Fossilization usually can only happen to hard parts of the organism's body, so that is another obstacle in the way of making fossils.
The pressure cements the sediments together and then as they dry out, a sedimentary rock forms around the fossilized part.Relative age dating also means paying attention to crosscutting relationships.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.Layers of volcanic ash are igneous deposits, while layers of rock these deposits surround are usually sedimentary. Igneous intrusions form when magma breaks through a layer of rock from beneath, or lava flows down from above. When igneous intrusion causes newer sedimentary layers to sink into older ones, it's called subsidence.When they break and engulf chunks of sedimentary rocks, it's called stoping. The original rock layers around subsidence areas are called wall rocks and the layers that xenoliths came from are called parent rocks.Unlike people, you can’t really guess the age of a rock from looking at it.