Tuesday, December 27, 2011

Geologic Pilgrimage: Mima Mounds

Mima Mounds

Early this fall I took a side trip pilgrimage to the Mima Mounds. Amazingly and to some extent embarrassing that I had never gone before. It really does not take that much time to leave I-5 south of Olympia and check the mounds out. For geologists the mounds are great fun because there have been no end of ideas on how the mounds formed. A perplexing problem that I will not hazard adding to at this time. It is my understanding that David Tucker will be including a chapter in his much anticipated field trip guide book nwgeology/my-book. He even has a great picture of a mound in cross section from the gravel quarry adjacent to the mounds, but alas not written it up on his blog. Washburn (1988) provides an over view of the mounds and an evaluation of the reasonable theories. I am not sharing my preferred theory, but I will say that I am in agreement with Washburn that there is no grand mound theory for all the various mound sites scattered around the country. 

While the Mima Mounds themselves are of great curiosity to geologists, there is another geologic aspect to this site. The Mima Mounds are located in Mima Prairie. Yep, we have grass land prairies in western Washington. The geology aspect of the Mima Prairie and some of the other prairies in western Washington is the very gravely glacial outwash underlying the site. So despite a wet climate, this area is exceedingly well drained and the substrate does not hold water. There are other factors as well that causes one to wonder about the anthropocene and the role of humans in regards to ecosystems.

The open prairie grass land ecosystem found at the Mima Mounds has become greatly diminished over the past 200 years. The Mima Mounds are a Natural Area Preserve managed by the Washington State Department of Natural Resources. The mounds themselves are worthy of protection, but the open grass land ecosystem has become an endangered ecosystem with a variety of rare plants and animals found only in these prairie areas. There are other mound sites nearby the Mima Mounds, but the ecosystem is in a better condition at Mima Prairie.


Riley said...

As a Thurston County native, the Mima Mounds were always one of my favorite hikes. A truly spectacular place.

Bob_Aegerter said...


My favorite theory is the one about the mounds being the result of shaking by an earthquake, the way sawdust segregates on a sheet of plywood when cut evenly with a hand saw.

I look forward to your discussion of the several theories.

Dan McShane said...

From Pat Pringle:

Some new evidence has come to light over the past decade or so. The Mima Mounds sit atop highest post-glacial "outwash" terrace, but themselves are composed of sediment that is more like a massive mudflow matrix. Those Mima Mounds along SR 99 between Olympia and Tenino are loaded with andesite, Mount Rainier rock. The sediment appears to be a remnant from a debris flow accompanying the breaching of an ice-dammed lake along the east side of the Puget Lobe glacier (~16,000 yr ago)--we called it Glacial Lake Carbon. The lake partially filled the Carbon River valley and nearby valleys. When it breached (at least once) it was a sort of mini Missoula Flood. The debris flow part may be the result of a landslide into the floodway--we know the deposits that slid in included till and volcaniclastics rich in andesite both from modern Mount Rainier and from the Lily Creek Formation (proto-Mount Rainier, ~1 Ma). Now what made the actual mounds out of the debris flow debris?

Barry and I have been poking around systematically mapping this for a decades, and a few of his students have done their BS theses on it too. Here's one abstract:

Pringle, Patrick T.; Goldstein, Barry S., 2002, Deposits, erosional features, and flow characteristics of the late-glacial Tanwax Creek-Ohop Creek Valley flood—A likely source for sediments composing the Mima Mounds, Puget Lowland, Washington [abstract]: Geological Society of America Abstracts with Programs, v. 34, no. 5, p. A-89. [Accessed on June 7, 2008 at http://gsa.confex.com/gsa/2002CD/finalprogram/abstract_35127.htm ]

Anonymous said...

Nobody will ever be able to convince me that Mima Mounds isn't the work of giant prehistoric woolly gophers.