Water was collecting in the basins, forming lakes of tremendous size. Lake Bonneville was one of the largest, growing to about the size of today's Lake Michigan. The lakes began to disappear as the last ice age came to a close, but left behind small remnants like the Great Salt Lake and Sevier Lake. Sevier Lake today is a playa lake, one that collects water in colder and wetter seasons, but dries up in warmer seasons. The evaporating water leaves behind vast stretches of salt flats.
Erosion strips down the mountains, and carries sediments down to the valleys creating alluvial fans.
These spread out from the canyons down to the valley floors and along with playa lakes, are classic geologic features of basin and range topography. Eventually the sediments are carried to the floor of the valleys, where they have accumulated in layers thousands of feet thick. The crust beneath the Basin and Range is still stretching today. Faults are active, mountains are pushing upwards, and basins are widening and filling with debris washed down from the high country. This landscape, which appears so everlasting, is actually in the midst of a geologic revolution, played out over millions of years.