One of the strongest evidences against coal formation taking thousands or millions of years is the presence of tree fossils in coal beds.
All over the world, geologists have found fossil trees embedded in coal seams. Sometimes these trees span several coal layers. Because these fossil trees cross several layers of strata, they are called polystrate fossils.
If coal is supposed to form slowly, then these trees should not be there—as the trees should have rotted. But the physical evidence shows that they are there, meaning the coal layers must have formed rapidly. Vast amounts of sediment must have been deposited in a very short period of time.
If coal takes many thousands and millions of years to form, then what would happen to these trees while they were waiting to be covered? Would they not quickly rot? We have a choice of what to believe. We can either believe that this formation took place over a 100 million year span of time or we can believe what we observe—the trees are there and had to be buried rapidly so they would not rot.