John Loudoun Macadam, the world famous road surveyor, was born in Ayr in 1756. As a businessman in New York he made a fortune and returned to live near Maybole in Ayrshire in 1783. Financial problems meant that he had to leave Scotland once again to pursue a career as a road surveyor in England for a while before returning home again, dying in Moffat in 1836.
Macadam’s contribution to road building was to recognize the greater stability of graded road surfaces with layers of increasingly smaller stones – a macadamised road. Although he was a partner in a tar works in Muirkirk he failed to recognize the value that a binding such as tar would bring to his techniques. It took another 100 years or so before tar-macadamised (tar-mac) roads began to be developed.
He is commemorated by two monuments Ayr: one in Wellington Square which the cache you are looking for is near, and another at Lady Cathcart’s House in the Sandgate (co-ords N55 27.822 W004 37.978, now the Tourist Information Office) where he is said to have been born.