Most fossils are formed when organisms or their traces are buried by sediment.

Sediments such as sand and silt, are formed by weathering of rocks due to abiotic factors such as wind, rain, ice, and sun. These sediments, when carried by water, landslides, or wind can accumulate over time and cover the remains of an organism or its trace.

As the layers of rock accumulate, their weight causes the lower layers to harden into sedimentary rock, trapping the organism or trace inside.