What are fossil formations, and how do they determine the age of landforms?

So, fossil formations are created when a plant or animal dies in a watery environment and is buried in mud and silt. The tissue decomposes and only the bones and shells are left behind. Over time, sediment builds up over the bones/shells and hardens into rock. Rock layers, and the fossils they contain toward the bottom of a stack of sediments are older than those found higher in the stack. This is called Radiometric Dating.