What It Is and Why It Is Important
While a visit to Whitman Mission may not initially scream out "Geology!", the Walla Walla Valley has a complex and interesting geologic history, a geologic history that even explains why Waiilatpu was such an attractive spot to the Whitmans when they were looking for a mission site back in 1836. Starting this week, the Photographer's Eye will take a two-part look at the park's geology. Part two will be posted in a few weeks.
The Whitmans traveled to the Oregon Country in order to bring Christianity to the peoples living here. They finally settled among the Cayuse. The Cayuse, though, like many northwestern tribes, followed a seasonal round of food gathering. The Whitmans realized it would be challenging to convey their message to a tribe that was only at their mission for part of the year. They hoped they could inspire the Cayuse to adopt a more settled lifestyle based on agriculture, which would then keep the Cayuse nearby year round for easier religious instruction.
So what does this all have to do with geology? In order to begin planting the crops that would sustain them, the Whitmans needed a site with fertile soils and an ample water supply. As anyone who lives in the Walla Walla Valley is aware, the valley is great for growing wheat, grapes, onions, and a variety of other crops. Dr. Whitman saw the fertility of this area also. He had noted the quality of various soils as his group traversed the continent. He wrote the following about this area: "We are located in the pleasantest valley of the upper Columbia on the Walla Walla River...The soil is better & more extensive on this than any other stream with which I am acquainted."
The reason why lies in the geology. If you look at the top few feet of soil just about anywhere in the Walla Walla area, chances are it looks similar to the picture at right: a dense, brownish-tan, fine grained soil, often with what look like vertical fractures or joints. This fine-grained silt, called "loess" (pronounced "luhs"), has formed since the last ice age ended about 15,000 years ago. During the last glaciation, enormous ice sheets covered North America as far south as Spokane and Seattle. As the ice advanced and retreated across the landscape, the weight of these immense glaciers and ice sheets pulverized and ground up bedrock into tiny particles. This broken down rock was then transported south by water melting out of the diminishing glaciers. Since the particles are so small, they are light enough to be carried by the wind, which has since spread the debris across much of the Walla Walla Valley, covering most of the area in a layer of loess that is normally several feet thick.
Were it not for the wind that created the thin veneers of loess, the Walla Walla Valley would be unsuitable for agriculture, because beneath the loess lies thick piles of volcanic rock. Volcanic rock doesn't easily form soils in arid climates such as we have here in eastern Washington. Additionally, unlike loess which has ideal hydrologic properties for growing crops, volcanic rock is generally extremely porous, meaning it can't retain enough water to nourish significant amounts of vegetation.
Without this loess the Whitmans would probably have decided to settle somewhere else and history may have taken a different course.
In part two, we'll talk more about the volcanic rocks underneath Whitman Mission and how some of the landforms in the park formed. Stay tuned!