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Nature
and Origin of Irish Blue Limestone:
The story of Irish Blue Limestone
began about 370 million years ago. At that time Ireland was not the green and rolling
countryside with which everyone is now familiar but rather part of the floor of a shallow
inlet of the sea. This inlet lay on the southern edge of a large landmass which
included the present day North America, Europe and Aisa and was about 5 degress north of
the equator. The water was relatively shallow and warm and provided the ideal
conditions for the precipitation of lime mud that in some cases incorporated the fine
debris such as clays and sands washed in from adjacent landmasses. Bivalves appear to
have been the most common macroscope fauna in the water.
As with all natural environments, there were periodic variations and numerous
temporary breaks in the deposition occurred. However, for much of the following 45
million years lime muds continued to form.
Although the relatively barren calcareous mud forms the
bulk of the Irish limestone deposits, there are exceptions and at a number of places
extensive colonies of crinoids developed. These thrived in the shallower water
adjacent to the surrounding landmasses. The masses of crinoids trapped particles suspended
in the water and built up reef-like structures, which protected the shallower inshore
areas. The crinoid skeleton had a number of tough calcareous structures that were held
together by the musculature. On the death of the animal, the skeleton broke down and
wave action carried the fragments into the protected inshore area where they became the
main component of the sediments.
About 325 million years ago the conditions in the shallow
inlet changed. The drainage pattern in the adjacent landmass altered and rivers began to
carry more clays and sands into the sea. The accumulation of calcareous sediments
ceased and, as the water became shallower, tropical forests began to encroach on the area.
The Dinantian period of the Carboniferous era, when the most important Irish limestones
were deposited had come to an end.
During the millennia
since the Dinantian period, the world has undergone radical changes. Continents have
moved; the dinosaurs have dominated the earth and disappeared and finally the mammals
began their inexorable rise culminating in modern man with his ability to reshape the
earth. During their long geological history the Dinantian sediments have changed from soft
unconsolidated muds into the limestone beds that underlie much of the central part of
Ireland.
The deeper water deposits have been transformed into a
tough, finegrained, grey limestone. The breaks in sedimentation are preserved as the
bedding within the sequence. Where appreciable amounts of clay or fine sand have been
present, the resultant limestone can be extremely siliceous. The uniformity of this stone,
and the potential to split out thin slabs of consistent thickness, made this material
attractive to the early stone masons. By the middle of the 19th century almost every
village on the outcrop of this limestone had a small quarry that was worked to provide
building stone for the local population.
Where the crinoid debris formed a major component of the
sediment, the nature of the resultant limestone was very different. The reefs tended
to have steeper dips with frequent discontinuities, intact crinoid stems, occasional
corals and numerous crystalline calcite inclusions filling what were originally voids in
the structure.
Many of the reefs,
particularly in the Irish Midlands, are associated with lead-zinc mineralisation. The
presence and diversity of structures makes recovery of commercial sized blocks difficult,
but when not mineralised they are frequently exploited for aggregate.
The crinoid-rich sediments from the sheltered waters have
transformed into massive crystalline limestone. Preservation of the fossils produces
an attractive surface when polished and their crystalline structure enhances the
appearnace of broken or sawn faces. The conditions under which the material accumulated
lead to an initially high water content and, during the early stages of consolidation,
solution and stylolite formation occured. This has tended to suppress any
discontinuities, which were originally present, and produce massive beds. Often the
presence of 'shell' beds, which formed during periods when the water became too deep for
the crinoids to thrive, provide the only indications of the original structure of the
deposits. The massive form of these limestones made extraction difficult for the
early masons although their consistency was desirable for carving.
The best of the crinoidal stones are extracted today to
produce Irish Blue Limestone.
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