Unit 3 Review (short): Relative Dating and Index Fossils

Which statement best describes the Law of Original Horizontality?

• Sedimentary layers are deposited in horizontal sheets and tilting or folding occurs after deposition.

• In any undisturbed sequence, the oldest layer is at the bottom and the youngest is at the top.

• A break in the rock record caused by erosion or other weathering factors.

• If a fault or intrusion cuts through another feature, the feature that has been cut is older.

Answer explanation

The Law of Original Horizontality states that sedimentary layers are initially deposited in horizontal sheets. Any tilting or folding occurs after this deposition, making the first answer choice the best description.

Tags

NGSS.MS-ESS1-4

According to the Law of Superposition, which layer in an undisturbed sequence is the oldest?

• The top layer

• The middle layer

• The bottom layer

• All layers are the same age

Answer explanation

According to the Law of Superposition, in an undisturbed sequence of sedimentary layers, the oldest layer is always at the bottom. Therefore, the correct answer is the bottom layer.

Tags

NGSS.MS-ESS1-4

Which description defines an unconformity?

• Layers of sediment extend laterally in all directions and were originally continuous.

• A break in time in an otherwise continuous rock record, usually caused by erosion or other weathering factors.

• Sedimentary layers are deposited in horizontal sheets.

• An igneous intrusion that cuts across layers.

Answer explanation

An unconformity represents a gap in the geological record, indicating a period of erosion or non-deposition. The correct choice highlights this break in time, distinguishing it from continuous sediment layers.

What does the Law of Cross-Cutting Relationships state?

• Older layers are always at the top.

• If a geological fault or intrusion cuts through another, the feature that has been cut is older than the one that cuts through it.

• Sedimentary layers are originally horizontal.

• Layers extend laterally in all directions and may later be eroded.

Answer explanation

The Law of Cross-Cutting Relationships states that if a geological fault or intrusion cuts through another feature, the one being cut is older. This helps determine the relative ages of geological formations.

Tags

NGSS.MS-ESS1-4

Which statement best matches the Law of Lateral Continuity?

• Layers of sediment initially extend laterally in all directions; they may later be eroded or broken up but were originally continuous.

• Igneous intrusions are always younger than faults.

• Index fossils occur in many layers at one location.

• Metamorphic rocks are always the oldest.

Answer explanation

The Law of Lateral Continuity states that sediment layers extend laterally in all directions and are originally continuous, even if later eroded or disrupted. This matches the correct choice perfectly.

Tags

NGSS.MS-ESS1-4

Several layers of sedimentary rock are illustrated. They are in the same order as when they first formed. Which layer formed first?

• Rock Layer W

• Rock Layer X

• Rock Layer Y

• Rock Layer Z

Tags

NGSS.MS-ESS1-4

Which of the following correctly lists the layers in order from oldest to youngest?

• A, B, C, D, E
• E, B, C, D, A
• E, D, C, B, A
• C, B, A, D, E

Tags

NGSS.MS-ESS1-4

What are the relative ages of the features in order from oldest to youngest?

• Limestone, Basalt, Fault, Siltstone
• Siltstone, Fault, Basalt, Limestone
• Limestone, Fault, Basalt, Siltstone
• Limestone, Siltstone, Fault, Basalt

Tags

NGSS.MS-ESS1-4

The best index fossils have which two traits?

• widespread geographically, long lifespan

• widespread geographically, short lifespan

• specific place, long lifespan

• specific place, short lifespan

Tags

NGSS.MS-LS4-1

These four outcrops show several fossils. Which fossil would be the best index fossil across these outcrops?

• 
• 
• 
• 

Answer explanation

The best index fossil is found in only one layer at each location and present at all locations, as it indicates a specific time period and widespread distribution, making it useful for correlating ages of rock layers.

Tags

NGSS.MS-LS4-1