kateegreen that's a great fact I didn't even know so thank you for teaching me something new
The early Australian explorers took a while to figure it out. The 'classic', tried-and-tested, method for exploring through mountain ranges to find out what is on the other side is to follow the valleys. In the Blue Mountains, that didn't work: every time they tried following a valley, they eventually hit a dead end looking at a waterfall. Eventually one of them thought that maybe they should climb up onto the 'ridges' and follow those instead, and suddenly they found it was really easy to get through.
The Blue Mountains are part of the Great Dividing Range, which was created by tectonic activity and is (mostly) sandstone with volcanic intrusions. The intrusions create the mountain 'peaks' where the sandstone has eroded down around it. Some of the Range are proper mountains (from memory the Australian Alps are proper mountains), but the bulk of it is an uplifted plateau.
Up in Queensland, north of Brisbane, the Glasshouse Mountains are a rather extreme example of the geology - something like 13 "mountains" that are in fact volcanic intrusions into what was once elevated sandstone, but all the surrounding sandstone has eroded away leaving flat land with these random mountains.