How do dinosaurs become fossils according to the theory of evolution?

I can understand how an animal can become fossilized when water and mud encase it (a flood) but I don't understand how the fossils in the geologic column got there? If marine life dies in the water, it rots! When animals die on land, they rot! Yet, they find all these animals in subsequent layers, one on top of another. Sometimes they find all the dinosaurs at one level as if they all died at one time.