🪨 What Are Sedimentary Rocks?

Sedimentary rocks are made from small pieces of other rocks stuck together. Very tiny bits of rock are called sediments.

These sediments come from other rocks that have been broken down over time by a process called weathering.

🌧️ How Rocks Break Down

Rocks are always being broken apart by weathering. For example, very hot and cold temperatures can make rocks crack, rainwater can dissolve minerals, and plant roots can push into rocks and split them apart.

When rocks are weakened by weathering, they can be worn away by rivers — this process is called erosion. The pieces of rock move along river valleys, and ice or wind can also break and carry rocks into small particles called sediments.

🌊 How Sediments Are Moved

All the sediments are carried or transported by rivers. When the river slows down near a lake or the sea, it drops the sediments. This process of dropping sediments is called deposition, and when they settle into layers it is known as sedimentation.

⏳ How Sedimentary Rocks Form

Over millions of years, the layers of sediment at the bottom get squashed by the layers above. The minerals in water help stick the sediments together to form solid sedimentary rock.

📚 Layers Tell a Story

Sedimentary rocks always form in layers, just like the walls of the Grand Canyon. These layers are an important feature that helps scientists understand Earth’s history.

🪨 Types of Sedimentary Rock

Three of the most common types of sedimentary rock are sandstone, shale, and limestone.

🏖️ Sandstone

Sandstone is a sedimentary rock made from grains of sand that are stuck together. Most sand is made of quartz, a very hard mineral that resists weathering at Earth’s surface. Sandstone can appear red, orange, yellow, or grey depending on the minerals it contains.

The photograph at the beginning of this topic shows sandstone. Notice the visible layers within the rock.

🪶 Shale

Shale forms from very fine-grained sediments that are softer than sand. It is usually grey in colour and leaves a mark when scratched. Shale is soft enough to break by hand and forms in clear, flat layers.

🐚 Limestone

Limestone is made from layers of shells that once covered sea animals. When these animals died, their shells sank to the sea floor and were pressed together over time. Limestone is usually white or grey, and chalk is a pure form of limestone.

🦴 Fossils in Sedimentary Rocks

Sedimentary rocks often contain fossils. Fossils are the preserved remains of animals and plants that lived long ago and became trapped in layers of rock.

🦕 How Fossils Form

Fossils can only form when an animal or plant is buried quickly in a place with little air. That is why fossils are usually found in sedimentary rocks formed under water, such as at the bottom of lakes or seas.

When animals that live in or near the sea die, their bodies sink to the sea floor. The soft parts of the body rot away, but over time minerals in the water replace the bones. The minerals harden and form rock, leaving a copy of the original animal — a fossil.

The fossil shown here was found in limestone and is about 150 million years old. It preserves the skeleton of a prehistoric animal beautifully in stone.

🦕 Fossil Moulds

Sometimes only the imprint or shape of an animal in the sediment is left behind. This kind of fossil is called a mould. The mould shows the outline of the animal or plant that once lived there.

For example, a mould of a dinosaur’s footprint in sandstone in Namibia is about 150 million years old. Another example is a fossil mould of a leaf in shale that formed around 350 million years ago.

🌀 Fossil Casts

Sometimes a mould fills with minerals that harden to make a solid copy of the animal or plant. This solid shape is called a cast.

The fossils below show two ammonites found in limestone. The moulds filled with minerals that hardened into the spiral shape of the original animals. Ammonites no longer exist — they lived about 240 million years ago, so these fossils are that old!

🌿 Making a Mould and Cast

In the next activity, you will create your own mould and cast of a leaf to see how fossils form. This experiment helps show how natural materials can capture and preserve shapes over time.