I recently made a geologic pilgrimage of sorts. As I was on one of my crazy trips for work (Port Discovery and Port Ludlow followed by a drive down the west shore of Hood Canal and then across White Pass to Kennewick), I made a minor detour and visited the Mima Mounds south of Olympia (or west of Littlerock).
The Mima Mounds are a landscape of mounds that are on the order of 2 to 6 feet high that cover a swath of glacial outwash gravels. A whole range of theories have been put forth on how the mounds formed. My own take on the mounds is that geologists have had a great time coming up with theories on the Mima Mounds and some other mound landscapes as well. One of the great things about the Mima Mounds is the theories. They have inspired creative thinking and great joy that there are landscapes that we still do not fully understand.
But this post is not about the Mima Mounds. That is for another day. This post is to note that there are lots of other mound sites in Washington State - particularly on the east side of the Cascades. One of my favorite routes in Washington State is along the upper, high portion of the northern side of the Columbia Basin between Wenatchee and Spokane. Lots of great geology and landscapes including several areas with mounds that are very similar in size to the Mima Mounds.
Mounds Along Highway 174 northeast of Grand Coulee
An approximately 3-foot high mound
This particular mound site is located on the northeast portion of the Watterville Plateau. I spotted these mounds just after reaching the plateau top above the Columbia River valley west of Grand Coulee Dam. This site has been glaciated by the Okanogan ice lobe and is underlain by thin glacial till over basalt bedrock.
A cursory read about various mound sites in western North America suggests that there is no single explanation for the mounds and a remarkable variety of explanations for each mound site are often proposed. This mound site on the northeast edge of the Watterville seems to be no different - it seems that a variety of explanations or theories are possible. A scattered number of basalt boulders left by the ice lobe in the vicinity suggests one explanation, but I would not want to place any bets. And perhaps the mounds at this site actually were formed in various ways but with a similar end result.
Basalt boulders on the Waterville Plateau
Of course there are anthropogenic explanations as well. The recently formed mounds pictured below are located a few miles west of the mounds pictured above.
Mounds in plowed field on the northern Waterville Plateau
And then there are the mounds in southern Wisconsin and other Midwestern sites. The mounds in Beloit, Wisconsin are similar in size. Prior to "development" of the area there were thousands of similar mounds in Wisconsin and elsewhere in the Midwest. Most, like the ones pictured below, are similar in size to the Mima Mounds and the mounds scattered around sites in eastern Washington. The Beloit mounds had some ritual meaning to the local First Nations. And perhaps a message to geologists that not all mounds were formed in the same way.
Mounds in the lawn, Beloit, Wisconsin.