Cretaceous rocks in the Southern High Plains, traditionally considered to be part of the High Plains aquifer and recharged by the overlying Ogallala aquifer, actually contain three aquifers with different recharge sources. Two separate subcrop areas of Cretaceous rocks underlie the Ogallala Formation. These subcrops are composed of sandstones, limestones, and shales and have a total thickness of as much as 250 ft (76 m). Three aquifers, the Trinity sandstone, the Edwards limestone, and the Duck Creek – Kiamichi sandstones and limestones, were identified within the Cretaceous section. Wherever the Ogallala aquifer is thin or missing, the Cretaceous aquifers are the main water supply. Major recharge sources are percolation through thick surficial alluvial deposits and sand dunes in New Mexico and northwestern Texas and leakage from the underlying Dockum aquifer in eastern New Mexico. Recharge from the overlying Ogallala aquifer is restricted to small parts of the study area. Elsewhere, differences in hydraulic heads and similarities in chemical and isotopic composition suggest cross-formational flow from Cretaceous aquifers into the overlying Ogallala Formation or the underlying Dockum Group.