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The Great Salt Lake is merely a tiny remnant of the much larger Lake Bonneville. Lake Bonneville existed at various sizes from about 32,000 to 14,000 years ago. At is maximum Lake Bonneville covered much of western Utah and extended across the borders with Idaho and Nevada and reached a maximum depth of 1000 feet.

During this time, the climate was wetter and colder than the current climate. Lake Bonneville was a relatively fresh water lake that was filled with rain water, rivers, streams, and melting glaciers.
Image source: Utah Geologic Survey, Lake Bonneville (http://www.ugs.state.ut.us/online/PI-39/pi39pg01.htm)

For most of its history, Lake Bonneville was a terminal lake because and had no natural outlet to the ocean. It continued filling until about 16,800 when it reached the elevation of Red Rock Pass in Idaho. Once the lake water began flowing over this pass, the water rushed out eroding out about 375 feet of rock in less than a year. The lake level stabilized here for about 600 years then began falling to approximately the level it is at now as the climate became drier and hotter and evaporated the water out of it.

The evaporation removed the water from the lake, but left the small amounts of salts that are transported into the lake by the rivers that drain into it. Over the thousands of years, more salt is transported to the lake by the rivers and concentrated in the lake by the evaporation of the fresh water.

From the coordinates, ancient shorelines of Lake Bonneville can be seen on the hillside to the south. The shorelines are the horizontal terraces that ring the hillside. Four major shorelines have been recognized for Lake Bonneville.

They are:

Each represents a time period where the lake levels remained relatively constant for a long period to allow erosion to create a terrace shore. These shorelines can be seen on the hills throughout the entire region since the lake eroded a shoreline all along its shores. On the other side of the island you can view the shorelines at N 41 1.141 W 112 11.694. From this point you should be able to pick out some of the shorelines along the mountains to the east.

However, the shorelines on Antelope Island are about 150 feet higher than on the surrounding mountains because of a concept called isostatic rebound. The weight of the water in Lake Bonneville actually pressed the earth’s crust down. As the weight of the water was removed the earth’s crust came up again, increasing the elevation of the island. In the diagram substitute lake water for the ice.

If you take the hiking trail around to the point (at N 41 1.665 W 112 15.551) the trail passes across one of these ancient shore lines. The rocks on the trail change from the very angular typical of freshly broken rock to the smooth rounded typical of well weathered rocks on a beach.

Logging requirements:
Send me a note with :

1. The text "GCZ9C3 Ancient Shorelines of Lake Bonneville" on the first line
2. The number of people in your group.
3. The number of shorelines you can see on the mountain. Include the criteria you used to identify them.

The above information was compiled from the following sources:

• Willis, Grant C. et al, Geology of Antelope Island State Park, Utah in Geology of Utah’s Parks and Monuments, 2003 Utah Geological Association Publication 28 (second edition) D.A. Sprinkel, T.C. Chidsey, Jr. and P.B. Anderson, editors
• Utah Geological Survy, Lake Bonneville, PI-39 Commonly Asked Questions About Utah’s Great Salt Lake and Ancient Lake Bonneville, http://www.ugs.state.ut.us/online/PI-39/pi39pg01.htm
• Tamara Jo Wambeam, ISOSTATIC REBOUND AND LAKE BONNEVILLE, University of Utah, http://www.earth.utah.edu/geoantiquities/ancient-evidence/rebound.htm
• Utah Geological Survey, Great Salt Lake, http://www.ugs.state.ut.us/utahgeo/gsl/index.htm
• New World Encyclopedia, Glaciation, http://www.newworldencyclopedia.org/entry/Glaciation

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