Uluru began to form over 550 million years ago when sand, eroding from the Peterman Ranges, built up into thick deposits. Some 50 million years later, these deposits were compressed into sandstone by the weight of a sea that eventually disappeared. Within another 100 million years, geological events caused the sandstone to tilt almost 90 degrees, resulting in Uluru.
Due to the way it was formed, the visible structure of Uluru is really only the tip of a huge sandstone slab. It is theorized that the the entire formation continues for 3 to 4 miles under the earth's surface. The layers of sandstone appear to run nearly vertical because of the uplift that shifted the rock. Over the millions of years since forming, the rock has continually weathered, forming ridges and ribs.