A geologic map represents the melding of field observations with various types of analytical data and earth science concepts. Choosing features to be portrayed is a reflection of the questions posed. Some would claim that in the mapping process, theory meets reality. However, a map is a more subjective product based on the sum of the geologist's prior training, aggregate field experience, and the stage of development of scientific concepts, the complexity of the mapped units, the extent and quality of exposures, the wealth of constraining ancillary data, and the time and thought expended in the mapping. The published map also reflects accommodations to the scientific reviewers' knowledge, and to technical compromises required by the printer-publisher. Because mapping style depends on a geologist's prior experience, it is necessarily a somewhat idiosyncratic process. My own field research has focused chiefly on the petrologic-structural development of Mesozoic and younger contractional orogenic belts, and through them, the tectonic evolution of continental margins. Mapping has been an essential step enhancing my understanding of processes that have shaped convergent portions of Earth's crust. (1) For instance, field relations combined with mineralogic analysis in the Panoche Pass area, southern Diablo Range, central California, indicated relatively high pressure–low temperature recrystallization and postmetamorphic, low-angle faulting of the Franciscan Complex. (2) Mapping of a similar Franciscan terrane in the central Diablo Range identified imbricate, subhorizontal, syn- to postmetamorphic bedding-plane thrust faults and implied accretionary growth in the Pacheco Pass quadrangle. (3) Field study of a structural inversion of high-grade metamorphic rocks tectonically overlying low-grade equivalents in the Sanbagawa belt, central Shikoku, Japan, led to the interpretation of postrecrystallization, ductile nappe emplacement and, as in California and the Western and Eastern Alps, (4) a progressive, relatively high P –low T metamorphism-exhumation subduction-zone model. (5) Mapping the interstratified distal turbidites and mafic lavas, and the discovery of pillow tops in the Sawyers Bar area, documented in situ stages of oceanic-island arc development in the North Fork terrane, central Klamath Mountains, northwestern California. Bulk-rock compositions of interlayered ocean-island basalts and island-arc tholeiites supported this interpretation. (6) Detailed geologic mapping combined with remote sensing in the central White Mountains, easternmost California, demonstrated that the Middle Jurassic Barcroft granodioritic complex is a steeply southeast-dipping slab that intruded previously deformed mid-Mesozoic arc volcanic rocks and Neoproterozoic–Lower Cambrian platform strata along a high-angle reverse fault. Conclusions derived from these studies, as well as more general plate-tectonic syntheses, depended on geologic mapping, and, for me, the field mapping was an enjoyable and scientifically fulfilling experience.

Abstract The 1999 Glen Creek Landslide provides an example of a negative result of human interaction with a complex geologic environment. Excavation during mass grading exposed the basal rupture surface of an old, static landslide and weak shear zones within the weak claystone, leading to translational failure. During remediation of the active landslide, the excavation for the buttress keyway exposed a downslope-dipping, deep-seated shear zone and reactivated it. The shear zone projected under an adjacent structure, and movement of the shear zone produced deflections in slope inclinometers located between the structure and the excavation. The events leading to the failure of the Glen Creek Landslide represent a failure in application of the four Jahnsian steps to geologic safety: (1) hazard recognition, (2) site characterization, (3) risk assessment, and (4) hazard mitigation. First, the developer's geotechnical consultant failed to recognize the old landslides. Second, the extent of shearing within the claystone was not characterized. Third, the risk of translational failure along weak shear zones was not realistically evaluated. Lastly, mitigative measures were not implemented to address the risk of slope failure. The postfailure mitigation of the Glen Creek Landslide was not planned with the Jahnsian steps in mind either. The potential for block failure along deep-seated shear zones was not recognized. Subsurface characterization for design of the keyway and a realistic translational failure analysis were not performed. If the first three steps were followed, the excavation plan should have been modified to be compatible with the adverse geologic conditions.