A massive Sahara-like desert geologists call an erg then invaded the area, covering it with 800 to 1,100 feet (240 to 335 m) of accumulated white to tan-colored fossilized sand dunes. The resulting formation, called the Navajo Sandstone, is composed of cross-bedded and very clean sandstone with well-rounded, generally very fine-grained and frosted sand. It reached its greatest thickness, 2,000 feet (610 m) in what is now Zion National Park (see geology of the Zion and Kolob canyons area). The cross-bedded make-up of the Navajo leads to the formation of curvilinear canyons and rounded domes such as Capitol Dome and Navajo Dome. In other places it forms massive cliffs and monoliths. Subsequent erosion leveled the tops of the sand dunes and left them in up to 60 foot (18 m) thick easy to discern layers.