Great Urswick - A choice of foundations

This photograph captures old cottages in the village of Great Urswick located at the north end of Urswick Tarn.  The land across which the cottages are seen would once be fenland that covered the marl bench surrounding the tarn.  As the village grew and cottages such as the ones seen here were constructed during the eighteenth and nineteenth centuries, it was necessary to elevate them above the fen by excavating into rising ground founded on limestone bedrock.  The excavated earth and rock was conveniently dumped on the fenland and, with time, this brought about a transformation in appearance, the former fen taking on the character of meadowland.  But the transformation was only of appearance, the deep deposits of waterlogged, paste-like marl remain below, sitting over sloping limestone bedrock dipping below the tarn’s water.  Marl forms by the precipitation of calcium carbonate from the water onto vegetation growing each year around the perimeter of the tarn.  The outcome of this is a marl bench which slowly encroaches into the tarn with a steep boundary with the water.  This means that the water deepens rapidly at this interface.  There is therefore an absence of any retaining force constraining movement in the marl bench towards the tarn.  The outcome of all this is that the bench of water-logged, paste-like marl is unsuitable for carrying any significant load as this would risk lateral as well as downward movement with the marl. The current meadow-like appearance of the tarn’s margin, which results from those housing excavations of old, is a beguiling false image of long term stability.  The builders of the old cottages may not have had an awareness of the formation process of the marl beneath the fen covering, but they would be conscious of the rapid increase in water depth and knew better than to attempt construction over the former wet fen that stretched before them to the water’s edge.