Fast-foward to 15 million years ago, when large eruptions of iron-rich lava engulfed the Palouse. The lava covered everything except the highest buttes, killing all the plants and animals in its wake. These eruptions formed layers of dark rock called basalt, which is still visible in many places today.
1.6 million years ago was the beginning of the Ice Age. From then up until only 10,000 years ago, glaciers covered the Palouse, reaching down from Canada. During this Ice Age, strong winds blew across the Palouse, carrying large volumes of silt. This silt, ground to a fine powder by glaciers, is known as loess. Most of the world’s best agricultural soils are in loess. When this silt settled, it formed the rolling, dune-like hills of the Palouse, with their rich topsoil.
Comments compiled by Chip Phillips: http://www.photocascadia.com/blog/geological-history-of-the-palouse-by-chip-phillips/