Abstract For more than a century, the Heart Mountain Detachment has been an important natural laboratory that has contributed to the education of thousands of students rep-resenting most of the colleges and universities of the nation. The purpose of this field trip is to provide a forum in which the important features of the Heart Mountain Detachment (HMD) can be observed and the various explanations for the origin of the structure can be discussed. The foremost questions to be addressed will likely include: What factors (e.g., paleotopographic slope; direction and magnitude of bedding dip; seismicity; eruptive processes, presence and pressure of fluids) triggered the formation of the HMD? What factors facilitated movement across the gently dipping detachment surface? Did the allochthon consist of numerous detached blocks of Paleozoic rocks or was the allochthon continuous and consist of predominantly Eocene volcanic rocks? Were the detached rocks emplaced gradually or catastrophically? What data can be used to constrain the rates of emplacement? This field guide begins with an overview of the regional setting of the HMD. Detailed site descriptions of the various detachments and their associated structures are integrated into the road log (Figure 1). Most outcrops that are viewed from a distance on this trip are accessible for detailed examination via hikes of 1 to 2 hours. Time constraints preclude visiting more than a few of these exposures for detailed examination during this field trip, but others meriting such examination are identified for future reference. The staging area for this trip is the Double Diamond X Guest Ranch in the upper South Fork Shoshone River valley. The ranch is owned by Russ and Patsy Frazier. We warmly thank Russ and Patsy for their friendship and hospitality.