The purpose of this research project is to determine the origin of the materials used to construct the Black Hills Dam in order to restore the landscape to pre-dam conditions. The Black Hills Dam site is located in northern Scottsdale, Maricopa County, at 33.75° North, 111.80° West. The goals of this project are to characterize the surficial deposits and local geology of the dam site. This report presents our findings, interpretations and conclusions based on background research, a site visit to the dam site, and technical discussions with the City of Scottsdale engineer and planners.

A series of small earth movements occurred along the slopes of State Route 87 at about mile marker 224 (between the Bush Highway and Route 188) throughout the winter of 2007‐2008, culminating with a landslide on Friday, 21 March 2008. This landslide buckled the southbound lanes, displaced the northbound lanes, and closed the highway for nearly a week. The mass movements occurred on slopes that were constructed with re‐vegetated, laid back slopes; soil nail walls; and rip rap‐lined channels. However, our reconnaissance mapping indicates that most, if not all, of the slope movements are located within a much larger, older landslide adjacent to, and cut by, SR‐87. No specific trigger for the landslides was immediately apparent, although a combination of factors (e.g., precipitation, groundwater levels, etc.) may have contributed to conditions for the slope failure. Headscarps of various types of landslides are present along both sides of the highway. Results from our preliminary investigation indicate that up to three other paleo‐landslides may be present in the immediate vicinity. The full extent and nature of these landslides are unknown and require further investigation to evaluate their potential to be reactivated and risk they pose to the highway.

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.