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Deposition

Deposition is the addition of sediment to a landform. Landforms are weathered and eroded all the time. Of course, their pieces have to go somewhere. They’ll be carried for a while by wind, water, or some other force. Yet, eventually, the sediment will be deposited onto a different landform.

A slope with rocks.

Rocks that have been depoited at the base of a slope. The resulting landform is known as a scree. Generally, screes form as a result of deposition by mass wasting.

The Science of Deposition

Deposition seems like a pretty simple concept, right? Things get picked up and eventually they get put down. That’s basically just how gravity works!

Well, things are actually a bit more complicated than that. A variety of forces move sediment in both patterned and messy ways. This can produce clear landforms, like deltas. Yet, it can also give us areas of plain sediment, like sandy deserts. The existence of these unique land features tells us that deposition isn’t totally random.

But first, we have to know where deposited sediments come from. In this article, we’re mainly going to focus on the ‘putting down’ side of sediment movement (deposition). You can read more about how sediment is picked up (weathering/erosion) in the articles to the left. For now, all we need to know is that weathering and erosion are part of the rock cycle. Their role in it is to break large rocks into the sediment. Outside of the rock cycle, they also take materials from soil beds.

Erosion in a small stream.

Surface runoff flowing over some soil. It breaks up the soil into smaller pieces.

The pieces created (sediment) have to settle down somewhere. Whenever they do, we call it deposition. Deposition creates both interesting and important landforms. A few of them, like screes, seem somewhat mysterious. Others are central to how humans beings live! With that, let’s explore a few examples of deposition.

Examples and Pictures of Deposition

Soil Deposition

A central concern for farmers is finding fertile soil for plant growth. They’re well aware that soil comes in many different types. That’s because it’s created by erosion and deposition. Soil is actually made-up of a mixture of different sediments, like weathered rock and clay.

Getting a good mixture for farming (and in the right order) can actually take thousands of years! For this reason, it’s really important that we can find good soil. Fortunately, the science of deposition tells us where to locate it.

Beautiful shot of agricultural fields.

Plants growing in a patch of soil. You might think that’s kind of boring. But, good soil is hard to come by!

One of the best kinds of soil for farming is silty soil. Silt is a very fine sediment that’s made when mineral-rich rocks are weathered. It’s often produced by wind erosion which is pretty common in desert environments. Then, during floods, the silt is picked up and deposited near topographical low points. These points are usually rivers or coastlines.

The Nile river.

The Nile River. Even though it’s in a desert, it’s a great place for farming.

As a result, the land around that water is some of the best for farming. This is true even in deserts! But, we would only know that if we understood how silt is deposited. In a sense, we need the science of deposition to produce food. And what else is more important than that?

Deposition of Rocks

If you’ve ever gone hiking, you might have seen lots of little rock piles laying around. Who put those there? A Sasquatch?

Obviously not. Instead, these piles were deposited by natural forces. As we said in the beginning, mass wasting tells us that rocks gradually fall or slide downhill. When they hit the ground, they might break into small pieces or piles.

Rocks are deposited in a number of other ways, too. For example, eroded rocks often find their way into rivers. If they’re small enough, the river will carry them downstream. All the way at the end, they may be deposited in a delta. A delta is a landform between a flowing body of water (here, a river) and one that’s not moving. They tend to contain a ton of sediment.

A river delta.

A river delta. You can see brownish sediment throughout the water.

Other Great Resources:

Deposition in the Rock Cycle: https://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3463.html

‘Deposition Facts for Kids’ by Sciencing: https://sciencing.com/deposition-kids-8512606.html

River Deltas by OneGeology: http://www.onegeology.org/extra/kids/earthprocesses/deltas.html

More on Soil Formation: https://www.ctahr.hawaii.edu/mauisoil/a_factor_form.aspx

Written by: Noah Louis-Ferdinand.