Mima Mounds Washington State Natural Area Preserve

Subtle Beauty

Some landscapes, like the Grand Canyon or Yellowstone, just knock your socks off, and rightly so. Many of us have seen pictures of those magnificent grand landscapes or have visited them ourselves. Less celebrated, and often known only locally, are unique natural areas that are both ecologically rich and possessing abundant subtle beauty. Mima Mounds, located near the town of Little Rock in Thurston county, only a short distance south of Olympia (the state capital of Washington), is one such place. I was fortunate to be able to visit this landscape during the "golden hour" in the summers of 2018 and 2019, when the low rays of the evening sun spread a golden cloak over the dormant grasses.

A paved path through the natural area makes this special landscape more accessible while at the same time reducing erosion due to foot traffic.

Landscape of Uncertain Origin

The Mima Mounds is a landform called a mounded prairie. It was named in 1845 when the area was first being explored. (Mima rhymes with dime-ah) In 1966, this area of 637 acres was designated as a National Natural Landmark. The mounds are around 8 ft tall and 30 feet wide. More than thirty explanations have been proposed for their origin but none proven. At one time they were thought to be Native American graves but no human remains have ever been found. A few of the more likely explanations are listed below.

  • Were the mounds created by collection of gravel and stone deposited by the melting Vashon glacier, 14,000 years ago?
  • Did erosion of the gravelly sediments by glacial meltwater create them?
  • Were they created by shock-waves generated by earthquakes?
  • Are they the work of pocket gophers over hundreds of years as they leave behind mounds of material excavated from their tunnels?

A recent computer model suggests that a mature mound is about 500 to 700 years old when it fully developed by the incremental work of generations of gophers. The size of each mound roughly matches the territorial range of a single gopher. There are 17-30 mounds per acre. Today, hundreds of mounds are arranged in a regular pattern. However, perhaps, in the past, as many as 900,000 mounds existed in this region.

Low clouds fill the sky although rain is unlikely during the summer months. Without precipitation, the drought-tolerant grasses become dormant.
The low summer sun turns the grasses on the Mima Mounds a golden color as evening approaches.
Summer vegetation on the mounds: Common hare-bells (Campanula rotundifolia), Bracken fern (Pteridium aquiliium), Coastal Reindeer Lichen (Cladinia minis), Kinnickinnick or common bearberry (Arctostaphylos ova-ursi).

Where did the other mounds go?

Some mounds have been lost to development - to houses, roads, ranches and fields, where they may have been plowed under. However, much of the mounded prairie may have become forest during the last 100 years as a result of the suppression of fire. Before the modern era, prairie fires started by lightning strikes kept trees and shrubs from invading the grassland. Perhaps fires were also started by native peoples as a management practice to assure the continued availability of important food plants such as the Camas Lilly (Cammasia quamash). Charcoal found in soil samples of the mounds supports a history of recurrent fires. The lack of frequent burns has lead to encroachment of the mounded prairie by Douglas fir (Pseudotsuga menziesii), Western Hemlock (Tsuga plicate) and opportunistic hardwoods such as alders, birch, and poplars. Volunteers at the preserve regularly rogue out invasive plants such as Scotch broom (Cystisus scoparius).

Douglas fir dominates the top of a mound at the edge of the prairie. The unusual forking of the tree indicates that its growing point was damaged, conceivably by fire, when the tree was young.
A ring of Douglas fir stand in a circle on the edge of a mound. Perhaps these trees are the offspring of an older tree, long since decomposed, which grew in the center.
Golden sunlight reaches the Douglas fir trees where the prairie meets the forest.


All photos by Christine Stockwell