Tuesday, May 8, 2012

Mima Mound Cross-Sections

I had some work last week in southwest Washington and took a little side trip to check out some prairies. I did the same thing during another trip this winter. Both trips involved getting very wet and very cold not only in my paid work but in my efforts to learn a little bit about this landscape that more typically is passed through on Interstate 5 with few or no stops. But on both trips I did get to see cross-sections of Mima Mounds. The first was an intentional cutting into a mound on the Mima Prairie that both Dave Tucker and Pat Pringle had alerted me to. 

Mima mound conveniently cut through showing the very thick organic soil (black) over non organic (tan) glacial outwash

Odd pocket of non organic soil surrounded by organic soil

Closer view of topsoil contact and the poorly sorted sediment

I checked out the pebbles to see the general makeup of the sediment. This area is a little outside my usual haunts so I am not sure what is typical glacial outwash composition in this area. Definitely saw andesite suggesting a Cascade volcanic source, but also saw greenstone and granite. Too nasty to site and do careful point counts. 

Purply andesite pebble

Metamorphic and granite pebbles

Back in January I spotted a cut through mound at Rocky Prairie. At this site the Douglas fir were invading the prairie.

Douglas fir capped mound cut by road

Same over thick organic layer of mixed organic silts and pebbles

I had a couple of theories regarding the mounds that I wanted to test out. I successfuly tested my theories. The theories failed the tests. More on the mounds at some future date.


Anonymous said...

I had a couple of theories regarding the mounds that I wanted to test out. I successfuly tested my theories. The theories failed the tests.

Yay for scientific method, and yay for being willing to get a "wrong" answer, and yay for honesty and integrity!

(Just had to argue with someone I know about what makes a "Theory" in science, and am still bruised. Your post reminds me what science is all about)

V. Hervet said...

I didn't know about the Mima mounds till about an hour ago, when I randomly read something about these. I looked up different pictures of the mounds on the internet as well as the surrounding area via Google Map. Looking at the mound x-section, we can see that the mounds are made of a very dark soil, on top of a horizontal rock bed. I think there are 2 possible explanations on how this soil was brought on top of this rock: 1) It was deposited on the rock into mounds; 2) A layer of soil was deposited on the rock, which then was eroded forming these mounds. I think we can exclude the previous hypotheses (e.g. the "sun cups" on ancient glacier that would have been filled with soil brought by the wind... is a quite unlikely hypothesis, sun cups are much smaller than these mounds, especially at this latitude, and when they would have been filled with soil then the ice between them would have started to meld and then get filled by soil there as well, etc). We can notice that there are many creeks around and the region is, or was, swampy. Considering how black this soil is, we can assume that there likely was a swamp there before and it’s how this layer of soil was created. Considering this, we still have to find out how this layer of soil eroded. Looking at the quite circular shapes of the mounds, wind erosion is unlikely (it would have created wavy patterns, or at least non-circular shapes, furthermore there is no mountain around that could have created a corridor of wind in this area…). I think the most likely explanation is water erosion, from when it was a swamp. It is not rare to see such mounds in large swamps with water streaming around. Maybe usually not quite as many nor so well scattered, but specific conditions like how large the swamp was, how fast the water streamed through it, etc, could explain this. To reinforce this hypothesis we can see that the rocks under and in the layer of soil has been rounded/flattened, like rocks that can be found in rivers. Pockets of these rocks in the soil can be explained by the water depositing these rocks while the layer of soil was formed. Just a few thoughts…

david cook said...

The mounds may be root balls and exfoliated bark of old growth trees that have decomposed completely to humus. They could be thousands of years old. In the Pacific North-west where I live old growth Douglas-fir (and to a lesser extent old growth western hemlock)create such mounds from their exfoliated bark and root balls.
David Cook