Scheffel؛ Wernet، المحررون (1980). Natural Wonders of the World. United States of America: Reader's Digest Association, Inc. ص. 189. ISBN:0-89577-087-3.Scheffel, Richard L.; Wernet, Susan J., eds. (1980). Natural Wonders of the World. United States of America: Reader's Digest Association, Inc. p. 189. ISBN0-89577-087-3.