MEASURING ORIENTATIONS OF ROCK FRACTURES IN INTRODUCTORY GEOLOGY FIELD TRIPS

The hypothesis that solar heating is a leading cause of fracturing and mechanical weathering of desert sediments has been recently revived (L. McFadden, et al, 2000, 2002). This hypothesis provides an excellent framework for using "discovery learning" to teach various aspects of the scientific process including use of a geologic compass, data recording and reduction, interpretation, and hypothesis formulation. The exercise has been used a part of an introductory-level field trip to the mouth of Clark's Fork Canyon (Northwestern Bighorn Basin, Wyoming) where, among other features, students observe a glacial moraine. At this stop students are grouped into pairs, and each pair is given an instruction sheet (which includes a table for recording data) and a geologic compass. Following a brief demonstration on the use of a geologic compass to measure azimuths of vertical and near-vertical fractures on boulders, the student pairs are sent off in different directions with instructions to each measure fracture orientations on 20 boulders (ten per student) and to take about 20 minutes to complete the task. Instructions also request a basic lithologic identification of each boulder studied. As part of their follow-up report, students plot fracture orientations into a rose diagram, and then speculate on the origin or cause of the fractures by formulating a hypothesis. The rose diagrams can be readily photocopied and compiled onto a few overhead transparencies and the students discuss their hypotheses in class the following week. Some of the more imaginative hypotheses, for example, are that stresses were caused by reversals of the Earth's magnetic field, or by impacts of boulders rolling eastward down the front of the glacier. Reports and discussions often reveal misconceptions. Rose diagrams from three different field trips show obvious north-south fracture orientations, quite consistent with results of McFadden, et al (2002), and some even show bimodal distributions attributed to seasonal variations in the solar azimuth.