The big canyons of Kata Tjuṯa are thought to be evidence of faults that formed while the rock was being folded millions of years ago. Many years of weathering has made the faults bigger and water has eroded the rock into the valleys and domes seen today. Uluṟu has deep parallel fractures that run down its sides. These are caused by erosion, mostly from rainwater running off the domed top. The caves around the base of Uluṟu were formed by water weathering, though the exact process is still not known. Erosion of the rock is very slow, because the surface is very hard, and because there is no major jointing or parting at its base.