de Vries, J. J.; Selaolo, E. T.; Beekman, H. E. (2000-11-30). "Groundwater recharge in the Kalahari, with reference to paleo-hydrologic conditions". Journal of Hydrology. 238 (1–2): 110–123. Bibcode:2000JHyd..238..110D. doi:10.1016/S0022-1694(00)00325-5.
Mazor, E.; Verhagen, B.Th.; Sellschop, J.P.F.; Jones, M.T.; Robins, N.E.; Hutton, L.; Jennings, C.M.H. (1977). "Northern Kalahari groundwaters: Hydrologic, istopic and chemical studies at Orapa, Botswana". Journal of Hydrology. 34 (3–4): 203–234. Bibcode:1977JHyd...34..203M. doi:10.1016/0022-1694(77)90132-9.
de Vries, J. J.; Selaolo, E. T.; Beekman, H. E. (2000-11-30). "Groundwater recharge in the Kalahari, with reference to paleo-hydrologic conditions". Journal of Hydrology. 238 (1–2): 110–123. Bibcode:2000JHyd..238..110D. doi:10.1016/S0022-1694(00)00325-5.
Mazor, E.; Verhagen, B.Th.; Sellschop, J.P.F.; Jones, M.T.; Robins, N.E.; Hutton, L.; Jennings, C.M.H. (1977). "Northern Kalahari groundwaters: Hydrologic, istopic and chemical studies at Orapa, Botswana". Journal of Hydrology. 34 (3–4): 203–234. Bibcode:1977JHyd...34..203M. doi:10.1016/0022-1694(77)90132-9.
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Wilmoth, Adam (2015-11-29). "Reading the Rock". The Oklahoman. Retrieved 2015-12-21 – via m.newsok.com/article/5463442.