Uluru and Kata Tjuta are millions upon millions of years old. Geologists say their formation began about 550 million years ago.
Back then, the Peterman Ranges to the west of Kata Tjuta were much taller than they are now. Rainwater flowed down the mountains, eroding sand and rock and dropping it in big fan shapes on the plains. One fan had mainly water-smoothed rock and the other was mainly sand.
Then 500 million years ago, this whole area became covered in sea. Sand and mud fell to the bottom of the sea and covered the seabed, including the fans. The weight of this new seabed turned the fans into rock. The rocky fan became conglomerate rock (Kata Tjuta) while the sand fan turned into sandstone (Uluru).
About 400 million years ago the sea disappeared. Rocks folded and tilted as the earth's tectonic plates shifted. Kata Tjuta tilted slightly. Uluru tilted 90 degrees.
Over the last 300 million years, softer rocks have eroded away leaving Kata Tjuta and Uluru behind.
What we see at Uluru and Kata Tjuta today, are just the tips of huge rock slabs that continue below ground for up to six kilometres!
Uluru is made up of a type of rock called arkose. If you take the Base Walk you'll notice the surface is actually flaky red with grey patches. These flakes are bits of rock left after water and oxygen have decayed minerals in the rock. The red is the rusting of iron found naturally in arkose and the grey is the original colour. Inside the caves you can see this original grey colour.