Why are dinosaurs buried so deep? Did rock form over them?
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Yes, exactly.
Fossils only form in sedimentary rock. Those are rocks formed from sediment (dirt, sand, clay, etc), so if something dies in the right conditions, it gets buried in sediment, that sediment forms into rocks, and the organism’s body decays and then gets replaced with different sediment, preserving the shape of the organism.
Igneous rock (made from lava) can’t form fossils because the heat of the lava destroys the organism
Metamorphic rock can’t form fossils because the heat and pressure necessary to make it (during the metamorphosis from either igneous or sedimentary rock) would warp and destroy any fossils that may have been there.
The layers of rock being used to date fossils is quite useful, and it is how we know an asteroid impact killed the dinosaurs. All dinosaur fossils we have ever discovered have been below a very thin sedimentary layer containing metals like iridium that would have come from an asteroid impact.
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