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Summary Full report
Site Type: Inland exposure
Site Status: PASSI
District: Moyle District Council
Grid Reference: D207360
Rock Age: Quaternary (Pleistocene)
Other interest: No data, Glacial feature, disappearing lake, glacial meltwater system, plunge pool, subglacial

Summary of site:

Loughaveema is a small lake sitting in the high and remote landscape between Cushleake Mountain and the east flank of Ballypatrick Forest in north-east Co Antrim. It is known locally as ‘The Vanishing Lake’ because from time to time its level falls progressively and sometimes suddenly. Often it will drain completely, revealing a well defined peat filled depression at its lowest point. Early mapping of the area showed that the lake bed rested partly on very old Dalradian rocks (about 500 million years old) of a kind called schists and partly on beds of Ulster White Limestone of more recent age (about 100 million years old). The schists are impervious to water and so too is the limestone but the limestone is susceptible to etching by acid water draining from the surrounding peat. The acid can dissolve the rock along lines of weakness such as natural fractures called joints. It was always supposed that the acid water of the lough had widened the joints sufficiently to allow it to drain, only refilling when the joints became plugged by silty peat. At that point it refilled until the depth of water exerted sufficient pressure to force the silt through, re-opening the joints to drain freely into the groundwater system of the limestone.

This interpretation is now challenged by re-examination of the evidence and an alternative explanation is considered more likely. Loughaveema sits in an expansion of a tight, steep-sided valley interpreted as a glacial meltwater channel, at a point where its course changes direction from north north-west to north-west. A glacial meltwater channel is cut by a sudden and massive surge of meltwater focused at a limited point during a period when glaciers are rapidly melting. In this case the channels in the ice-filled Glendun to the south suddenly and catastrophically spilled over the watershed towards the Carey River system to the north diving beneath the ice cover and rapidly cutting the narrow steep-sided channel. Huge quantities of glacial sands and gravels were carried by the surge and deposited as deltas in a ponded lake in the Carey Valley. A powerful sub-glacial waterfall formed at the site of Loughaveema and rapidly excavated the lake basin. As the flow eased, irregular patches of sands and coarse gravels filled the channel and much of the Loughaveema basin. Later a new, smaller channel was cut into these deposits and the basin flooded on the site of the modern lake. This interpretation suggests that the Ulster White Limestone probably does not occur on the bed of the lough and that the fluctuating level is the result of seepage through the extremely porous gravels to align with the local water table. In general this would result in low water levels during drought and high levels in periods of high precipitation.

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