This report contains files that provide a digital version of USGS map I-1853-A, "Precambrian Basement Map of the Northern Midcontinent, U.S.A.," compiled by P.K. Sims and published in 1990. The files are provided in two formats: (1) Arc export (e00) files, which can be imported directly into a number of applications, and (2) Arcview shapefiles and a related Arcview project, which allow direct viewing and manipulation of the map information.
The intent of this release is solely to make the original map data available digitally, not to make updates or modifications to the map based on new data acquired in the past decade since the original map was published. Thus, the information in the database files is distilled from the original map and preserves the terminology used therein. To further preserve the original map, ancillary information such as the Correlation of Map Units and Description of Map Units are included as images that were scanned from the printed map.
The data are presented in several data layers. A polygon coverage presents the distribution of map units in which each polygon is attributed with 15 geological parameters. An accompanying line coverage shows faults. Additional coverages show location of drill holes used in the original compilation and structure contours on the Precambrian basement surface. --USGS