The Mima Mounds
Mysterious mounds
Walk around weird, wildflower-covered hillocks in southwest Washington.
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 magnitudebefore agriculture and development encroached, the mounds extended a remarkable 20 square miles.
You can ponder the Mima Mounds' quirky mystery as well as Mima Prairie's unusual ecosystem at a 450-acre preserve set aside by the Washington State Department of Natural Resources. Call for a schedule of the Nature Conservancy's monthly guided nature walks to learn about the native prairieone of the rarest ecosystems in Washington.
If you'd rather explore on your own, start at the mound-shaped interpretative kiosk. From here, a second-level deck looks out over miles of neat, oblong hillocks covered with tawny grasses, purple camass, and blue violets. Listen for the bubbling, flute-like song of the western meadowlark, and watch for ground-nesting savannah sparrows, western bluebirds, and northern harriers.
From the kiosk, a network of easy hiking trails wend their way through the mounds. Start on the 1/2-mile paved, wheelchair-accessible nature path; from here you can branch onto a 2-mile loop trail. 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 examplehave been refuted.
Perhaps, after a visit, you'll come up with your own theory for these strange mounds.
More on the mounds:
WHERE: From I-5, take exit 95 and then Maytown Rd. SW west through Littlerock. After 3.8 miles, turn right on Waddell Creek Rd. Continue 1 mile to Mima Mounds Natural Area Preserve.
COST: Free.
CONTACT: Washington Department of Natural Resources: (360) 902-1600. Nature Conservancy: (360) 956-9713.
Original story by: Beth Geiger
Geology of the Prairies:
The last glacial advance into the Puget Sound Lowland, the Vashon Stade of the Fraser Glaciation, reached its maximum extent approximately 14,000 years ago. Deglaciation appears to have involved a succession of meltwater-channel and ice-margin-lake stages which probably lasted less than 1000 years. Glacial outwash was deposited by meltwater from the retreating ice mass in the meltwater-channels. The prairies of the Puget Sound Lowland are located on these glacial outwash deposits.
The high flow of the meltwater is reflected in the coarse texture of these glacial outwash deposits. The glacial outwash deposits of the Mima Prairie area consist primarily of well-sorted sands and gravels. The majority of the smaller silt- and clay-size particles remained in suspension in the meltwater and were deposited elsewhere. The gravelly Spanaway soils are found on the upper mounded terrace of Mima Prairie, and the gravel-free Nisqually soils are found on the lower, unmounded terrace surfaces.
Origin of the Prairies:
Conifer forests are thought to have dominated the area of the present prairies soon after the recession of the ice and the deposition of glacial outwash (Tsukada et al. 1981). This forested period was terminated on portions of the Puget Sound area by the development of drier and/or warmer climatic conditions. The climatic conditions may have started to become drier or warmer as early as 10,000 years ago.
During the drier and/or warmer conditions, the forest ecosystem was replaced by prairie ecosystems on portions of the glacial outwash terrain in the Puget Sound Lowland. The forest ecosystems were replaced by prairie ecosystems during this period of drier and/or warmer climatic conditions because of the doughtiness of these coarse-textured, somewhat excessively drained outwash deposits.
The climate appears to have remained drier and/or warmer until about 5,000-7,000 years ago when it began to moderate toward present conditions. The increased moisture and/or cooler temperatures associated with present climatic conditions have not resulted in a complete replacement of the prairie ecosystem for two major reasons: 1. Undisturbed prairie vegetation tends to completely cover the soil surface, minimizing suitable seedbed and limiting the establishment and survival of seedlings from adjacent forests; 2. Aboriginal burning minimized the encroachment of forest vegetation by destroying forest seedlings at the forest-prairie interface.
Soils of the Prairie:
The Spanaway Soil Series includes the soils formed under native prairie vegetation on somewhat excessively drained, gravelly outwash terraces (Soil Survey of Thurston County, Washington, 1991). The horizons that make up the profile of the unmounded Spanaway soil display a great deal of contrast in both color and texture. The A horizon at the surface of the Spanaway soil is a black gravelly sandy loam. The B horizon from 15 to 20 inches in depth is dark yellowish brown very gravelly sandy loam. The C horizon from 20 to 60 inches in depth is a dark yellowish brown extremely gravelly sand. The horizonation of the mounded Spanaway soil is dramatically different and will be discussed below.
The Spanaway A horizon's black color is primarily a result of the high charcoal content. The high charcoal content in the Spanaway A horizon is the result of centuries of aboriginal burning of the prairies. The charcoal contributes to the dark color of the Spanaway A horizon, but it is relatively inert and adds little to the nutrient status of this soil. Only small amounts of charcoal have penetrated to the lighter colored B and C horizons of the Spanaway soil.
Textural contrasts between the Spanaway soil horizons appear to have been influenced by differences in the rate of movement of the glacial meltwater that deposited them. The Spanaway C horizon formed in extremely gravelly glacial outwash material which was deposited from rapidly moving glacial meltwater. The upper soil horizons formed in finer textured outwash material (less gravel and more silts and clays) deposited later by slower moving meltwater.
Characteristics of the Mima Mounds:
The Mima Mounds of the Puget Sound Lowland are reported to vary from 8 to 70 feet in diameter and from 1 to 7 feet in height. The height of all the mounds within a specific area tends to be uniform. The mounds occur at densities as great as 8-10 per acre.
Viewed from above or in air photos, the mounds are generally circular to elliptical in outline; and certain groups of mounds show elongate and curving trends, apparently reflecting drainage patterns.
Cross-sections of the mounds and the adjacent intermound areas commonly reveal a biconvex outline of the black Spanaway A horizon. The A horizon commonly extends deeper into the extremely gravelly C horizon near the center of the mound, reaching thicknesses of as much as 7 feet. In contrast, the A horizon in the intermound area is commonly less than 6 inches thick.
The A horizon soil within the mound contains some small rounded pebbles, but is well mixed and show no stratification in contrast to the bedded, rounded outwash gravels in the C horizon below. Mound roots are extensions of the black A horizon soil into the bedded outwash gravel and have been observed and described under several mounds.
Origin of the Mima Mounds:
Many hypotheses have been proposed over the years to explain the origin of the Mima Mounds. Hypotheses on the origin of the Mima Mounds range in character from whimsical theories (i.e. "buffalo wallows" or "sucker nests") to credible theories with significant scientific support. Washburn (1988) prepared a critical review of the most promising hypotheses regarding the origin of the Mima Mounds and the reader who desires additional information on these hypotheses is encouraged to review this publication.
Washburn indicates that for Mima-like Mounds (that is, mounds apparently similar to the Mima Mounds of the Puget Sound Lowland, but occurring elsewhere) there are two hypotheses that are potentially widely acceptable, but remain to be proved. One of these hypotheses explains the Mima and Mima-like Mounds as the work of fossorial (burrowing) rodents, such as pocket gophers. The other hypothesis explains these mounds as the result of runoff erosion combined with vegetation anchoring.
Washburn feels that runoff erosion combined with vegetation anchoring may best explain the Mima Mounds. He suggests that a former forest existed and could have served as anchoring vegetation, retaining soil particles in place against the forces of erosion and creating the mounds, although other types of anchoring are not excluded.
The fossorial rodent hypothesis was primarily developed by Dalquest and Scheffer (1942). They cite the following evidence:
The Mima Mounds are constructed by entirely by soil materials small enough to be moved by gophers.
Materials too large to be moved by gophers appear beneath the mound or in the intermound region.
Mound roots extending into the gravel bed correspond to the size and shape of tunnels and nest excavations occupied by living gophers.
Mima Mounds are found only on prairies where gophers now live or quite certainly once lived, but are absent from prairies which, though geologically similar, yield no traces of gophers.
The characterization features of the mounds (namely, aerial distribution, size, and shape) are in conformity with the habits of pocket gophers.
The thickness of the black A horizon within the mounds (as much as 7 feet) is much greater than that on unmounded prairies (15 inches) or in the intermound areas (less than 6 inches). The greater thickness of the A horizon and the high degree of soil mixing within the mounds indicate that a major redistribution of A horizon soil material has occurred since the soil parent material was deposited by flowing water. These soil characteristics strongly support the fossorial rodent hypothesis for mound formation.
While the gopher theory is appealing, there remain several reasons to question whether it is correct, including:
No modern analogs of large mounds are known to have been formed by gophers;
The presence of gophers does not prove conclusively that the soil was mounded by gophers, the mounds may have been occupied after they were formed;
Many Spanaway soils occur in an unmounded phase and if the gopher population was as prolific as would not required, there is no known basis given for why gophers preferentially occupy only some areas.
Perhaps the most recent mound hypothesis is that of Berg (1990), who observed that small-scale Mima Mounds can be produced experimentally by subjecting a plywood board covered with a thin veneer of loess to impacts that produce vibrations in the board. Based on these observations, Berg suggests that Mima Mounds may have formed as a result of seismic activity in conjunction with unconsolidated fine sediments on a relatively rigid planar substratum.
Vegetation of the Mima Mounds:
Prairies on glacial outwash gravel occupy large areas of west-central Washington. These prairies, including the Mima Mound prairie became established about 9,000 yr. BP, prior to the entrance of Douglas fir into this area. Summer drought, initial low soil fertility and coarse-textured substrate, along with frequent fires enabled these prairies to exist. With fire suppression and reduced grazing, trees began to invade about 1850. With increase agriculture and disturbance, weedy species have invaded including: Scot's broom (Cytisus scoparius), St. John's wort (Hypericum perforatum), and Cat's ear Hypochaeris radicata).
The common native species include grasses Festuca idahoensis, Poa pratensis, and Agrostis diegoensis. Common forbs are Cammassis quamash, Dedecatheon hendersonii, and Viola adunca in the spring. Other common forbs include Achillea millefolium, Rumex acetosella, and the seedy species Hypericum perforatum and Hyopchaeris radiata. The mosses Rhacomitriuum canescens and Polythichem juniperinum and the lichen Cladonia mitis occur throughout.
In addition to natural fires, the Indians burned these prairies to aid the growth and abundance of the camass bulbs that provided an important source of starch. As you walk over the mounds, note some species are more abundant in the troughs between mounds and other species are more common in the slopes and tops of these mounds.
Timing of the Formation of the Mima Mounds:
This section follows Hallet and Sletten (1994).
Mima Prairie of western Washington is the type locality for conspicuous mounds composed of A horizon soil material that dot prairies in this area. These mounds range in height up to 7 ft and are spaced typically 30 ft apart. They are restricted to recessional outwash deposits dating to about 14 Ka following the maximum extent of the Vashon Stade of the Fraser glaciation. Assuming that soil formation was spatially uniform and that the A-horizon material was subsequently heaped into mounds, an estimated period of several thousand years must have elapsed between deglaciation and the onset of mound formation for a ~0.5 m thick A horizon to develop over the featureless gravelly substrate. This A-horizon thickness is similar to that found in unmounded prairies in the area, and it is sufficient to form mounds of realistic dimensions.
The conspicuous micro-relief characteristic of the mounds suggests that insufficient time has elapsed since their formation for significant degradation to occur due, for example, to soil creep, or that they are sustained in spite of erosive wear. A simple diffusion model of the mound relief suggests that, in the absence of constructional processes or of continued preferential erosion of low areas, the mounds "diffuse out" quickly, attaining heights only a tenth of their original heights within a few millennia.
This work favors formative mechanisms that either operated much more recently than the Vashon deglaciation or are continuing to operate. These include convergence due to fossorial rodent activity, and seismically induced soil mounding.
Significant soil development is expected to have occurred prior to soil mounding in order to account for the large volume of A horizon material (left column). If the soil were mounded prior to soil development, it is expected that the soil horizons would follow the surface in these well-drained soil.
insert diffusion model
Model of mound diffusion using diffusion coefficients typical of soil creep. The asymmetry of the mounds may be accounted for by slightly different diffusion coefficients on north and south facing slopes.
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