Science · 6th Grade · Earth's Changing Surface · Weeks 28-36

Sedimentary Rocks and Fossil Formation

Students explore the formation of sedimentary rocks and how they preserve evidence of past life.

• Common Core State Standards
• MS-ESS2-1

About This Topic

Sedimentary rocks are Earth's record books, forming when layers of sediment, fragments of minerals, shells, and organic material, are deposited and compacted over millions of years. In the US 6th grade curriculum (MS-ESS2-1), students move beyond rock identification to understand the processes of weathering, erosion, deposition, and lithification that create these layered formations. Each layer tells a story about the environment that existed when it was laid down, whether a shallow sea, a desert dune, or a river delta.

Fossils form when organisms become buried quickly in sediment before decay removes all traces. Minerals gradually replace organic material, creating a cast of the original creature. This makes sedimentary rock the nearly exclusive host of fossils, since the low heat and pressure of sedimentary environments preserve delicate structures that would be destroyed in igneous or metamorphic processes.

Active learning is especially effective here because students can physically simulate the layering process, examine real rock samples with hand lenses, and construct evidence-based arguments about ancient environments.

Key Questions

1. Explain how a grain of sand can eventually become part of a mountain.
2. Analyze what the characteristics of a sedimentary rock tell us about its formation environment.
3. Justify why fossils are primarily found in sedimentary rocks.

Learning Objectives

• Classify different types of sediments based on their size, shape, and composition.
• Explain the processes of weathering, erosion, deposition, and lithification in the formation of sedimentary rocks.
• Analyze the characteristics of a sedimentary rock to infer the depositional environment.
• Justify why fossils are predominantly found in sedimentary rock layers.

Before You Start

Weathering and Erosion

Why: Students need to understand how rocks break down and are transported before they can grasp how sediment forms and is deposited.

Earth's Materials: Minerals

Why: Understanding the basic building blocks of rocks, minerals, helps students comprehend the composition of sediments.

Key Vocabulary

Sediment	Small pieces of rock, minerals, shells, or organic matter that have been broken down and transported by natural forces.
Lithification	The process by which loose sediments are transformed into solid sedimentary rock through compaction and cementation.
Deposition	The geological process in which sediments, soil, and rocks are added to a landform or landmass, often by wind, water, or ice.
Fossil	The preserved remains or traces of ancient organisms, typically found embedded within sedimentary rock.
Compaction	The process where the weight of overlying sediments squeezes the sediments below, reducing pore space and increasing density.

Watch Out for These Misconceptions

Common Misconception

All rocks with visible layers must be sedimentary.

What to Teach Instead

While layering is a hallmark of sedimentary rocks, some metamorphic rocks develop foliation (banding) under pressure. Having students examine actual samples of both types and compare their textural characteristics helps them distinguish compositional layering from pressure-induced banding.

Common Misconception

Fossils are always intact skeletons or bones.

What to Teach Instead

Many students expect fossils to look like museum skeleton mounts. Correct this by showing examples of trace fossils such as footprints and burrows, mold and cast fossils, and chemical fossils like biomarkers. A gallery walk with diverse fossil photographs makes the range of preservation types concrete.

Common Misconception

Rocks and fossils form quickly.

What to Teach Instead

Students often underestimate geological timescales. Help them build intuition by connecting sedimentary rock formation to measurable rates, such as 1 mm of sediment deposited per year requiring 100,000 years to accumulate 100 meters of rock, making the timeframes visceral rather than abstract.

Active Learning Ideas

Fossil Dig Simulation

Groups excavate plastic organisms from layered plaster blocks that were assembled in advance, with each layer labeled as it was built. Students record the depth of each find, identify which organisms lived in which era, and present evidence-based conclusions about the sequence of ancient environments.

45 min·Small Groups

Gallery Walk: Reading the Rocks

Station posters display photographs of real sedimentary formations such as the Grand Canyon and the White Cliffs of Dover. Students rotate and record what each rock layer communicates about ancient environments, grain size, and depositional energy.

25 min·Small Groups

Think-Pair-Share: Sand to Stone

Students trace the journey of a sand grain from a desert landscape to its eventual incorporation into a sedimentary rock. Partners quiz each other using the vocabulary of weathering, erosion, deposition, compaction, and cementation before sharing with the whole class.

15 min·Pairs

Inquiry Circle: Sediment Settling

Groups fill clear tubes with mixed sediment including gravel, sand, and clay, then add water, shake, and observe the settling sequence. Students connect the sorted layers they produce to the graded bedding seen in real sedimentary outcrops.

30 min·Small Groups

Real-World Connections

• Paleontologists at the Smithsonian National Museum of Natural History study fossils found in sedimentary rock formations across the United States, like the Morrison Formation in Colorado, to reconstruct ancient ecosystems and understand evolutionary history.
• Geologists working for oil and gas companies analyze sedimentary rock layers, such as those found in the Permian Basin of Texas, to identify potential reservoirs where fossil fuels formed and accumulated over millions of years.
• Construction engineers examine the properties of sedimentary rocks, like sandstone and shale used in building materials, to assess their durability and suitability for foundations and infrastructure projects.

Assessment Ideas

Exit Ticket

Provide students with a diagram showing layers of sediment and a fossil. Ask them to write two sentences explaining how the fossil became preserved in the rock and one sentence describing the likely environment when the sediment was deposited.

Quick Check

Present students with images of different sedimentary rocks (e.g., conglomerate, sandstone, shale). Ask them to identify one characteristic of each rock and relate it to a specific process of formation (e.g., large grains in conglomerate suggest rapid deposition).

Discussion Prompt

Pose the question: 'Why are fossils almost never found in igneous rocks like granite or metamorphic rocks like marble?' Facilitate a class discussion where students use their knowledge of rock formation processes to justify their answers.

Frequently Asked Questions

How do fossils form in sedimentary rock?

Fossils form when an organism is quickly buried in sediment before it fully decays. Over time, minerals replace the original organic material through permineralization, or the organism decomposes leaving a mold that later fills with minerals. The low heat and pressure of sedimentary environments preserve these delicate structures across geological time.

Why are fossils mostly found in sedimentary rocks and not igneous or metamorphic?

Fossils require gentle formation conditions. The extreme heat of igneous rock formation and the intense pressure of metamorphic processes would destroy organic remains. Sedimentary environments trap organisms at low temperatures and pressures, allowing mineralization to preserve their structures over millions of years.

What does the Grand Canyon tell us about Earth's history?

The Grand Canyon exposes nearly 2 billion years of rock layers, each representing a different past environment. Geologists read these layers like pages in a book, identifying ancient sea beds, deserts, and river systems based on rock type, grain size, and fossil content found at each level.

How can active learning help students understand sedimentary rock formation?

Hands-on activities like simulating sediment settling or excavating model fossil sites give students a physical experience of processes that unfold over millions of years. When students build or excavate the layers themselves, they develop stronger mental models of how rock sequences preserve evidence about ancient environments than any diagram alone can provide.