Kid-friendly explanation

Sediment is tiny bits of rock, sand, and dirt that break off from bigger rocks. Water, wind, or ice can pick up these bits and move them to new places. When the bits settle down and pile up, they can make new land or even new rocks over a long time!

Examples to show what sediment is

Example 1 (younger grades): After a heavy rain, a puddle in a dirt driveway looks muddy. When the water sits still for a while, the water on top looks clearer and you can see a layer of brown material on the bottom. That layer is sediment, made of tiny pieces of soil and rock that washed into the puddle and then settled.

Example 2 (upper grades): A river flows from mountains toward the ocean. As it moves downhill, it picks up gravel, sand, and clay from the riverbanks. In fast, steep sections, the river can carry larger pebbles. When the river slows down near its mouth, the heavier pebbles drop first, then sand, and finally clay settles in the calmest water. Over thousands of years, these layers of sediment can be buried and compacted, helping form sedimentary rock as part of the rock cycle.

Going deeper with sediment

Sediment forms when rocks are broken down by weathering, which includes physical processes like freezing and thawing, and chemical processes like minerals dissolving in water. Once broken into smaller pieces, these materials can be transported by erosion, which is the movement of sediment by wind, water, ice, or gravity.

Where sediment ends up depends on the energy of the environment. Fast-moving rivers and waves can carry and drop larger particles like gravel, while slow, calm water in lakes or deep oceans tends to collect very fine mud and clay. Over time, layers of sediment build up, and the size and type of particles in each layer tell a story about the environment when that layer was deposited.

Scientists use sediment layers to learn about Earth’s past. For example, fossils found in sedimentary layers can help with relative and absolute dating, which is figuring out the order of events in Earth’s history and sometimes estimating how many years ago they happened. Sediment also shapes landform features such as deltas, beaches, sandbars, and floodplains.

Common mistakes about sediment

  • Mistake: Thinking sediment is only sand on a beach.
    Correction: Sediment includes all loose particles, from large gravel and pebbles to fine silt and clay, as well as bits of shells and plant material, not just sand.
  • Mistake: Believing sediment always stays where it first forms.
    Correction: Sediment is often moved by erosion, so material that forms in one place can be transported far away by rivers, wind, glaciers, or gravity before it settles.
  • Mistake: Confusing sediment with solid rock and thinking they are the same thing.
    Correction: Sediment is loose and unconsolidated, while sedimentary rock forms later when layers of sediment are buried, compacted, and cemented together over long periods of time.

How sediment shows up in everyday life

Sediment is easy to spot in daily life. Muddy water in a puddle, sand that collects at the bottom of a bucket after rinsing dirty toys, or the layer of dirt that builds up at the bottom of a jar of water mixed with soil are all examples of sediment settling. Beaches, riverbanks, and many backyards are covered with sediment your child walks on every day.

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