Friends of the Pleistocene

Lake Bonneville and the Making of the Bonneville Salt Flats

The Bonneville Salt Flats are one of the flattest places on Earth — a blinding white plain where the curvature of the planet is visible along the horizon. They are also a fossil. The salt crust is the residue of Lake Bonneville, the largest of the Pleistocene lakes of the American West, which filled much of western Utah during the last Ice Age.

A lake the size of a modern sea

At its height, around 18,000 years ago, Lake Bonneville covered roughly 20,000 square miles and reached depths of more than 1,000 feet. Its surface stood over 5,000 feet above sea level, lapping against the flanks of ranges that are now bone dry. The Great Salt Lake, shallow and shrinking, is only a remnant of that former inland sea.

The Bonneville Flood

The lake did not simply evaporate. Around 17,000 years ago it rose high enough to spill over a divide at Red Rock Pass in southern Idaho. The outlet cut down rapidly through soft sediment, and the lake released a catastrophic flood — one of the largest known in the geologic record. In a matter of weeks, an immense volume of water surged north into the Snake River, carving canyons and stranding house-sized boulders far downstream. When the flood stabilized on bedrock, the lake had dropped by about 350 feet to a new level geologists call the Provo shoreline.

From water to salt

After the flood, the climate warmed and dried. Evaporation outpaced inflow, and Lake Bonneville retreated toward its lowest basins. As the water disappeared it left its dissolved minerals behind, concentrating them into the salt crust we see today.

  • The salt flats are mostly common table salt (halite), with gypsum and other minerals.
  • The crust is seasonally recharged: winter water dissolves and re-levels it, summer sun bakes it dry.
  • Old shorelines are still visible as terraces on the surrounding mountains.

Standing on the lakebed

To walk out onto the flats is to walk across the deepest floor of a vanished lake. The perfect flatness is not an accident — it is the settled bottom of standing water, planed level over thousands of years. The Pleistocene did not end cleanly; it left its edges and floors written across the landscape, waiting to be read.