A stack-unit map is a lithostratigraphic map that provides information on the presence and distribution of one or more geologic materials to a selected depth. Each stack unit represents a particular succession of units (designated by standard symbols in a stacked sequence) present to the specified depth. Stacks units can be represented by pattern and color combinations as well as by an outline map with each stack unit labeled within the limit of its boundary. Mapping detail and depth limit are governed by various factors including availability of data and criteria for each anticipated land-use interpretation.

Stack-unit maps, when combined with physiographic information (a terrane map) and hydrogeologic data and principles, provide the basis for the preparation of a new generation of interpretative maps. These interpretive maps depict such hydrogeologic information as regional ground-water recharge rates and estimated depth to the top of the zone of saturation. In turn, incorporation of these hydrogeologic evaluations improves traditional interpretive maps that show suitability of geologic materials for waste disposal and general construction, and other land uses.

Three-dimensional mapping has evolved to meet the expanding need for geologic information that can be interpreted directly for land use or resources development. Some stack-unit maps and mapping symbols are tailored specifically to the engineering geologist and other informed users; interpretive maps based on stack-unit maps provide specific land-use evaluations directly and can be used by planners, developers and engineers with little geologic background.

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