This report describes the construction and operation of a mobile gamma ray logging system used to locate areas and structures containing elevated levels of natural uranium decay chain radionuclides. The detector system consists of a sodium iodide crystal coupled to four photomultiplier tubes and associated electronic system which permits both an audible indication and a strip chart recording of gamma ray photon count rates. Under normal operations, the detector assembly is mounted in a van at about two meters above the ground surface and gamma ray logs are obtained while travelling at about 16 kilometers per hour (10 mph). This system has been used extensively in the Pocatello and Soda Springs, Idaho areas and identified about 1900 locations where phosphate slag has been used for various construction purposes. The system has also been used in several other communities to identify locations where uranium mill tailings or pumice, containing slightly elevated concentrations of natural radioactivity, was used for construction purposes.