Tree-ring dating in several avalanche tracks in Galena Creek valley, northern Absaroka Mountains, Wyoming, is used to determine the frequency of large snow avalanches that pass below the forest line. The following criteria are used: (1) datable scars on the trees, (2) changes in growth-ring pattern from concentric to eccentric, caused by tilting, (3) changes in growth rate due to increase in photosynthesis when adjacent trees are destroyed, and (4) age of trees within a given reforested avalanche track. The first two of these criteria are most reliable. Many young trees within the avalanche tracks are protected by snow during avalanches, and they thus survive to reforest the track immediately following destruction of the larger trees. Above the forest line, avalanche boulder tongues are one of the most reliable indicators of persistent activity in alpine regions, for they are formed over a period of many years by the accumulation of debris swept out of avalanche chutes. A peculiar linear feature on the surface of the tongues is the avalanche debris tail, which consists of fine debris that was deposited by large snow avalanches downslope 5 to 10 m from a large boulder. They are thought to be formed by a mechanism similar to that by which sand shadows are formed in the lee of obstacles in river channels or on desert dunes.

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.