Flat-bottomed sinkholes in limestone areas, in particular in that barren limestone pavement landscape in western Ireland called the Burren, can become flooded in wet weather as underground systems fill up and the water table rises.
The resulting lakes, called turloughs, can appear quite quickly after a spell of wet weather, vanishing again within a few days of the rain stopping. Some are seasonal, water-filled in winter but reliably dry enough in summer to be used for pasture. Turloughs empty through parts of the floor of the sinkholes and also through swallow holes — small depressions filled with loose rocks.
Researchers in Ireland argue turloughs are unique, quite unlike others, such as some English meres, and types variously called dolines, uvalas and poljes. The Irish ones are more erratic in the way they appear and disappear, as the result of climate, underlying rock type, and depth of soil in the sinkholes that holds back drainage.
The word is from Irish turloch, deriving from tur, dry, plus loch, lake, the second element being pronounced with a guttural ch as in the Scots loch.
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