@book {verstappen1973geomorphological,
	title = {A geomorphological reconnaissance of Sumatra and adjacent islands (Indonesia)},
	volume = {1},
	year = {1973},
	publisher = {Wolters-Noordhoff},
	organization = {Wolters-Noordhoff},
	author = {Verstappen, Herman Theodoor and Genootschap, Koninklijk Nederlands Aardrijkskundig}
}
