We are all familiar with plants and the roots that anchor them into the ground and provide them with nutrients and moisture. What you may not have thought about is how they can show stratigraphic up if the roots are preserved in the rock record. The photo on the left is an example of a modern plant and how it grows. You can see the main trunk of the tree and - because the river has eroded the soil from underneath - the branching nature of the root below the surface. Typical of all plants, the part of the plant that is at the top of the bed is a single, large column. As you go deeper into the earth, the root mass branches out and gets thinner, eventually coming to very fine, tapered ends.
Typically, the top part of a plant is not preserved. The roots, however, sometimes do get saved. The photograph on the right shows just such a feature. There is a thin orange-colored horizon under which very fine roots can be seen. The orange layer is known as a paleosol, or old soil, and marks a surface that once saw the light of day and was exposed as topsoil. The roots that are found below are all that remains of the plants that once populated this area. You can tell that this photo is in the stratigraphic up position in two ways. First, the roots start as single, larger structures towards the top and branch out and get thinner towards the bottom. Second, the paleosol layer overlying the roots also shows that this is stratigraphically up in orientation.
These two photos show root structures that are 300 million years old. The one on the left is the original photograph, and the one on the right has had red lines added to outline some of the roots to make them easier to find. (Click the photo to enlarge it and see the red lines.) These photos also show rocks in the stratigraphic up position.
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