One of the first things that I read in Helen Lawrence’s fascinating How Hampstead Heath was Saved, was about the unusual geology that has created the heath’s rolling hills: a layer of clay covered with sand. (This map comes from her book – with thanks.)
I was reminded of the sands when I walked passed the clay pond the other day – looking rather lovely in the sun.
And this time I noticed a battered but still just about legible sign at the edge which gave a load more fascinating information.
This pond was created in 2010 around a natural spring emanating from a line encircling Hampstead Heath. Spring lines are governed by geology and occur where a clay rich layer is overlain by a sandier layer. At the top of Hampstead heath are the Bagshot Sands, laid down close to the cost about 50 million years ago. Rainwater percolates through the sands until it meets with the more clayey Claygate Beds beneath. Notable springs from this junction would once have been seen at ‘the Old Bath house’ at Kenwood House and the well that supplied the health Spa on Well Walk in Hampstead. The water is very iron-rich and was supposed to be a cure for all manner of ailments. This upper spring line provides the headwaters of both the Hampstead and Highgate branches of the River Fleet which runs through the ponds below and eventually enter the Thames at Blackfriars.
The lower spring line is where the Claygate Beds meet the impermeable typical London Clay (about 52 million years old). The water from this junction is a pure soft spring water that was formerly supplied to the people of Hampstead from Shepherd’s Well in Fitzjohn’s Avenue. Here were the headwaters of the now ‘lost’ River Tyburn, on the same spring line which feeds this small pond.
And while I was reading, a friendly duck came to join me.
For today’s tree… A massive willow just south of the clay pond.
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