Strata Sleuths Assessment Item
ATLAS Science Practice Grade 8 • Earth & Space Science
Standards: MS-ESS1-4 • MS-LS4-1
Student Name:
Date:
Period:
Field Stimulus: River Valley Geological Outcrops
Read the passage and examine the diagram before answering Part A and Part B.
Geologists surveyed two exposed canyon walls located 30 kilometers apart. Both sites display undisturbed sedimentary rock strata containing index fossils. At Location 1, an undisturbed volcanic ash bed was radiometrically dated to 220 million years ago (Ma). At Location 2, an ancient fault line cuts through lower rock layers but terminates beneath the limestone.
LOCATION 1: North Ridge Outcrop Layer 4: Sandstone 🌿 Fern Frond ★ Volcanic Ash Bed (Dated: 220 Ma) Layer 3: Limestone 🌀 Ammonite Layer 2: Shale 🦞 Trilobite Layer 1: Siltstone (Bottom) Fossil Tie LOCATION 2: South Fork Canyon Layer C: Limestone 🌀 Ammonite Layer B: Shale 🦞 Trilobite Layer A: Siltstone Layer B (offset) 🦞 Trilobite Layer A (offset) Fault F MAP KEY / LEGEND Sandstone Ash Bed (220 Ma) Limestone Shale Fault Line F 🌀 Ammonite Fossil 🦞 Trilobite Fossil 🌿 Fern Frond Fossil
Part A
Based on the rock strata and fossil correlation across both locations, which claim about relative age is scientifically supported?
A The limestone containing ammonite fossils is younger than 220 million years old because it sits below the sandstone.
B The limestone containing ammonite fossils is older than 220 million years old, and Fault F occurred before Layer C formed.
C Fault F is younger than all strata at Location 2 because fault movements always occur after all rock deposition has ceased.
D Trilobite fossils are younger than ammonite fossils because the shale layer is denser than the limestone layer.
Part B
Which TWO statements provide the strongest scientific evidence to support the claim selected in Part A? (Select exactly two.)
1 By the Law of Superposition, Layer 3 was deposited beneath the 220 Ma ash bed and must therefore be older than 220 Ma.
2 By cross-cutting relationships, because Fault F cuts through Layers A and B but stops beneath Layer C, the fault is older than Layer C.
3 The fern frond fossils in Layer 4 prove that terrestrial plants existed before marine organisms appeared in Earth's oceans.
4 Shale and limestone always deposit simultaneously, meaning all fossils in both layers are exactly the same absolute age.
Student Response Grid:
Part A:
A B C D
Part B:
1 2 3 4
ITEM ID: ATLAS-8-ESS-042
Fill in response bubbles completely. Select 1 option for Part A and 2 options for Part B. Page 1 of 1
Strata Sleuths Scoring Guide
ATLAS Teacher Scoring Guide Grade 8 • Science Assessment Item
Item ID: ATLAS-8-ESS-042
Standard Aligned MS-ESS1-4 & MS-LS4-1
Item Format EBSR (Part A & Part B)
Cognitive Complexity DOK Level 3 (Strategic)
Total Points 2 Points Possible
Answer Key & Scoring Matrix
Scoring Model: Dichotomous / Multi-Part
Part A Correct Answer: Choice B
Key Concept: Layer 3/C sits below the 220 Ma ash bed in undisturbed sequence (older than 220 Ma). Fault F is truncated by Layer C, meaning faulting occurred before Layer C was deposited.
Part B Correct Answers:
Choice 1 Choice 2
Key Concept: Law of Superposition directly dates strata beneath dated ash. Cross-cutting relationships prove features cutting across rocks are younger than the cut layers, while overlying uncut strata post-date the fault.
2 Points: Correct response for Part A (B) AND both correct responses for Part B (1 & 2). 1 Point: Correct response for Part A only, OR Part A correct with only 1 correct Part B selection. 0 Points: Incorrect response for Part A regardless of Part B selections.
Part A: Distractor Rationale & Student Misconceptions
A:
Misconception (Inverted Superposition): The student recognizes the ash date (220 Ma) but confuses relative orientation, believing strata below an ash bed are younger instead of older.
B:
CORRECT: Applies superposition correctly (strata beneath 220 Ma ash are older than 220 Ma) and applies cross-cutting logic (Fault F is terminated at the base of Layer C, establishing the fault is older than Layer C).
C:
Misconception (Universal Event Faulting): The student assumes tectonic faults always post-date all local deposition, failing to recognize unconformities where upper strata remain continuous and undisturbed.
D:
Misconception (Physical Density vs. Chronology): The student confuses rock density or physical thickness with chronological deposition, ignoring stratigraphic position and the Law of Superposition.
Part B: Evidence Evaluation & Distractor Analysis
Choice 1 — CORRECT (Superposition)
Directly justifies the age constraint of the ammonite layer by anchoring it beneath the absolute date (220 Ma) of the volcanic bed.
Choice 2 — CORRECT (Cross-Cutting)
Directly justifies the timing of Fault F by noting it truncates at the boundary below Layer C, proving deposition occurred after movement.
