Complex sentence: They left Joshimath, well within the mountains, on the 26 May, 1812 and proceeded along the valley of the Dauli, a tributary of the Ganges river.
Split into simpler sentences: They left Joshimath, well within the mountains, on the 26 May, 1812. Proceeding along the valley of the Dauli, a tributary of the Ganges river.