SELF-GUIDED GEOLOGY TRAIL AT CITY OF ROCKS NATIONAL RESERVE, SOUTHERN IDAHO

The City of Rocks National Reserve showcases a spectacular landscape of granite spires eroded from the 28 MA Almo pluton and 2.5 GA Green Creek Complex. Outcrops within the reserve display textbook examples of weathering phenomena such as case hardened crusts, tafoni, panholes, and exfoliation. To date, interpretive resources have focused on the human history of the area, especially the California trail, and not on the unique geology that gives the reserve its name. This situation is largely the result of the absence of layperson accessible information on the geology of the area and the lack of studies that focus on the evolution of the region’s peculiar landforms. Keck Geology Consortium research projects were conducted at the City of Rocks National Reserve and adjacent Castle Rocks State Park during the summers of 2001 and 2002 with the goal of providing easily digested information to park management that could be used to communicate the geology to park visitors. During the course of these studies, a search was made for the best location for a self-guided interpretive trail. The ideal trail should not only lead a hiker past excellent exposures of all of the weathering phenomena listed above but should also feature easily-recognizable examples of intrusive contacts, dikes, joints, and hydrothermal alteration. In addition to these geological criteria, the trail should be located and constructed to enhance accessible and avoid potential damage to park resources. After careful consideration of these factors, a route was located a short distance from the Circle Creek overlook parking area near the intrusive contact of the Almo pluton with the Green Creek Complex. The trail route forms an approximately 1.5 km loop through scattered inselbergs in low-relief terrain. When completed, the trail will feature approximately 12 numbered stops that are tied to a trail guide available at park headquarters. Digital photographs of stops on the trail combined with text and figures from the trail guide will be used to construct a virtual geology trail linked to the reserve’s web site.