Mushroom stones

These rocks take the form of solitary sentinels of limestone which still bear the unmistakable marks of long-continued erosion by ice, soil or lapping waves at the edge of lakes which have since vanished or retreated.

These stones are sometimes shaped like mushrooms, others have an overhang facing in just one direction, but all are notched and undercut in such a fashion as to suggest prolonged exposure to standing water at some time in the past.

He made a drawing of these water-worn limestone blocks ‘eroded up to a certain height by the solvent action of former lake water, showing the level at which Lough Ree once stood – 10 to 15 feet higher than the present summer height: the lake shore, moreover, is now some 300 yards distant.’ To date, wavestones have been recorded in Counties Offaly, Tipperary, Galway, Clare, Cork, Limerick, Roscommon and Westmeath (see map below in External Links).

A remarkable example is located near Crancreagh Bridge in County Offaly, which marks the position of what must have been the north-western edge of ancient Lough Boora, on whose shores the first people of the Irish midlands camped in Mesolithic times 8,500 or so years ago.

It has been noted that the Irish landscape has been profoundly altered by drainage operations,[3] that signs of peat cutting are common in the Burren fens[4] and that extensive deforestation has occurred in Ireland,[5] all of which can lead to a lowering of the land surface.

Three examples of ‘wavestones’ that illustrate mushroom rock formation in a lost-soil environment and a mid-lake, non-undercut Carboniferous limestone erratic occur respectively at Gortlecka, Creevagh and Cornaseer, and in Inchiquin Lough.

The base of the ‘wavestone’ at Gortlecka (Plate 1) is winter-flooded by Lough Gealáin (pH 8.2), and the horizontal surface of the water contrasts strongly with the sloping and undulating lip.

In contrast, the ‘wavestone’ at Creevagh (Plate 2), which occurs inside a meander of the river Shannon (pH 8.3), seems an obvious candidate for formation in a lake since its lip is horizontal.

The Cahir Rock (Plate 4) is a large Carboniferous limestone erratic found in Inchiquin Lough (pH 7.8), which shows no sign of notching by present day lake water.