Lake Bonneville and the Bench Beneath the City
Near Provo, Utah
Look east from almost anywhere in Provo and you can read geology in the hillside. About halfway up the Wasatch front, above the last houses and below the steeper rock faces, there is a horizontal line, a faint terrace, a notch cut into the slope. That line is the Provo terrace, the ancient shoreline of Lake Bonneville, and BYU's campus is built on top of it.
Lake Bonneville was not a small lake. At its peak, between thirty thousand and fourteen thousand years ago, it covered twenty thousand square miles of western Utah and stretched into parts of Nevada and Idaho. Its deepest points were nearly one thousand feet.
The entire valley floor here was submerged. About seventeen thousand five hundred years ago, a natural dam at the northern edge of the basin failed. The lake drained catastrophically through what is now southern Idaho, dropping roughly four hundred feet in a matter of months before stabilizing at a new level, the Provo level.
That second shoreline cut a bench into the rock at the elevation where the university now sits. When the lake finally evaporated, the weight of the water had been so great that the land itself had been pressed down. As the water left, the crust slowly rebounded, meaning that what you see as a level bench is actually slightly tilted, rising toward the center of the old basin.
The mountain has been rising for fourteen thousand years, imperceptibly, making up for what the lake took.
