Stack-Unit mapping of geologic materials to a depth of 15 meters. Stack-unit maps show the distribution of earth materials vertically from the surface to a specified depth and horizontally over a specified area. They also show succession of geologic units in order of occurrence. Primary source is ISGS Circular 542, Stack-unit Mapping of Geologic Materials in Illinois to a Depth of 15 Meters, Berg and Kempton (1988). Attributes include stack-unit coding and map symbols. ISGS, 1988 (revised 1995, 1998).