During World War II, the River Mole comprised part of the fortified GHQ Line B. This defensive line ran along the North Downs from Farnham via Guildford to Dorking, before following the river to Horley. Between Betchworth and Box Hill, the north bank of the River Mole was stabilised and made steeper to prevent wheeled vehicles from crossing. At Boxhill Farm, where access to the river from the north bank was required for the herd of dairy cows, a row of twelve concrete cylinders were cast as an anti-tank measure. Gun mounts were also installed to protect both Boxhill and Deepdene bridges and several pillboxes were constructed. The Stepping Stones at the foot of the hill were removed as an anti-invasion measure.