Geology PhD candidate Mike Mohr spent a week this summer rafting through the Grand Canyon with a team of 16 collaborating geologists from University of New Mexico, Utah State University, University of Las Vegas Nevada, Denver Museum of Science, and Boise State University studying the Grand Canyon’s Tonto Group. The Tonto Group is a Cambrian package of sedimentary rocks directly overlying the Great Unconformity that contain clues of geologic and biologic changes that occurred ~500 million years ago—a time known for the Cambrian explosion. Mike sampled sandstones and fine sandy layers within shales and limestones to extract detrital zircons for U-Pb geochronology. Mike works with Dr. Mark Schmitz in the BSU Isotope Geology Laboratory, where he measures geologic time with the mineral zircon. Wielding this technique on the samples from the Grand Canyon, Mike and his collaborators will refine the timing of transgression (sea level rise) that lead to deposition of the Tonto Group, as well as refine the timing of biostratigraphic zones of trilobite fossils which are used to calibrate the Geologic Time Scale.