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Delta: formed by the deposit of sediment at the mouth of a river flowing into a lake, ocean or sea.  ... more

Depositional environment: a specific environment where sediment is deposited by the combination of physical, biological or chemical processes. 

Floodplain: an area next to a river where sediments are deposited when flooding occurs. ... more

High-energy environment: a place that has fast and strong flowing water or wind.

Low-energy environment: a place that has very slow and calm flowing water or wind. 

To be buried or not to be, that is the question

You stare out over an African grassland. It is made up of grasses two to three feet tall, a few trees, and a river running through the flat valley into a small lake. Crocodiles, fish, and hippos are a few of the lake inhabitants. Close to the riverbank, a group of lions are eating dinner, while some hyenas lounge under the shade of a tree waiting for any of the lions’ scraps. Dotting the landscape are zebras, giraffes, and elephants grazing on any tasty plants they come across. If this scene was frozen in time, only some of these animals would become a part of the fossil record. What largely determines this is where the bones end up on that landscape and how long they remain there before they are moved or buried. 

A glacier. Click for more detail.Bones can break down to become a part of the soil, but some may be preserved below the surface. What determines the fate of these bones? Bones can be preserved if they were buried and left undisturbed or if they happened to end up in specific environments that protected them. Bones that are left on the surface for too long will be broken up by the weather or by animals walking on them.

The fate of a bone can be determined by what depositional environment it ends up in. There are several kinds of depositional environments including rivers, lakes, beaches, deltas, glaciers, or wind-blown sediment (sand dunes). Each depositional environment has a different rate of burial. It also has different wind or water currents that can move objects like rocks and bones. The strength and speed of the wind and water is known as the transport energy of an environment.  

This lake is an example of a low-energy depositional environment. Image by Wing-Chi Poon.Bones that end up in low-energy environments where the current is very slow, like a lake or a delta, will have a lower chance of being broken up. High-energy environments, like a fast flowing river or a mountain side where rockslides occur, are more likely to break fossils down into small pieces. This would lessen the chance for preservation.

This rock slide is an example of a high-energy depositional environment. Image by August Bernardo, USFS. Another factor is the length of time the bones are exposed to water currents.  Even though floods can be fast and turbulent, they deposit a lot of sediment fast. This means that whatever bones would have been sitting on the ground quickly got buried and protected. Fast deposition can occur on floodplains, next to rivers where sediments pile up when the river floods.

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