This is the first geologic field investigation of ancient, heavily cratered terrain on Mars: Husband Hill in Gusev Crater, as investigated by the Mars Exploration Rover (MER) Spirit, which explored this site from the ~150th through ~700th sols of her 90-sol mission (a sol is a martian day). This dissertation characterizes all of the rock classes that the rover encountered on Husband Hill and the West Spur, and presents a geologic map and cross-section. A microtextural analysis is presented, followed by descriptions of the occurrence, major element geochemistry, iron-bearing mineralogy, spectral characteristics, and origin of each rock class. The structure and stratigraphy of Husband Hill are investigated, using stereogrammetry applied to Pancam stereo image data. All potential bedding planes of all in-place outcrops are examined. A stratigraphic section is produced: the Peace Class unit onlaps the Watchtower Class unit. The Watchtower Class stratigraphic unit has a minimum thickness of 9 m, and can be correlated over a distance of ~200 meters. If this volcaniclastic material was emplaced conformably to the topography at the time of deposition, tens of meters of material may have been removed from Husband Hill. Variations within the Watchtower Class outcrops of Cumberland Ridge and the Husband Hill summit are analyzed. Subclasses, defined by previous workers on the basis of Mössbauerderived iron mineralogy, Pancam visible and near-infrared (VNIR) spectra, and Mini-TES thermal emission spectra are described, and a new set of subclasses, defined on the basis of microtexture, is presented. The size of knobby protuberances, quantifiable in Microscopic Imager (MI) data, is correlated with iron oxidation state and the abundance of nanophase and amorphous material. The variation across Watchtower Class targets may be due to gel weathering resulting from acid fog, with the degree of alteration varying inversely with the amount of insolation. In addition to the science, this dissertation presents detailed descriptions of the Mars Exploration Rover vehicle and all of its instruments and payload elements, including the physics behind each instrument; data analysis techniques, capabilities and limitations; and archived data products. Resources for finding MER data and the context of Spirit's mission are listed.