Our interdisciplinary team of a geologist and volunteers, working with the City of Scottsdale and the McDowell Sonoran Conservancy (MSC), has confirmed that an unusual rock outcropping in the McDowell Mountains within the McDowell Sonoran Preserve in Scottsdale is travertine, a form of limestone. Limestone has not been identified previously in the McDowell Mountains or in the metro Phoenix area. This research is part of a coordinated effort on the part of MSC and the City of Scottsdale’s Preservation Division to use the talents and energy of MSC volunteers to better understand the geologic setting of the McDowell Sonoran Preserve. Travertine is a whitish sedimentary rock consisting of calcium carbonate. Travertine forms when calcium carbonate is deposited by mineral springs. There also is evidence of possible plant fossils and algal residue associated with the travertine. Our preliminary analysis indicates that the travertine was deposited 2 to 20 million years ago, during the formation of the McDowell Mountains.