Sedimentary rock classification helps students identify rocks by how they form, what they are made of, and what environments produced them. This cheat sheet covers clastic, chemical, and organic sedimentary rocks in a clear reference format for Earth Science. Students need these patterns to interpret rock samples, sedimentary layers, and clues about Earth’s past environments.
It is especially useful for labs, field observations, and review before tests.
Key Facts
- Clastic sedimentary rocks are classified mainly by particle size: gravel is greater than 2 mm, sand is 1/16 to 2 mm, silt is 1/256 to 1/16 mm, and clay is less than 1/256 mm.
- Conglomerate has rounded gravel-sized clasts, while breccia has angular gravel-sized clasts.
- Sandstone is made mostly of sand-sized grains, and its texture, sorting, and mineral content can reveal transport history.
- Shale forms from compacted clay-sized particles and commonly splits into thin layers called fissility.
- Chemical sedimentary rocks form when dissolved minerals precipitate from water, such as rock salt from halite and limestone from calcite.
- Organic sedimentary rocks form from the remains of living things, such as coal from plant material and some limestone from shells or coral fragments.
- Common sedimentary structures include bedding, cross-bedding, ripple marks, mud cracks, and graded bedding.
- A basic classification rule is: identify texture first, then grain size or composition, then use structures and environment to support the rock name.
Vocabulary
- Clastic rock
- A sedimentary rock made from fragments of preexisting rocks that were transported, deposited, compacted, and cemented.
- Chemical sedimentary rock
- A sedimentary rock that forms when minerals crystallize or precipitate from dissolved materials in water.
- Organic sedimentary rock
- A sedimentary rock made mostly from the remains or products of once-living organisms.
- Sorting
- The measure of how similar sediment grains are in size within a rock or deposit.
- Rounding
- The degree to which sediment grains have smooth edges from abrasion during transport.
- Depositional environment
- The setting where sediment is deposited, such as a river, beach, desert, lake, or deep ocean.
Common Mistakes to Avoid
- Calling every layered rock shale is wrong because sandstone, limestone, and other sedimentary rocks can also show bedding.
- Using color alone to identify a sedimentary rock is wrong because color can change due to impurities, weathering, or iron staining.
- Confusing conglomerate and breccia is a common mistake because both contain gravel-sized clasts, but conglomerate has rounded clasts and breccia has angular clasts.
- Classifying limestone as clastic just because it contains visible pieces is wrong because many limestones are chemical or organic and are identified mainly by calcite composition.
- Ignoring grain size leads to incorrect clastic rock names because shale, siltstone, sandstone, and conglomerate are separated primarily by particle diameter.
Practice Questions
- 1 A rock sample contains mostly rounded particles larger than 2 mm. What sedimentary rock is it most likely to be?
- 2 A clastic rock has grains that are 0.5 mm across. Based on grain size, should it be classified as shale, sandstone, or conglomerate?
- 3 A rock forms when dissolved halite crystallizes as seawater evaporates. Is it clastic, chemical, or organic, and what rock type could it be?
- 4 A sedimentary layer contains mud cracks, ripple marks, and very fine grains. What do these features suggest about the depositional environment and changes in water conditions?