Yellow Creek Rhizoliths
Thank you to Daviess County Parks & Recreation for permissions to place this Earthcache.
Geography, Topography, and Geologic Time
This Earthcache brings you to Yellow Creek Park. The channel of Yellow Creek here is slowly dissecting (eroding) the Quaternary geology here, consisting of river, lake, and dune deposits. When we talk about Quaternary geology, we are talking about the time period when the deposition of the sediments here occurred. The Quaternary Period is the most recent period in geologic history, beginning about 2.58 million years (Ma) ago. The period is divided into two epochs, the Pleistocene, beginning at 2.58 Ma and most recent Holocene, which began approximately 12 thousand years ago (Ka). The map below points out your location and your relationship with the geologic units, in this case, layers of sediment, at this site.
Geology
At your current location you are looking at 3 separate geologic units…
- Qal, the modern alluvium, deposited during the Holocene, is the unit forming the creel basin for Yellow Creek. Alluvium is the product of river sediment deposition, and consists of sands, silts, and gravels.
- Qlt, lacustrine terrace, is the unit making up the steep banks of the creek. This clayey silt, deposited during the Pleistocene, is the result of glacial outwash settling out here when glaciers caused impoundment, or damming, of many surrounding watersheds. It is the yellow in this creek’s namesake.
- Qes, sand dunes, laid down after the lacustrine layer, were also formed in the Pleistocene. Brought here by winds, the dunes can be recognized here as the sinuous ridge running through the wooded area on the opposite side of the creek.
Note: Outside the scope of this Earthcache, there is an interesting phenomenon occurring here that I would like to point out. Typically, in geology, we most often see the oldest geologic units the deepest with the younger units overlying them. But because the Ohio River has overprinted the area, bringing in flood deposits and dropping them atop the older units, we see a bit of inversion of what we would expect to find, with the Holocene material on the stream bed and the older Pleistocene material above it.
Rhizoliths
So, let’s talk about rhizoliths, after all, that’s why we’re here. Another name for a rhizolith is a root cast. These root casts are scattered throughout the yellow clay banks of the creek in this area. A root cast is produced by the roots of a plant growing down into soil or sediment. When the plant dies, the roots decay, occasionally leaving a void. In a clayey soil, the voids can maintain the shapes and textures of the roots, essentially creating a mold of the original object.
A rhizolith is formed when minerals, dissolved in water, begin to precipitate out of solution within the void. In this case, calcium and iron-bearing minerals slowly filled the voids, from the outside in, creating castings of the original root which had created the voids.
At this location, these rhizoliths may be what geologists refer to as zombie fossils, because like zombies, these trace fossils have become reanimated and moved around, long after their formation and deposition. The casts, likely in much larger sections, were likely brought here with the other sediments that make up the geologic units here. This would also explain why they are so sparsely distributed through the clay here. Trying to find one in the creek bank is almost a fool’s errand, but because they are much heavier than the clays here, they end up in notable concentrations in the pebbles and gravels of the creek bed. As the water moves the sediment, the lighter clays and sands are carried away, leaving behind the pebbles, gravels, and most importantly here, the rhizoliths!
Here are some of the Yellow Creek Rhizoliths I collected at this location.
Logging Tasks
- Please take a picture of yourself at or near ground zero for the Earthcache. You should be able to get Yellow Creek in the frame.
- Why do you think a rhizolith is considered a “trace fossil”? Think about what makes a fossil, and what is considered a “trace” of a fossil.
- When you read about the Qlt, the yellow clay in the banks of the creek here, what would suggest the rhizoliths are “zombie fossils?” Think about where the clay came from and why there might not be plants expected to be in this material.