Evidence of ancient Lake Bonneville is prevalent throughout the region of the Wasatch Front. Shoreline deposits rim the valley floor and indicate a time when water levels were much higher than those of the present day Great Salt Lake.

Four main shorelines are associated with the fluctuating levels of the ancient lake. The Stansbury, Bonneville, Provo and Gilbert shorelines each mark a time when lake level remained constant long enough to deposit massive accumulations of sand and gravel.

The Stansbury is the oldest of these shorelines and is thought to have formed during an the early phase of the Bonneville transgression from about 21,000 to 20,000 radiocarbon years ago. The Stansbury shoreline can be seen above the junction of Victory Road and Beck Street just north of Salt Lake City and at the south end of Stansbury Island. This shoreline is often marked by a rocky shore platform with an apron of tufa-cemented gravel.

Lake Bonneville reached a maximum level of 5,090 feet above sea level approximately 15,000 years ago. At this time the lake was an open basin covering approximately 20,000 square miles and was over 1,100 feet deep. In the vicinity of Great Salt Lake, the Bonneville shoreline can now be seen at altitudes ranging from 5,160 to 5,230 feet due to isostatic rebound of the earth's crust. One of the best places to view the Bonneville shoreline is at "the great bar at Stockton" (Gilbert, 1890), located 6 miles southwest of Tooele. Also, the popular Bonneville Shoreline Trail is developed on the ancient deposits of the highstand of Lake Bonneville.

Approximately 14,500 radiocarbon years ago, the natural threshold of Lake Bonneville failed and a catastrophic flood occurred causing the lake to drop over 350 feet to the Provo shoreline level (~ 4740 feet). The Provo level is the most easily recognized shoreline feature throughout the valley and is distinguished by thick accumulations of tufa that formed nearshore during the 500 years that the lake was at this level.

About 14,000 years ago, the lake started to drop again due to changing climate conditions and by 12,000 years ago, the lake reached a level even lower than that of the modern day Great Salt Lake. A slight transgression or rise in lake level occurred about 10,900 to 10,300 years ago and formed the Gilbert shoreline. The Gilbert shoreline is the least conspicuous of the major shorelines but evidence of it remains at Antelope Island and in large coastal features, such as the Fingerpoint Spit near the Hogup Mountains.

REFERENCES

Burr, T. N. and Currey, D. R. (1988). The Stockton Bar. In In the Footsteps of G. K. Gilbert--Lake Bonneville and neotectonics of the Eastern Basin and Range Province. Guidebook for Field Trip

Twelve, The Geological society of America, p. 66-73.

Gilbert, G. K. (1890). Lake Bonneville: U. S. Geological Survey Monograph 1, 438 pp.

Green, S. A. and Currey, D. R. (1988). The Stansbury Shoreline and Other Transgressive Deposits of the Bonneville Lake Cycle. In In the Footsteps of G. K. Gilbert--Lake Bonneville and neotectonics of the Eastern Basin and Range Province. Guidebook for Field Trip Twelve, The Geological society of America, p. 55-57.

Oviatt, C. G., Currey, D. R. and Sack, D. (1992). Radiocarbon chronology of Lake Bonneville, Eastern Great Basin, U.S.A. Palaeogeogr., Palaeoclimatol., Palaeoecol., 99: 225-241.