material.
Here in Ahenny a symposium was held in 1992 to use slate as an artistic medium. Many sculptures can still be seen around the site. It is said that the slate from the nearby quarry is reputed to have been used to roof the English Prime Minister's residence, No 10 Downing Street.
Please replace carefully – the cache is a de-con container please take time to replace the lid properly, it will make a satisfying snap when in place.