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.