What is a fossil and how does it form?
A fossil is an organism, or trace of an organism (like a footprint or worm tube), that has been preserved in rock. When an organism like a clam dies, its shell slowly gets covered by sediment. Once the sediment becomes thick enough, it begins to compact together and form a rock. The organic material in the shell slowly gets replaced by another mineral, like silica (quartz), calcite, or pyrite (fool’s gold), but keeps valuable information like isotope data for measuring temperature, precipitation, and age in the fossilized shell. Sometimes a shell gets filled with sediment before it gets compacted and the shell dissolves away. However, the sediment inside the shell can harden and preserve an internal mold, or steinkern. The sediment outside of the shell can preserve an external mold, too. These molds are examples of trace fossils.
