Sedimentary Rocks and Environments of Deposition

Rocks built grain by grain

Sedimentary rocks are built from the broken remains of older rocks and living things, pressed and cemented together over long spans of time. Read carefully, they record the world that made them. This module works through the story of a sedimentary rock: - From loose to solid: how weathering, transport and deposition make sediment, and how it hardens into rock. - Reading the grains: how the size, shape and sorting of grains reveal how far they travelled. - Naming the rock: the main sedimentary rocks and how to tell them apart, including the acid test. - Reading the place: how sedimentary structures reveal the environment of deposition. Fossils and graphic logs are handled in a separate module. Here the focus is the rock itself and the environment it records.

Words for sedimentary rocks

Five terms you need before you use them. Learn what each one means.

Match each sedimentary rock to its feature

Well-sorted against poorly-sorted

How even the grain sizes are, called the sorting, is one of the strongest clues to how far sediment travelled. Learn to read it.

Read the transport story

A rock is made of large, angular grains of many different sizes that were clearly not carried far. What does this suggest?

• The sediment was dropped quickly, close to where it came from
• The sediment travelled for hundreds of miles down a river
• The grains were rounded and sorted by very long transport
• The rock formed as crystals from a dried-up salty sea

Tap the two clues to a high-energy environment

Tap the TWO features that point to a high-energy environment of deposition.

• Large pebbles that only fast-moving water could carry
• Cross bedding left by strong currents or wind
• Very fine mud that settles only in still water
• Thin, flat clay layers laid down very slowly

Reading the environment from the rock

A sedimentary rock is a kind of record, and its structures are the writing. Learn a few and you can name the place it formed. Ripple marks show water or wind that was moving gently back and forth, as on a shallow bed. Cross bedding, where layers sit at an angle inside a bed, is the mark of migrating dunes or strong currents. Graded bedding, coarse at the bottom and fine at the top, records a single pulse of sediment settling out. Desiccation cracks, the cracks of dried mud, show that the surface was exposed to the air and dried out. Put the structures together with the grains, and the rock gives up its environment.

Pick the true sedimentary facts

Select the TWO statements that are true.

• Limestone fizzes when a drop of weak acid is placed on it
• Rounded, well-sorted grains suggest long transport before deposition
• Sedimentary rocks form by cooling from molten rock
• Coarse pebbles always settle out in the calmest, stillest water

Order how a sedimentary rock forms

Put the stages that turn a solid rock into a new sedimentary rock in the correct order.

• An older rock is broken down by weathering into fragments
• The fragments are eroded and transported away
• The grains are deposited when the energy drops
• Layers build up and squeeze the lower grains together
• Minerals cement the grains into solid rock

One beach, written in stone

Imagine picking up a single rock and reading the story of where it formed.\n\nThe grains: the rock is full of rounded grains all of a similar size, which means they were carried a long way by water before they came to rest. The structures: gentle ripple marks cross its surface, the sign of shallow water moving back and forth. The clue in the fizz: a drop of weak acid makes it bubble, showing it holds the broken remains of shells. Put together, the grains, the ripples and the fizz all point to one place: a warm, shallow shore where sand and shell built up and slowly hardened into stone. The rock did not simply appear. It quietly recorded the shore that made it.

Complete the sedimentary rock facts

Loose grains of broken rock and shell are called _____. They settle out of moving water or wind in a process called _____. Turning those loose grains into solid rock by compacting and cementing is _____. A rock made largely of shell that fizzes with weak acid is _____.

Build a sedimentary point

Choose the words that complete this statement about reading grains.

The sedimentary rocks run

Five questions on sedimentary rocks. Three lives.

Name the environment

Read each rock and work out the environment it most likely formed in, then think about why.

• A rock is full of large, angular grains of very mixed sizes. Where did it most likely form?
• A rock is made of thin layers of very fine mud, with no ripples and no coarse grains. What environment fits best?
• A rock fizzes with weak acid and is packed with coral and shell fragments. Where did it form?

Explain how sedimentary rocks form and what they record

A student in the year below does not understand how a sedimentary rock forms or what it can tell us. Explain it using what this module has covered.

• Explain, in order, how loose sediment becomes solid rock
• Explain what grain size, shape and sorting reveal about transport
• Name two sedimentary rocks and give a way to tell them apart
• Explain one sedimentary structure and the environment it suggests
• Finish by explaining how a rock can record the place it formed