Sedimentary Rocks

Key Words: sedimentary, weathering, erosion, transport, deposition, limestone, sandstone, mudstone, chalk, igneous, metamorphic, slate, freeze and thaw.

This summer Virginia and I took our three cats Carrie, Sambal and Sampson camping in West Wales. I took some photographs to show you what the cliffs look like on the Pembrokeshire coast. We camped on the top of the cliffs and were able to walk down to the seashore in a few minutes, though it took a little longer to climb back up again.

Here is a photo of the beach at Broad Haven.

Where did all this sand come from? It might have been blown here by the wind or it might have been washed up by the waves. If you put it under the microscope you will see that some of the sand particles are made of broken seashells. I spent a bit of time watching the seagulls paddling; but they were not paddling for fun. I tried doing what they were doing. The seagulls were wriggling their feet in the sand at the edge of the water. When I took my shoes off and did the same I found lots of tiny bivalves (little molluscs with two shells) in the puddle between my toes. When the seagulls eat the bivalves, the shells get broken up in the seagulls beaks.

Some of the sand particles are made by the effects of the weather on the rocks. In the next photograph you can see that some of the rocks have broken off the cliffs and fallen down to the shore. When the rocks get heated up by the sun or cooled down by the rain, snow and hail, the rocks crack. If water gets into these tiny cracks it can open them up when it gets cold and the water freezes.

Weathering and erosion of the cliffs at Broad Haven.

The rocks which have recently fallen down the cliff look slightly orange. Eventually they will turn a darker black colour. “Weathering” is the breaking of rocks into smaller pieces by the effects of wind, rain, and sunshine. “Erosion” is the transport of rocks, stones, sand and mud from one place to another. Here it is gravity which has pulled the rocks down the cliff. Wind might blow sand from one place to another, and rain water might wash sand or mud from one place to another.

When you go close up to the cliffs and have a careful look at the rocks, you will see that they are made up of millions of layers of stone. Each layer of stone is a slightly different colour from the other layers. Some of the layers are harder than others. We call these layers “strata”: They are made by deposition of gravel, sand or clay. Each new layer of sand or clay squeezes the layers below so that they get turned into hard rock. Rocks made in this way are called “sedimentary rocks”. E.g. limestone, sandstone, chalk, mudstone. Sedimentary rocks are generally softer than igneous rocks (made when molten lava cools down) and softer than metamorphic rocks (made by heating and squeezing a sedimentary or igneous rock). How hard the rock is depends upon what it is made of (gravel, sand or clay) and what sticks it all together.

Strata in the Rocks.

You don’t have to go close up to the rock to see the strata; if you do, you will be able to see the cracks caused by the weather. (Remember to wear a hard hat if you do go near the bottom of the cliff – I was nearly killed by an idiot throwing rocks over the cliff.) The first time you see the strata in the cliffs you will be amazed to see what has happened to them. They get folded up into the most incredible shapes.

Folding up the rocks.

As the Earth’s crust moves the layers of rock get folded up. Fortunately we do not get big earthquakes in England or Wales. However millions of years ago these rocks were laid down in flat level sheets. Movements of the Earth’s crust have folded the rock strata. You only see the folds where they have been exposed by the action of the weather.

Some of the strata are softer than others. The softer strata are more easily broken up by the weather. This leads to the formation of caves. At the top of the cliffs you can see “earth” which has not yet been squeezed to make rock. The tops of these cliffs are very crumbly. Here is a photograph of a cave at Little Haven (just next door to Broad Haven).

Cave at Little Haven, Pembrokeshire.

When the tide comes in, the water goes right into the cave. If the weather is bad, the waves will wash the softer rock away. Eventually the harder rock on top will “cave in”. I rather liked this big “whale” of rock sticking out into the sea. The waves have already started to tear it apart.

A “whale” of a rock.

This great big roll of rock sticks out into the sea: it looked like a great big whale stranded on the shore. Someone has taken a bite out of it. The waves on the Pembrokeshire coast can be very fierce. The next photograph shows the hole in more detail.

Detail of the hole in the “whale”

You can see that a soft layer of rock has been eaten away. You can also see cracks in the rock caused by the freeze-thaw effect. When water freezes, it expands. This makes the crack bigger so that more water can get into the crack when the ice melts. By freezing and thawing many times the water in the crack eventually splits the rock in two.

The three cats enjoyed their holiday but not the car trip there and back.

Sedimentary Rocks are formed when materials like gravel, sand, silt or clay are deposited in layers and then compressed until they become hard. Fossils are formed when plants or animals fall into the deposit before it turns into rock. A sediment is something which sinks to the bottom of a lake or ocean. Carrie is my eldest cat; she is nearly 12 years old and she spends a lot of time lying down asleep; she is like a sediment deposited on the most comfortable cushion. She has to be carried up stairs to bed at bedtime.

Igneous Rocks are formed when molten lava cools down and crystalises. These rocks e.g. granite, are very hard. They NEVER contain fossils because any plant or animal which fell into the molten lava would by completely burnt up by the intense heat of the lava before it turned into rock. Igneous means “fire”. Sambal is my middle cat. He is very fierce and fiery: “sambal” is a Malay word meaning hot.

Metamorphic Rocks like slate and marble are formed when either a sedimentary or an igneous rock is changed by the effects of extreme heat or compression. Metamorphic means “changed”. Sampson is my youngest cat. He is changing from being a young kitten into an adult cat, but he is really a bit of a baby. He needs a lot of attention.