Offaly

Heritage Sites of Co. Offaly

Clonmacnoise

The early buildings in Ireland generally utilized local stone for their constructions.  Stone is heavy and as a consequence it is more economical in terms of effort and cost to use locally quarried materials or collected field stones rather than imported stone.  This imparts a distinctively regional feel to early Christian buildings.  As building techniques developed from the 1100s onwards there were good reasons to import some rock types where they were required for specific purposes; in this regard one thinks of Dundry Limetone from Somerset, which was favoured by Norman stone carvers, and it is seen in a number of religious and other buildings in the southeast of the country where it was employed for carved elements of the fabric.

The Midlands of Ireland are underlain with a Mississippian (Lower Carboniferous) succession dominated by different limestones with lesser horizons of yellow to white sandstone. Some of the limestones are dark grey and muddy, such as the well-known Calp, while others are pale, comprising almost pure calcium carbonate, and may contain plenty of fossils, or not. 

At Clonmacnoise a variety of local limestones and sandstones were available to the builders.  The ecclesiastical complex contains six small church buildings, a larger cathedral and two round towers, all situated within a walled enclosure overlooking the River Shannon just to the west. Several High Crosses, carved in sandstone, were in situ until recently but are now to be seen in the Visitors Centre where they were removed on conservation concerns.  Five hundred metres to the northeast stands the Nun’s Church.

In Temple Melaghlin (or Temple Rí) the walls are constructed of alternating courses of a grey limestone and a browner sandstone showing well developed cross-bedding in some blocks; this duality of stone now provides an attractive polychromic banding in this building.  Whether this was the intention of the builders is unknown but it may be due to consignments of the two stone types arriving on site at staggered times and being used immediately before the next load arrived regardless of type.  The walls may have been covered by whitewash or perhaps even rendered and the alternation now seen is purely coincidental.  In other buildings limestone and sandstone is used for walling but alternation of courses is not as obvious.

Clean grey limestone was dressed for use in doorways, windows openings, and arches in most structures and stylistically these remained rather plain, the exception being the fifteenth century north door of the cathedral with its elaborate figurative carvings in plaques above, lateral finials, and ornate arches.  Close examination of some of the carved stones in the arched doorway reveals the use of Clorhane crinoidal limestone—this may have been originally polished but it has since weathered and this finish has dulled.

For other openings, especially those in the older Romanesque style, a yellow to beige coloured sandstone has been used and skillfully carved by masons into a variety of designs, including a chevron motif.  Examples here include the doorway and a window in Temple Finghin and the west door of the cathedral, which was extensively restored in replacement sandstone by the Office of Public Works.  The use of carved sandstone by the early builders at Clonmacnoise suggests that it was either easier to carve than limestone at the time or it was not as plentiful and so was reserved for specific purposes. 

The Clorhane limestone is by far the most distinctive limestone quarried in Co. Offaly on account of it containing some beds packed full of the fossilized remains of crinoids. This group of filter feeders lived attached to the sea bed; they consisted of a stem that held up a small cup in which the soft body tissue was located and over which a five-fold array of arms could open out to trap food.  They were conspicuous members of the Echinoderms, which included Sea Urchins and Starfish.  Over a period of nearly one hundred years the Clorhane limestone was actively extracted from two quarries in an area just south of the monastic site. Stone was transported on the Shannon to marble mills, especially that at Killaloe, which operated in the mid- to late-1800s and then transported onwards to various markets.  The quarries closed in the 1950s but latterly a slab of stone was obtained, cut and polished, and is used to super effect for the reception desk in the Visitors Centre which opened in 1993.  

Further reading:

  • Declan Ryan (2013) The Marble of Clonmacnoise: limestone quarrying at Clerhane (Clorhane) Shannonbridge, Co. Offaly. Offaly County Council, Tullamore. 
  • Andrew Tierney (2019) Central Leinster. The Buildings of Ireland. Yale University Press, New Haven and London, pp. 271–81.

Geology of Co. Offaly

The oldest rocks in Offaly are those of Slieve Bloom, an inlier of older rocks surrounded entirely by younger rocks. The core of the inlier are the hills of Silurian sedimentary slaty rocks, but these are poorly exposed. They are protected by thick sandstones of Devonian age, originally deposited by rivers around 380 million years ago, these can be seen in the eroded trenches of river valleys such as the Silver River. In the Carboniferous Slieve Bloom was probably an island in the shallow tropical seas.

Today, few of Offaly’s rocks are exposed. They have been eroded down over millions of years to a low lying plain and are now blanketed by bogs and glacial deposits left by the last Ice Age. Offaly has some of the largest and finest bogs in Ireland’s midlands.

The most recent development occurred during the last 1.6 million years when ice ages came and went, the last ending about 10,000 years ago and leaving Offaly with some of the finest glacial deposits of sand and gravel across the lowlands. Eskers (the international name for these features comes from the Irish name: eiscir) formed, around Tullamore in particular, from rivers flowing beneath the ice, leaving a long narrow ridge of sand and gravel such as that at Kilcormac (pictured above). The Eiscir Riada is famous in history, providing a dry route across the boggy plains of the Midlands and to the monastic centre at Clonmacnoise.

Further reading: L. Dunne & J. Feehan 2003. Ireland’s Mushroom Stones. Relics of a Vanished
Lakeland. Environmental Resource Management, University College Dublin

Map legend: Geological Map of Limerick (based on GSI 1:1,000,000 map 2003). Pale green: Silurian; Beige: Devonian sandstones and conglomerates; Pale blue: Lower Carboniferous limestone