In 1911, when Tolkien was only 19, he travelled with a on a walking tour of the Swiss Alps. It was his first and only experience of truly large mountains, and as he later made clear in several of his letters, the experience inspired the mountain landscapes in his Middle-earth writings.
`Only once before have I seen them from afar in waking life, but I know them and their names, for under them lies Khazad-dûm, the Dwarrowdelf, that is now called the Black Pit, Moria in the Elvish tongue. Yonder stands Barazinbar, the Redhorn, cruel Caradhras; and beyond him are Silvertine and Cloudyhead: Celebdil the White, and Fanuidhol the Grey, that we call Zirak-zigil and Bundushathûr.
Gimli, The Lord of the Rings.