Rocks and the Fossil Record

In 1788 a man named James Hutton wrote Theory of the Earth. In it he hypothesized that all the processes that we observe today, such as erosion and deposition, remain uniform. That is, they don’t change over time. They happen repeatedly in the same way. We call this theory uniformitarianism. Basically, it is the idea that the same geological processes have been shaping the Earth throughout its history.

Hutton’s theory caused controversy, since it meant that the Earth was much older than people had previously thought. People believed that the Earth was only a few thousand years old. That wouldn’t be enough time to have allowed for the processes that Hutton proposed.

Most scientists at the time believed in another theory, called catastrophism. This theory proposes that all geologic change on the Earth happens suddenly, as in a catastrophe.

It wasn’t until the geologist, Charles Lyell published his books, called Principles of Geology (1830 – 1833), that scientists began to believe that uniformitarianism was the guiding principal of geology. Geologists today realize that neither theory explains all of the geological changes that have occurred during Earth’s long history.

While we are studying the geology of the Earth we need to also look at the organisms that inhabit it and the conditions under which they live. Paleontology is the branch of biology that studies the forms of life that existed in former geologic periods. Paleontologists are the scientists that engage in this study. The data that they use is mainly fossils.

Fossils are the remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities. Fossils have been found on every continent on Earth.

The word fossil comes from the Latin word fossilis, which means "dug up." Most fossils are excavated from sedimentary rock layers.

Sedimentary rock is rock that has formed from sediment, like sand, mud, small pieces of rocks. Over long periods of time, these small pieces of debris are compressed (squeezed) as they are buried under more and more layers of sediment that piles up on top of it. Eventually, they are compressed into sedimentary rock. The layers that are farther down in the Earth are older than the top layers.

Determining whether an object or event is older or younger than other objects or events is called relative dating.

Layers of sedimentary rocks form one on top of the other, as seen in the photo on the right. The principle that states that younger rocks lie above older rocks in undisturbed sequences is called superposition.

Superposition

Forces within the Earth have disturbed some rock sequences. These forces can push other rocks into the sequence, tilt or fold rock layers, and break sequences into movable parts. They can even tilt them until they are upside down, making it difficult for geologists to determine the relative age of the rocks.

To help with this problem, geologists have combined data collected from all the known undisturbed rock sequences from different parts of the world. From this data geologists created the geologic column.

Since scientists assume that sediment is deposited horizontally to form layers, if the rock layers are not horizontal, something must have happened to disturb them.

Folding and tilting are two types of events that disturb rock layers. These events are always younger than the rock layers they effect.

Faults, intrusions, and the effects of folding and tilting can make dating rock layers a challenge. Sometimes, layers of rock are missing altogether, creating a gap in the geologic record.

Missing rock layers create breaks in rock-layer sequences called unconformities. An unconformity is a surface that represents a missing part of the geologic column. Unconformities also represent missing time — time that was not recorded in layers of rock. When geologists find an unconformity, they must question whether the “ missing layer ” was never present or whether it was somehow removed. This figure shows nondeposition.