The River Thames once began as a spring, in this field, on the lower slopes of the Cotswold Hills.
The Cotswolds are made of limestone, the same type of rock that we encountered in Wharfedale, where the river Wharfe disappears into the ground. On the Cotswolds you won’t find much in the way of rivers for the same reason. Limestone is full of cracks, enabling rainwater to sink into the ground. It is a permeable rock.
The water flows down through the permeable limestone until it can’t go any further, either because the rock has changed and no longer has holes, or because the holes are already full. The water will then flow sideways, out of the hillside, forming a spring, the source of a surface stream.
So if there hasn’t been rain in a while, water stops flowing out and the spring dries up.
But where the Thames is concerned, that is only part of the story. Find out more next week.