Rivers move back and forth across their floodplains, eroding here and depositing there. They cut channels down into the rocks beneath them at one time and later deposit fresh river sediment into those channels. Rivers can dislodge fossils from the rocks along their beds and banks and deposit them in their water-cut channels. This effect, like bioturbation, juxtaposes material of different ages.
As long as the channel-deposited rocks can be distinguished from those the channel is cut into, no one is led astray, but if, say, both are sandstones, it may not be easy to tell them apart. When we do not recognize the channel deposits for what they are, younger fossils carried downward into older rock appear to belong there and to have originated before they actually did. In the famous dinosaur beds of Montana, for example, fossils of mammals that were to be important in the Tertiary have been said to occur well down in Cretaceous rocks, suggesting that the replacement of dinosaurs by mammals began well before K-T time, which likely means that the dinosaurs were going extinct long before the boundary. But if the mammal fossils were washed off a Tertiary landscape and deposited into channels cut by Tertiary streams down into the Cretaceous rocks below, then the mammals and the channel deposits in which they are found are Tertiary, not Cretaceous, and the early replacement is an illusion. Here is another difficult puzzle for paleontologists and sedimentary rock specialists.
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