The Palouse is a geographically distinct region in the northwestern United States, stretching across southeastern Washington and into northern Idaho. It is recognized for its remarkably uniform, steep, rolling hills, which create a striking visual landscape. This unique topography is composed of fertile soil sculpted by ancient forces of nature. The formation of these hills is rooted in deep geological history, involving volcanic eruptions, massive ice sheets, and persistent wind action, resulting in a major agricultural area.
Setting the Stage: The Basalt Bedrock
The foundation for the Palouse hills was laid millions of years ago, long before any soil began to accumulate. The entire region rests upon the Columbia River Basalt Group (CRBG), a vast series of flood basalt eruptions that occurred primarily between 17 and 6 million years ago. These eruptions involved enormous volumes of molten rock that poured out of fissures, covering over 81,000 square miles across the Pacific Northwest. This continuous, durable igneous rock created a relatively flat and deep base layer. This bedrock provided the necessary structural integrity for the entire Columbia Plateau, establishing a low-relief surface over which subsequent sediments could be deposited and shaped.
The Building Material: Glacial Silt and Loess
The material that forms the hills is a fine, wind-blown silt known as loess, not soil in the traditional sense. The origin of this material is tied to the massive continental glaciations of the Pleistocene Epoch. As immense ice sheets advanced and retreated, they ground bedrock into fine, pulverized mineral sediment. Glacial meltwater and powerful outwash streams carried this fine sediment away, depositing it in vast floodplains across the Columbia Plateau. A significant source was the sediment left behind by the cataclysmic Missoula Floods. When these areas dried out during colder, drier periods, the exposed silt was highly susceptible to wind erosion and was lofted into the atmosphere by strong winds, beginning the process of transport and deposition.
Sculpting the Palouse Hills
The final, defining step in the Palouse’s formation was the persistent action of wind, which deposited the airborne silt in massive quantities. Prevailing southwesterly winds carried the loess northeastward, resulting in layers that are incredibly thick, reaching up to 250 feet deep in the eastern parts of the region. The characteristic shape of the hills is due to the continuous airfall of silt, giving them a dune-like appearance rather than being shaped by water erosion. The wind deposited more silt on the downwind, northeast-facing slopes, creating asymmetrical hills with a gentler southwest side and a steeper northeast side. The hills are composed of multiple layers of loess separated by ancient, buried soils called paleosols, which formed during warmer interglacial periods. This layering confirms that the hills accumulated episodically over hundreds of thousands of years, resulting in fine, uniform loess highly valued for its agricultural productivity and moisture retention.