Viereck, A.; Rudner, J. (1957). "Twyfelfontein: A Centre of Prehistoric Art in South West Africa". The South African Archaeological Bulletin. 12 (45): 15–26. doi:10.2307/3886436. JSTOR3886436.
Viereck, A.; Rudner, J. (1957). "Twyfelfontein: A Centre of Prehistoric Art in South West Africa". The South African Archaeological Bulletin. 12 (45): 15–26. doi:10.2307/3886436. JSTOR3886436.