A Rock and a Hard Place

by Aron Ralston

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Student Question

What can the sequence of tree and clam fossils in different rock layers tell us about past climates in that area?

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Sedimentary rocks are often created by fluvial processes. First, sediment is carried by rivers into water basins. Then, once the water dries up, the sediment hardens into rock. Fossilization, the process by which a living organism becomes a fossil, usually happens catastrophically. Most plants and animals will never become fossils because the conditions will not permit it. Soft animal tissues are scavenged. Bones become fragile and wear away, especially in the sun, wind, and water. Shells are ground into fragments by mechanical forces, like waves. The hard parts of living things will turn into sediment unless something happens quite quickly—catastrophically—to bury these organisms before they decay. On land, it could be a volcano, mudslide, or haboob. Near water, quick burial could result from storms that cause the sediment on the floors of oceans and riverbeds to shift. When this happens, organisms trapped within the sediment are protected from the surface elements that would otherwise wear them away entirely.

Shells, bones, and wood fragments are most commonly fossilized through a process called permineralization. Permineralization occurs when mineral deposits from water fill the empty spaces (pores) of trapped organisms. The minerals harden and help preserve the shape of the solid material that remains.

It would be difficult to classify how and where a fossil was formed without more details about the composition and appearance of the sedimentary rock, but we can make some general conclusions about the sequence of climates in this scenario. A layer of shells is often created by waves or tides, so it’s likely that the region in question was once a beach. The clams were suddenly covered by too much sediment to dig themselves out. Eventually, as sea levels changed, the beach became a forest. What happened next depends upon the geography of the area and what lies above the layer of trees. The sea could have risen again, or a river could have overflowed or changed course. Whatever the cause, sediment quickly entrapped the trees before they could deteriorate at the surface.

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