2013-07 - Urswick Tarn on marl bench


This photograph is taken on private land at the north-west corner of Urswick Tarn looking in a south-easterly direction.  This is a rare marl tarn holding a body of water which is rich in calcium carbonate dissolved from the local limestone geology as its water flows underground before emerging through springs at the bottom of the tarn.  The flat land forming the perimeter of the tarn consists of an accumulation of this calcium carbonate following its precipitation from the water onto plants growing in the tarn.  When the plants die each year the calcium carbonate, known as marl in these circumstances, sinks.  This process has taken place since the last ice age, slowly reducing the size of the body of open water whilst forming the flat perimeter on which vegetation gradually takes hold to create a marl fen.  Entering the tarn from the water's edge is dangerous because, as a consequence of this rare process of marl deposition, the transition between shallow and deep water is very sudden.

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