In Ordinary Ngondro we have the topic of impermanence, in it there is the subtopic of impermanence of the outer world, meaning impermanence of Earth, impermanence of the Solar System and of the Milkyway Galaxy. Study of geology is essential to get anywhere in the understanding of impermanence of Earth. Especially useful are Plate Tectonics, and how rocks and mountains are slowly formed, and how they then slowly wear away completely. It is very rewarding, it gives you a deep sense of impermanence, when you start to fathom it. The earth we walk upon is not as solid as it seems, it changes and moves very slowly but continuously. You must study and meditate on it long enough to get a real sense of impermanence of Planet Earth and its continents, its mountains and its rivers. This leads further on to the formation and impermanence of the solar system and to the formation and impermanence of stars/suns and of galaxies.