OVERVIEW: This report considers the relative sensitivity of Porter County aquifers through identification and mapping of the geologic framework, the three-dimensional geologic terrain, within which aquifers occur. Aquifers are beds of saturated, granular material such as sand and (or) gravel that are sufficiently permeable to transmit usable quantities of water to pumping wells. The various terrains consist of landforms and associated stacked sequences of deposits related to relatively recent lakeshore environments and to older glacial, glacial outwash, and glacial lake environments. The sequences are composed of unconsolidated materials ranging in total thickness from about 50 to over 200 feet, lying above bedrock. The purpose of this terrain mapping is twofold: first to form the areal (map) background for a system of relative sensitivity ranking, and second to illustrate the nature and degree of geologic variability within mapped terrain-sensitivity units. Geologic terrain is mapped and terrain map units are ranked with respect to the relative ease (or speed) with which aquifers contained within might receive contaminants from surface or near-surface sources. The ranking process is based upon a "point-method" calculation of sensitivities in which a score is given to individual well records. Sensitivities of mapped terrain units are characterized on the basis of means and variations of scores associated with the units. These scores are essentially a measure of aggregate thickness of aquifer-capping "clays," with varying weights given to these materials according to position and individual thickness. Aquifers may be thick (>100 ft) or think (<5 ft); they may be simple, discretely mappable units, or they may be ill-defined aspects of a larger package of units; they may be capped by thick "clays", or they may extend to the surface, capped only by a think surficial soul profile. Therefore, relative sensitivity may be affected by variables that are not simply summarized in terms of character and thickness of "capping clay." For these reasons, illustration of the variability of aquifer-enclosing sequences is of great importance. These are graphically illustrated here in great detail. The intent is to illustrate the many nonquantifiable variables involved in true sensitivity analysis, to illustrate the assumptions involved in this mapping process, and to illustrate the degree of variation of materials and sensitivities. In these examples, accompanying geologic models are intended to provide conceptual background for future geologic evaluations. Aquifers within the unconsolidated geologic sequences of Porter County are, in fact, "used" aquifers across the entire county (with very limited exception). For this reason, the sensitivity of deeper bedrock aquifers is not a consideration herein.