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.