Friday, December 31, 2021

Stromatolites in the snow

To close out 2021, here are some stromatolites in the snow.

A big stromatolitic eye; no scale bars today (I couldn't reach this one anyway, but it's a couple of feet across).

At this location there's a large wall of Prairie du Chien Group rocks that was exposed a few years ago during the construction of a bike trail. The bike trail descends from north to south, exposing more of the section as you go until a few dozen feet of the Lower Ordovician are visible. There is a distinct stratigraphic break near the top of the section, and just above this break is a stromatolitic interval with some large colonies, like the eye-shaped example above. I haven't seen any definite fossils below the break, but there might be some (there are numerous small voids, at least some of which could represent dissolved shells). There are some nice sedimentary structures, though, such as cross-bedding and planar bedding (sand grains in the carbonate).

The center of this photo shows a sort of bi-lobed pillow that smooths out into a single mound. It is not far from the eye-like stromatolite in the previous photo. In both cases, the large colony is about a foot above the stratigraphic break.

There are also some blocks that have fallen. In fact, I suspect that the stromatolitic interval is more prone to shedding blocks than the rest of the interval, because the stromatolitic interval has more opportunities for fracturing.


  1. Found my second crinoid in fall. First was a tiny barely 2 inches long but remarkable complete Ohiocrinus attached to a brachiopod. The second one I'm not sure of yet - have lichenocrinus holdfast, few if any plateshead with feeding tentacles coming from the attachment to the stem. Don't know if feeding tentacles are preserved or absent yet as a lot of cleaning need to be done.

    1. Congratulations! I hadn't heard of Ohiocrinus being out this way.