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Whale Fossils on a Mountain Top
In 1987, a team of scientists hiked through the Andes Mountains, more than 1,524 meters (5,000 feet) above sea level. They were on a mission: to find fossils of whales and other marine organisms to bring back to a museum to study. 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. Sedimentary rock often holds fossils because its layers built up over time. As sediment was deposited, it often trapped and preserved remains of living things. These remains include whole plants and animals, as well as traces of organisms such as footprints.
Fossils can tell scientists a lot about Earth’s past, including what the environment was like and how it changed. For example, the whale fossils told scientists that the land was once covered by an ocean because whales can only survive in the ocean. The marine fossils on top of the mountain also told scientists about a major change to Earth’s surface. They are evidence that the mountain used to be much lower because it was once covered in water. At some point, two tectonic plates collided, pushing the land upward and forming the mountains.
Geologic Detective Work
Geologists are like detectives. They look for clues in the layers of rock that tell them how Earth has changed since it first formed 4.6 billion years ago. For example, the geologists who traveled to the Andes learned a lot of information from the fossils they found.
They gathered fossils from a 305-meter (1,000-foot) thick rock section of one of the mountains. Within their rock sample, they found fossils of both marine and land animals in different layers. They found oyster beds and sand dollars, which were both evidence of a marine environment. Right above these layers were fossils of land organisms.
Understanding layers is an important part of this geologic detective work. The rock layers and the fossils found in them are called the geologic column. Geologists refer to the rock strata when talking about the layered arrangement of rocks in the geologic column. Scientists know that when they look at rock strata, the bottom layer is generally the oldest layer. The top layer is the newest, most recent layer. This is because of the process of sedimentation. Sediment formed from weathering and erosion slowly accumulates in layers in oceans, lakes, and valleys over time. Each new layer is deposited on top of an older layer. This process also results in layers that are horizontal. In other words, the layers are parallel (or mostly parallel) to Earth’s surface.
The layers of rock and fossils in the geologic column told the geologists in the Andes that the land was once covered by an ocean. It then transitioned to a land environment, which scientists believe happened as tectonic plates pushed the land upward, causing the ocean to disappear.
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