sentences:  Before the Andes were formed, the region was likely covered by temperate forests.However, after the formation of the Andes, ash from nearby volcanoes covered the forests and mineral-saturated waters seeped into the logs, thus fossilizing the trees and creating one of the world's best preserved petrified forests in the desert's center.The Patagonian is mainly composed of gravel plains and plateaus with sandstone canyons and clay shapes dotting the landscape, sculpted by the desert wind.