How Are Sedimentary Rocks Formed?

Sedimentary rocks are one of the three main types of rocks on Earth, and they form right on or near Earth’s surface. These rocks begin their journey as tiny pieces of other rocks. Over long periods of time, these pieces are moved, dropped, and pressed together until they become solid rock again.

Where Do the Sediments Come From?

Sediments are small bits of rock that break off larger rocks. This happens through a process called weathering, which means rocks are broken down by wind, water, ice, or even changes in temperature. Once the pieces break off, they don’t just stay in one place. They are carried, or transported, by natural forces such as wind, flowing rivers, ocean waves, or moving glaciers.

How Sediments Are Deposited

Eventually, the moving sediments slow down and settle in a new place. This is called deposition. Sediments can be deposited:

• On dry land

• Along riverbanks

• At the bottom of lakes

• On the seabed

• On top of older layers of rock

Over time, more and more layers build up.

Layers of Rock

Sedimentary rocks form in layers, which scientists call strata. Each layer may contain different sizes or types of sediments depending on where they came from. When you look at a cliff or canyon wall, you can often see these layers stacked like pages in a book. Each layer tells a story about what Earth was like when that layer formed.

How Sediments Become Rock

As new layers pile on top of older ones, the weight pushes down on the sediments below. This pressure squeezes the sediments together in a process called compaction. Then, minerals in water seep between the tiny pieces and act like glue. This “glue” hardens and cements the sediments together. This process is called cementation. Together, compaction and cementation turn loose sediments into solid sedimentary rock.

Examples of How Sediments Move

River

A river can carry sediments of all sizes—from tiny grains of sand to larger pebbles. These sediments come from weathered rocks upstream. As the river flows, it transports the sediments until they settle somewhere else.

Seabed

When sediments reach the ocean, they may sink to the bottom and collect on the seabed. Over millions of years, these layers can harden into sedimentary rock.

Glacier

A glacier is a slow‑moving river of ice. As it moves, it picks up rocks and sediments and carries them long distances. When the glacier melts, it drops the sediments in new locations.

TEXT: A TRIP TO THE COLORADO PLATEAU

The Colorado Plateau was lifted many thousands of feet, mostly straight up, from an ancient sea bed. After that, millions of years of weathering (breaking down) and erosion (moving) by the Colorado River left high river cliffs. The exposed surface of the cliffs shows the formation of many layers of sedimentary rock.

On a hard bedrock, layers of sediments (matter that settles to the bottom) built up over millions of years. Each layer of sediment depends on whether the area was covered with a sea, dry land, or shallow muddy shore. The layers are: greenish shale from seas, tan sandstone from sandy shores, yellow-gray limestone from the remains of sea creatures, red iron- rich layers of left by muddy flood zones, and more greenish sandstone when the area was covered by a deep sea. Between wet times, a sandy desert blew across wide areas, laying down the Coconino Sandstone layer. Great pressure from layers above pressed the layers together. Along the Colorado River, you can see that deposition (laying down) of

sediments, compaction (pressing together), and cementation (sticking together) forms sedimentary rock.