Charles Darwin on his visit to the Blue Mountains in 1836: "...It is not easy to conceive a more magnificent spectacle than is presented to a person walking on the summit plains, when, without notice, he arrives at the brink of one of those cliffs … Certainly, the most stupendous cliffs I have ever seen…"
Subsidence, Darwin thought, must have caused those stupendous hollows in the ground.  But the great man was wrong. Erosion did it. 60 million
years of wind and weather removed three quarters of what had been,
and left today's skeletal landscape.
The Blue of the Blue Mountains is caused by the Rayleigh Effect.
The leaves of gumtrees exude minute droplets of ethereal oil into the
atmosphere. Together with tiny dust particles and moisture they scatter the short waves of the spectrum, the blue ones. On warm hazy days the air around the mountains catches the blues.