"Mike Lyle" writes:

Right, here we go on an Arthur Ransome trip. It was in one of the
ones away from the Lake District and the Broads, no? They kept
finding these mysterious prints in the mud at low water, and had
begun to theorize that there was a brontosaurus

"Secret Water" and the Mastodon. Loosely based on the area just north
of Walton on the Naze.

Anthony