The climate of Central Asia became dry after the large tectonic collision between the Indian Plate and the Eurasian Plate. This impact threw up the massive chain of mountains known as the Himalayas. The Himalayas, Greater Khingan and Lesser Khingan mountains act like a high wall, blocking the warm and wet climate from penetrating into Central Asia. Many of the mountains of Mongolia were formed during the Late Neogene and Early Quaternary periods. The Mongolian climate was more humid hundreds of thousands of years ago. Mongolia is known to be the source of priceless paleontological discoveries. The first scientifically confirmed dinosaur eggs were found in Mongolia during the 1923 expedition of the American Museum of Natural History, led by Roy Chapman Andrews.