CLAY_ILITH_IN is a grid that shows total thickness of clay, as derived from logs of water wells in the state of Indiana. (It presents the same data as shown in a point shapefile named "CLAY_ILITH_PTS_IN" but in a grid format.) The data were obtained from the iLITH Database of the Indiana Geological Survey. The following explanation of the iLITH Database appears in Open-File Study 00-8 of the Indiana Geological Survey: "iLITH is a Microsoft Access database containing GIS capabilities that allows users to interact with a portion of the Indiana Department of Natural Resources (IDNR) Water-Well Record Database. iLITH differs from the IDNR database in that lithologic descriptions and drilling contractor names have been standardized, locational information for ~104,000 water-well logs has been verified, and the spatial information in the database has been standardized for use with USGS digital elevation data and US Census Bureau Tiger data. The iLITH graphical interface facilitates efficient interaction between the user and the database." Using Microsoft Access, queries were constructed that summed the total thickness of clay units for each well. Units with the following designations were included in the summation: "clay," "clay and gravel," "clay and sand," "clay and stone," "clay with gravel," "clay with gravel seams," "clay with sand," "clay with sand streaks," "mud," "mud and gravel," and "mud and sand." Some inconsistencies in the database were eliminated. The resulting table was then exported from Microsoft Access and imported into ESRI ArcView to construct a point shapefile (CLYTHKPTS). CLYTHKGRD was created by running the inverse-distance-weighted interpolation routine with default settings on the point shapefile, using ESRI ArcView Spatial Analyst. A cell size of 200 meters was chosen. The cells of the grid outside the state boundary were set to NODATA.