The official source of the River Thames, Thames Head, is 4.8km south-west of Cirencester, in a field called Trewsbury Mead. There an ancient ash tree grows by a stony hollow in the ground from which the river springs to flow down the sloping meadow. It is marked by a block of stone with the inscription:

The Conservators of the River Thames
1857 - 1974
This stone was placed here to mark the
Source of the River Thames