While walking along the River Raisin near Blissfield, Michigan, I came across this sun-baked stretch of riverbank. This particular location floods often, and it’s usually wet and muddy here. In the late summer it can completely dry out, leaving the thick mud parched and cracked. Observations here reminded me of a number of fossils I’ve seen from tens and hundreds of millions of years ago. It made me realize this location is perhaps actively in the process of preserving new fossils. Flooding here introduces new sediments, which bury existing sediments. Over time, this burial and compaction can lead to lithification (formation of rock) and the preservation of whatever is in the sediments. While fossilized mud cracks are perhaps being preserved, there are other potential fossils-in-the-making.
Looking at this black willow leaf (Salix nigra, Salicaceae) pressed into the dried mud, it’s easy to see the similarity with a number of plant fossils. If it gets buried before it has a chance to decompose, its impression can be sealed into the mud. Add some additional sediments, burial, compaction, and a few million years, and voilà, you could have a plant fossil.
Animals tracks can be preserved as ichnofossils. These dried footprints could be filled with silt and mud in the next flood. Drying of that new layer could then preserve both the positive imprints and negative casts of the tracks. Additional burial and lithification could make the record more permanent. In the distant future these tracks could provide clues about the animals that walked here today.
Over time the sediments here could form some type of mudstone, a product of the frequent influx of flood-borne mud alternating with dry periods. Between the mud cracks, leaf imprints, bird tracks, and raccoon tracks, a geologist in the future could begin to decipher the ecology of this location. Wooded, hosting vertebrates, and muddy, just as it is today.