A confined aquifer in Triassic Dockum Group sandstone beneath the southern Great Plains was isolated from hypothesized paleorecharge areas in eastern New Mexico by Pleistocene erosion of the Pecos and Canadian river valleys and formation of hydrologic divides. Truncation of the flow system left meteoric water in the confined aquifer with mean δD and δ18O values that are 17‰ and 2.0‰,respectively, lighter than those in the overlying High Plains aquifer. Thick upper Dockum mudstone retards downward flow from the High Plains aquifer, which has been recharged by isotopically heavy precipitation during the Holocene. Recharge to the confined aquifer occurred at altitudes of 1600 to 2200 m in proximal Dockum sandstone facies since eroded in eastern New Mexico, at a mean temperature 3 °C cooler than present temperature across the southern High Plains. Effects of Pleistocene climatic change on isotopic composition of Dockum ground water could be superposed over geomorphologic effects.