This geodatabase includes spatial datasets which represent the Lower Cretaceous aquifer system in the States of Iowa, Kansas, Minnesota, Montana, Nebraska, North Dakota, South Dakota, and Wyoming. Included are: (1) a polygon dataset that represents the aquifer system extent, (2) raster datasets for the altitude of each aquifer subarea, (3) altitude and thickness contours (plus thickness fault lines) used to generate the surface rasters, (4) georeferenced images of the figures that were digitized to create the altitude and thickness contours. The images and digitized contours are supplied for reference.

The extent of the Lower Cretaceous aquifer for all subareas was produced from the linework of the Lower Cretaceous aquifers maps in U.S. Geological Survey Professional Paper 1414-E (USGS PP 1414-E) Plate 6, U.S. Geological Survey Water-Resources Investigations Report 84-4153 (USGS WRIR 84-4153) Plate 1, and the U.S. Geological Survey Hydrologic Atlas 730 (USGS HA-730). A digital version of the HA-730 Lower Cretaceous aquifer extent is presented in the Groundwater Atlas of the United States (the U.S. Geological Survey Hydrologic Atlas 730-D,(USGS HA 730-D), 730-I (USGS HA 730-I), and 730-J (USGS HA 730-J.

The Lower Cretaceous Aquifer System has 4 aquifer subareas: (SA1) Great Plains aquifer, (SA2) Iowa aquifer, (SA3) Minnesota aquifer, and (SA4) Northern Great Plains aquifer.

The altitude lines for the top of SA1 and SA4 were digitized from georeferenced figures of altitude contours in USGS PP 1414-E Plate 4 and USGS PP 1402-B Plate 1. Altitude lines for the top of SA3 were digitized from WRIR 84-4153 Figure 5. The top of SA2 was derived from the thickness contours for SA2, USGS PP 1414-E Figure 16. Areas of overlap between subareas were deleted, based on available original data. The altitude values and thickness values were interpolated into surface rasters within a GIS using tools that create hydrologically correct surfaces from contour data, derives the altitude from the thickness (depth from the land surface), and merge the subareas into a single surface. The primary tool was "Topo to Raster" used in ArcGIS, ArcMap, Esri 2014. The raster surfaces were corrected for the areas where the altitude of the top of the aquifer exceeded the land surface, and where the bottom of an aquifer exceeded the altitude of the corrected top of the aquifer. For a more detailed description of the processes used by the data compiler, please see the Supplementary Information section.