Sometimes odd geography intersects with odd geology. One particularly rare example occurs on the island of Ireland. It’s called a Turlough or Turlach. Described very simply, it’s an ephemeral lake that appears during the wetter months of autumn through springtime and dries-up during the summer. Most of the examples happen west of the River Shannon. Here the exposed karst limestone is so riddled with cracks, drains, holes and subterranean channels that rainwater simply disappears into the landscape and gurgles to surface lowpoints, forming Turloughs.
A Turlough has no visible outlet to the sea. It rises and recedes with the local aquifer, at the whim and mercy of prevailing weather patterns. The annual cycle of filling and draining differentiates a turlough from an ordinary spring. Right now the academicians in the crowd are cringing at my over-simplified explanation. There are precise geological and ecological hallmarks of turloughs that I’ve glossed over for expedience. Hopefully the basic idea still gets across.
The etymology of turlough listed in Oxford Dictionaries is "late 17th century: from Irish turloch, from tur ‘dry’ + loch ‘lake’". Others sources build a case that it’s probably something closer to "pasture lake" (also provides some good information on turloughs in general, too).
Turloughs are relatively uncommon even in Ireland — totaling in the low hundreds — and they are disappearing. Farmers have cut channels across them over the centuries to drain them permanently. This makes the land available throughout the year. Draining however destroys a rather unique turlough ecosystem of mosses, algae and fauna that have adapted to the wet-dry cycle. Conservation efforts have expedited in recent decades to preserve what remains.
Republic of Ireland
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All but four turloughs on the planet are found in the Republic of Ireland. Accordingly, the largest and best representations of the phenomenon can also be found here. This example is the Glenamaddy Turlough, associated with the Lough Lurgeen Bog in County Galway. Glenamaddy derives from Gleann na Madadh, or Valley of the Dogs, with speculation that it’s due to its shape at winter extreme. I’m not sure I see the resemblance but I have trouble with these kinds of things.
Irish conservationists consider the entire Lough Lurgeen Bog – Glenamaddy Turlough area to be extremely significant. It is both large in size and relatively undisturbed, a rare combination of factors after hundreds of years of intensive agriculture.
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Still on the island of Ireland but now across the border in Northern Ireland are three small turloughs grouped in close proximity west of Lower Lough Erne. They are known as Fardrum Lough, Roosky Lough, and Green Lough and have been designated a European Union "Special Area of Conservation" (SAC). This is the northernmost extreme of a rather unusual turlough habitat.
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Perhaps most unique of all, and the only example found outside of the island of Ireland, is the Pant-y-llyn Turlough in Wales. It is part of the protected Cernydd Carmel in Carmarthenshire. It’s a small turlough by Irish standards. Nonetheless it’s been studied extensively because it’s such a geographic outlier: one of only four instances of a true turlough in the UK and the only one located on the island of Great Britain by many accounts. I wonder if people traveling through the Carmel Woods SAC even notice it, much less ponder its significance?