Mima Mounds and similar landforms
The geological process that created the rolling landscape known as Mima Mounds has been the subject of ongoing debate. Although named after a prairie in Washington State, the structures are seen in many other areas of North America (i.e.. Texas), China and Australia.
Mima Prairie, WA Photo by Dave Schiefelbein
There are several features which can be associated with the Carolina bay landforms, not the least of which is the repetitive geographic distribution. Another notable similarity is that there seems to be no new ones being created. The Mina Mounds of Washington are dated from 7,000 to 9,000 BCE, after the retreat of the glaciers in the area. Although glaciers area considered a possible contributor, we note that that would not have been an effective agent in Texas.
By Beth Geiger, published on line in Sunset
One of Earth's strangest landscapes can be found in the humble pastures near Littlerock, Washington, 12 miles south of Olympia. The Mima Mounds, at Mima Prairie, look like a sea of giant, half-buried bowling balls 8 feet tall and 30 feet across.
"There's no obvious reason why they should be there," says University of Washington geology professor Bernard Hallet. Indeed, after two centuries of speculation, scientists are still baffled by the mounds' origin and magnitude—before agriculture and development encroached, the mounds extended a remarkable 20 square miles.
As you explore, consider the two most viable current theories for the mounds' formation: Some geologists believe that violent earthquakes shook the loose prairie soils into neat heaps. Zoologists have also studied this area extensively; some believe that ancient potato-size pocket gophers created the mounds over generations of frenzied territorial construction. Most other theories—that these are Native American burial mounds, for example—have been refuted.
The Puget Sound area of Washington is home to many of these landforms; they stand out best on the flat - level and un-plowed plains (where they are left!). This is, again, a similar situation with the Carolina bays, as they are typically found on level-fat terrain which befits their thin-veneer structure. Here is a Google Earth composite snapshot, along with a GE place mark to locate the site. The view is ~350 meters across. For reference, we suggest the reader access the Washington State Universities' Quaternary Research Center, which had done some thorough soil research in this area.
Mima Mound View keyhole file
Mima Mounds in GE
Included below is a beautiful farmland b&w photo from the early 1900's, showing the expanse of the formations. This graphically depicts the mound as having a Perigee: Zero generic ejecta shape: the teardrop. The representation allows for a directionality to be deduced.
Close up aerial view of Mima Prairie mounds. Note directionally preferenced teardrop
shapes of these particular mounds (not all have this feature). *** If the reader has any information relating to the source of this photo, we would appreciate hearing from you.
Further west, another broad expanse of Washington State is studded with these mounds - the area around Manastash Ridge. This landscape is far more diverse in elevation, and the setting offers an opportunity to view the mounds in different settings. Our proposal is that these mounds of unconsolidated and unstratified gravels and soils are the emplacements of ejecta splash form a distant PZ cratering event. As such, their low angle of arrival should correlate to the emplacement sites. For instance, in deep ravines, there may only be emplacements on one side if the ravine runs normal to the trajectory line. On steep slopes, the ejecta may break up and slough off, leaving a different signature behind. On rolling terrain, we may see "shadows", as the ejecta pattern is distributed on the forward facing and top areas of the hills.
Given these facets to search for, Google Earth is employed as an exploration tool. Our first graphic shows the general Mima Mound formation in the area of the Manastash Ridge. Note the present of oriented dunes in the vicinity.
Mima Mounds and dunes
As we noted, this area is far from "flat". However, the viewer should note that the GE graphic has a 3x elevation exaggeration. As we look up into various gullies, we expect to see a variety of mound emplacement scenarios displayed. Our next example shows a slope with mounds that are sloughing off down a steeper grade.
The "shadow" effect can be seen in the next GE graphic, where the rolling hill in the foreground is covered with mounds, but the back slope is bare. The mounds again appear on the higher areas of the next rolling ridge. We interpret the arrival azimuth as coming from behind the "camera".
We would expect that an arriving droplet of ejecta that strikes "over the edge" of a slope would become elongated, as a function of the increased angle of incidence. The next graphic presents our proposal for such a scenario, where the structures are more easily identified as being "aligned", given their more oval profiles.
George W. Cox and Jodee Hunt, in their paper Nature and origin of stone stripes on the Columbia Plateau discuss an anomalous correlated structure - mima mounds on ridge tops, connected to long down-slope ridges of stones:
Beds of size-sorted stones forming stripes perpendicular to the contour are conspicuous on hillsides of the Columbia Plateau. Stripes occur on terrain ranging from 0" to about 30" in steepness, often beginning amongMima-type mounds on mesa tops and extending downward onto steep, unmounded slopes
Here is a supporting graphic. Our interpretation is that the bright spot at the top of the down-trace is a disrupted mina mound, unable to hold it's shape due to the steepness of this slope.
In some valleys, we note the down-slope ridges asymmetrically distributed between the two faces, interpreted by us as a sign of the arrival angle.
The proof would, of course, require significant field work to properly identify the structures and their alignments and correlations. Within the scope of our work herin, we feel the data is highly supportive of the proposed Mima Mound morphology.
The Manastash Ridge Keyhole file
As is the situation with Carolina bays, the mima mound geology is seen in numerous locations, with a variety of names and attributions. Here is a map of suggested mima mound (or "pimple mound" structures in the US.
From Science Frontiers #119, SEP-OCT 1998. © 1998-2000 William R. Corliss
The mounds, being comprised of good soils in regions that are otherwise lacking, have become sources of farming land and have been destroyed and obliterated as a result. Presented next is a next photograph from 1928 showing the area currently occupied by San Diego State University.
From Current Distribution and Historical Extent of Vernal Pools in Southern California and Northern Baja California, Mexico
by ELLEN T. BAUDER AND SCOTT MCMILLAN
The most-reference paper discussing various solutions to the enigma is A.L.Washburn's 1988 report Mima Mounds, An evaluation of proposed origins with special reference to the Puget lowlands. Washington Division of Geology and Earth Resources Report of Investigations 29, 53 p.
General Pimple Mound References by Paul V. Heinrich