The Geologic Column
The concept of the Geologic Column is that layers of sedimentary rock containing fossils demonstrate the story of evolution chronologically, from bottom to top. There is a tendency for fossils to be found together in certain groups and for these groups to be found one after the other in a certain sequence. For example trilobites are found in the Cambrian region and dinosaurs in the Cretaceous region: the two aren't usually found together.
However, the Geologic Column is a concept, not an actual series of rock layers. The complete sequence of layers has not been found in one location, there is always just a fraction of the total column. According to evolution, the Cambrian trilobites died out millions of years before the dinosaurs evolved, so they appear lower in the column. However, there are other explanations for their separation. For example, if trilobites and dinosaurs were alive today, they wouldn't be found together geographically as they live in different ecological zones. Trilobites dwelt on the bottom of the sea, whereas dinosaurs were land animals.
During the time that parts of the Geologic Column were still being worked out in the mid 19th century, the Victorian philosopher Herbert Spencer commented on the illogical nature of the Geologic Column in his appropriately-named essay, "Illogical Geology". One of the things Spencer challenged was the use of fossils for the correlation and dating of strata. Specifically, he took issue with the practice of using particular fossils as supposed time- markers (index fossils) for the global correlation of strata, and then not questioning the whole procedure when frequently finding such fossils in the 'wrong' strata with further collecting of fossil specimens.
An example of such "misplaced" fossils are plants in the Cambrian period. According to the Evolutionary Model, land plants did not appear until over 100 million years after the Cambrian period. However, more than sixty genera of woody-plant spores, plus pollen and wood have been found in the Cambrian layer across the world.