This bulletin serves not only to introduce the non-geologist to the rich geology of Millard County, but also to provide professional geologists with technical information on the stratigraphy, paleontology, and structural geology of the county. Millard County is unique among Utah’s counties in that it contains an exceptionally complete billion-year geologic record. This happened because until about 200 million years ago the area of present-day Millard County lay near sea level and was awash in shallow marine waters on a continental shelf upon which a stack of fossil-bearing strata more than 6 miles (10 km) thick slowly accumulated. This bulletin summarizes what is known about these strata, as well as younger rocks and surficial deposits in the county, and provides references to scientific papers that describe them in greater detail. Mountains North 30 x 60 (1:100,000-scale) quadrangles. These companion maps and this bulletin portray the geology of Millard County more completely and accurately than any previously published work.

General geology papers and road logs for the Millenium Field Conference in Utah.

Geologic exposures in the Salt Lake City region record a long history of sedimentation and tectonic activity extending back to the Precambrian Era. Today, the city lies above a deep, sediment-filled basin flanked by two uplifted range blocks, the Wasatch Range and the Oquirrh Mountains. The Wasatch Range is the easternmost expression of major Basin and Range extension in north-central Utah and is bounded on the west by the Wasatch fault zone (WFZ), a major zone of active normal faulting. During the late Pleistocene Epoch, the Salt Lake City region was dominated by a succession of inter-basin lakes. Lake Bonneville was the last and probably the largest of these lakes. By 11,000 yr BP, Lake Bonneville had receded to approximately the size of the present Great Salt Lake.