Drive South of London across the North Downs

We drove around London on the circular M25 then turned across the North Downs. Why are these hills called downs and not ups?  I don't know. The South Down are on the South coast whilst the North Down are North of them nearer London. The area in between has been eroded into a hollow or plain called the Weald. I think of weald rhyming with field.

Having suffered many dull journeys along motorways I was delighted by the views over the treetops and hilltops through the forested North Downs. We drove towards Sevenoaks and Tunbridge Wells and Tunbridge which I shall describe later. 

Signs on the road warned us of cows and sheep.