Fossils & Relative Dating — Practice · Grade 9 Science

Chapter: Earth’s History

What this practice covers

By the end of Fossils & Relative Dating you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: fossil, relative dating, index fossil, cross-cutting relationship.

Problem 1

Which of these is the best example of a fossil?
  1. A dinosaur footprint preserved in hardened mud
  2. A shiny piece of quartz
  3. A smooth river pebble
  4. A cloud formation
Answer and explanation

A dinosaur footprint preserved in hardened mud

A fossil is a preserved remain or trace of a once-living organism, and a footprint (trackway) counts even without any body parts.

Problem 2

Relative dating tells you...
  1. The exact age of a rock in years
  2. Whether a rock layer is older or younger than another layer
  3. The chemical composition of a rock
  4. The temperature at which a rock formed
Answer and explanation

Whether a rock layer is older or younger than another layer

Relative dating only compares the order of events — older versus younger — not exact ages in years.

Problem 3

A geologist finds rock layers tilted at a steep angle. Which principle tells her the layers must have been disturbed after they formed?
  1. Original horizontality
  2. Superposition
  3. Cross-cutting relationships
  4. Index fossil correlation
Answer and explanation

Original horizontality

Original horizontality says layers are deposited flat, so tilted layers were bent or tipped sometime after they formed.

Problem 4

A drill core pulls up 6 distinct horizontal, undisturbed rock layers labeled 1 (top) through 6 (bottom). Which layer number is the oldest?
Answer and explanation

6

Undisturbed layers follow the law of superposition — the deepest, bottom layer (6) is the oldest.

Problem 5

A volcanic dike (a wall of hardened magma) cuts straight across five sedimentary rock layers. What can you conclude?
  1. The dike is younger than all five layers
  2. The dike is older than all five layers
  3. The dike formed at the same time as the layers
  4. No conclusion can be drawn
Answer and explanation

The dike is younger than all five layers

By cross-cutting relationships, anything that cuts through existing rock must have formed after that rock existed.

Problem 6

Explain why an index fossil needs to have lived over a wide geographic area but only for a short span of geologic time.
Answer and explanation

A wide range means the same species could get fossilized in many different places, so geologists can find it in rock layers far apart; a short time span means finding that fossil narrows down the age of the layer very precisely, since the species wasn't around for very long.

Problem 7

Species X was widespread for 1 million years; Y was local for 100 million years. Which is a better potential index fossil? Explain the reasoning in your own words.
Answer and explanation

X is more useful, provided it is identifiable and occurs in the sampled layers.

A short time range narrows the possible age.

Wide distribution permits correlation across many sites.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

A fault cuts a fossil-bearing bed. Which event is later? Show the reasoning, not just the conclusion.
Hints

Reworking and identification errors must be considered.

Answer and explanation

The faulting is later than deposition of the bed it cuts.

Problem 9

Why are widespread, short-lived species useful index fossils?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Their limited time range constrains age and broad distribution permits comparison across locations.

Problem 10

The same time marker can occur in different rock typesThe same time marker can occur in different rock typesYoungerIndex group XOlderYounger, different rockIndex group XOlder, different rockSite ASite BRestricted time range + broad distribution + reliable identification
Original hypothetical two-site correlation. Fossil group X is invented; its restricted range is a supplied assumption, not a claim about a real named species. Rock color and vertical position are not the time marker by themselves.
Interpret the supplied visual model. Why might identical sandstone appearance be weaker correlation evidence than a well-established short-range fossil marker?
Answer and explanation

Similar sedimentary rock can form at many times. A suitable reliably identified fossil with a narrow known range constrains time more specifically, with habitat and preservation qualifications.

The dashed connection follows the supplied group X interval across sites with different heights and material types. It is a time relation, not a continuous physical layer drawn between them.

Similar sedimentary rock can form at many times. A suitable reliably identified fossil with a narrow known range constrains time more specifically, with habitat and preservation qualifications.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 11

Beds A then B were deposited, a fault cuts both, and later bed C covers the fault without being cut. Order these four events and state what remains unknown.
Answer and explanation

A deposition precedes B deposition; faulting follows both; C deposition follows faulting. Exact ages, durations and any unrelated event history remain unknown without additional evidence.

A deposition precedes B deposition; faulting follows both; C deposition follows faulting. Exact ages, durations and any unrelated event history remain unknown without additional evidence.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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