The objective of the Upper Colorado Regional Aquifer Systems Analyses (RASA) project is to assess quantitatively the occurrence, movement, and availability of water stored in rock formations underlying the basin. This is one of a series of preliminary reports that describe what is currently known (1984) about the geometry and hydrologic properties of the aquifer systems, and the chemistry of the water in known or potential aquifers. This report provides the foundation needed for the assessment of the ground-water resources of the region. The report describes only part of the hydrologic system; that is, formations of Mesozoic age. Younger and older formations significant to the hydrologic system are described in other reports.