In addition to active karst which is still functioning and developing today, there is evidence of much older karst also preserved in various parts of the country.
Relict karst is karst which formed under different conditions from those now active, but which is still exposed and being modified by present processes. Examples include remnant towers, which might once have looked like the classic towers of modern south-east Asian karst. |
Burren terraces (David Drew)
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They are difficult to recognise as they have been modified by glaciation which may have knocked off the tops and smoothed the sides. This may be the origin of some of the small hills around Lough Gill, Co. Sligo, the small hillocks in the Cork valleys and the Stradbally hills south-east of Portlaoise (e.g. the Rock of Dunamase).
Palaeokarst is buried, inert and fossilised karst which is often difficult to find. There are buried poljes (enclosed depressions, 1 km or more wide) across the country, often only detectable by geophysical surveying, such as that at Dunshaughlin, Co. Meath. These are often infilled by sediments associated with the breakdown of rocks in a tropical climate; a clue to the remarkable appearance of the landscape of Ireland some 60 million years ago, when hot and humid conditions prevailed.
On a smaller scale there are buried dolines, several of which are known to contain identifiable plant remains which were deposited during the Tertiary. Some are connected to buried cave systems but others are isolated. These infilled palaeokarst dolines, pipes, fragments of cave passages and channels have been found across the country from Headford, Co. Mayo; to Hollymount, Co. Carlow; to a group at Tynagh, Co. Galway. Infilled karst features have also been encountered in boreholes near Tipperary and Rathdowney, and at Ballymacadam, Co. Tipperary.
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A doline 10m deep in the Ulster Chalk at Garron Point, Co. Antrim. The doline is infilled with a mixture of flints from the chalk, clays and volcanic ashes and lavas from overlaying basalt rock. Thus the doline was formed before the lavas (basalts) of the Antrim plateau outflowed (John Kelly)
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Further south, the infilled deposits are older still, e.g. deposits of Cretaceous limestone at Ballydeenlea, Co. Kerry, and Jurassic clays at Cloyne, Co. Cork. Ancient palaeokarst is also known at Portrane, Co. Dublin, where a 15 m wide, 10 m deep, sediment-filled doline in very ancient (Ordovician; 440 million years) limestones has been recognised. In the Lower Carboniferous (the same age as the limestones), palaeokarst formed at Feltrim, Co. Dublin and Ballykane, Co. Kildare, when sea levels fell.
However, best known to Irish geologists are the 'clay wayboards' recorded all over the country, but classically from the Burren, Co. Clare and the Aran Islands, Co. Galway. These thin clay layers, usually rich in volcanic ash, represent periods when the seafloor was elevated above sea level for relatively short times, allowing karstification of the surface and the accumulation of a thin soil. They give rise to the distinctive terraced hillsides of the Burren: an excellent example occurs at Aillwee, where the show cave was initiated along and above a wayboard.
Location of presumed ancient karst features, caves, isolated limestone hills and enclosed depressions (usually sediment-filled), In Ireland (David Drew and Gareth Ll. Jones)
Du Noyer watercolour of Ross Quarry, south of Lough Sheelin, Co. Meath
A GEOLOGICAL TIME CHART
Summarising Ireland's geological history in relation to limestones and karstification in Ireland