By about 500 million years ago, a shallow inland sea formed in the basin. Over many hundreds of years, sand and mud fell to the bottom of the sea, covering the alluvial deposit. This layer cemented everything together, forming the arkose over Uluṟu and the conglomerate over Kata Tjuṯa. The sea dried up between 400 and 300 million years ago. A long period of folding tilted the horizontal rock layers. The layers of Uluṟu arkose were folded to the near vertical position they are in today. The conglomerate of Kata Tjuṯa was tilted about 15 to 20 degrees from the horizontal. This means that Uluṟu and Kata Tjuṯa are the exposed tips of huge rock slabs that run far below the ground.