How do geologists put rock layers in order and read the history recorded in a cross section?
Apply the principles of relative dating (superposition, original horizontality, cross-cutting relationships, inclusions and unconformities) to sequence events in a geologic cross section (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on relative dating for the Virginia Earth Science EOC: the law of superposition, original horizontality, cross-cutting relationships, inclusions and unconformities, the difference between relative and absolute age, and how to sequence the events in a geologic cross section, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standard ES.9 asks you to read Earth's history from rocks, starting with relative dating, putting events in order without numbers. The EOC tests this with a geologic cross section and a sequencing or ordering item: you apply a handful of principles to decide what happened first. This is one of the most reliably tested Earth-history skills, and it pairs with absolute dating (numbers from radioactive decay) and fossils.
Relative versus absolute age
The principles of relative dating
These principles are simple rules, but the EOC tests whether you can apply several at once to a single diagram.
Reading an unconformity
An unconformity is the trickiest principle, because it represents time that is missing from the record. It forms when rock is deposited, then uplifted and eroded (removing some layers), then buried again by new deposition. The eroded surface between the old and new rock is the unconformity. On the EOC, an unconformity is a clue that the record is incomplete: layers that "should" be there were eroded or never laid down.
Sequencing a cross section
To work out the order of events in a geologic cross section, build the history step by step:
- Use superposition to order the undisturbed sedimentary layers (bottom is oldest).
- Use original horizontality to spot tilting or folding, which happened after those layers formed.
- Use cross-cutting relationships to place faults and intrusions after the rock they cut.
- Identify any unconformity (erosion surface) as an episode of uplift and erosion.
- Read the whole story from oldest to youngest.
Try this
Q1. State the law of superposition. [1]
- Cue. In an undisturbed sequence of layers, the oldest rock is at the bottom and each layer above is younger.
Q2. A layer of rock is tilted at a steep angle. What principle tells you it was disturbed after it formed, and why? [2]
- Cue. Original horizontality: sediments are deposited horizontally, so a steeply tilted layer must have been tilted by later movement.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Earth Science SOL 2023 (style)1 marksIn a sequence of undisturbed sedimentary rock layers, which layer is the oldest? (A) the top layer. (B) the bottom layer. (C) the middle layer. (D) they are all the same age.Show worked answer →
A 1-point multiple-choice item on superposition.
The correct answer is B. By the law of superposition, in undisturbed layers the oldest rock is at the bottom (it was deposited first) and each layer above is younger. The top layer (A) is the youngest, not the oldest.
The test rewards the rule: in undisturbed layers, oldest is at the bottom, youngest at the top.
VA Earth Science SOL 2024 (style)2 marksA geologic cross section shows horizontal sedimentary layers cut by a vertical fault, and the fault is cut by an igneous intrusion. (a) State which is younger, the fault or the intrusion, and the principle you used. (b) Explain why the sedimentary layers must be older than both.Show worked answer →
A 2-point sequencing item.
(a) 1 point: the intrusion is younger than the fault, by the principle of cross-cutting relationships (a feature that cuts across another is younger than what it cuts).
(b) 1 point: the sedimentary layers were already there to be cut, so they must have formed before the fault and the intrusion that cut through them; they are the oldest.
Markers reward naming cross-cutting relationships in (a) and the reasoning that a feature must exist before it can be cut in (b).
Related dot points
- Explain radioactive decay and half-life, and calculate the age of a sample or the fraction of parent remaining using the number of half-lives that have passed (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on absolute dating for the Virginia Earth Science EOC: what radioactive decay and half-life mean, the parent-to-daughter ratio, how to count half-lives to find an age or the fraction remaining, why carbon-14 dates young organic material and uranium dates ancient rock, and how Earth's age (about 4.6 billion years) is known, with worked calculations.
- Explain how fossils form and how index fossils correlate rock layers, and describe the divisions and major events of the geologic time scale, including in Virginia (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on fossils and geologic time for the Virginia Earth Science EOC: how fossils form, what makes a good index fossil, using fossils to correlate distant rock layers and infer ancient environments, the eon-era-period divisions, major events like mass extinctions, and Virginia's fossil record, with worked exam questions.
- Classify igneous, sedimentary and metamorphic rocks by how they form and explain the rock cycle, including how cooling rate, lithification, and heat and pressure transform rock (Virginia 2018 Earth Science SOL ES.5).
A SOL-level answer on rocks for the Virginia Earth Science EOC: how igneous, sedimentary and metamorphic rocks form, the link between cooling rate and crystal size, clastic versus chemical sediment, foliated versus nonfoliated metamorphic rock, and how the rock cycle transforms one type into another, with worked exam questions.
- Distinguish mechanical and chemical weathering, identify the agents of erosion and deposition, and explain how particle size, sorting and water velocity control where sediment is deposited (Virginia 2018 Earth Science SOL ES.6 surface processes).
A SOL-level answer on surface processes for the Virginia Earth Science EOC: mechanical versus chemical weathering and what speeds each, the agents of erosion (water, wind, ice, gravity), how water velocity controls the size of sediment carried and deposited, sediment sorting and rounding, and landforms like deltas and moraines, with worked exam questions.
- Read a topographic map: interpret contour lines and the contour interval, find elevation and relief, judge slope steepness from contour spacing, and use the rule of Vs to find stream direction (Virginia 2018 Earth Science SOL ES.1 and ES.6).
A SOL-level answer on topographic maps for the Virginia Earth Science EOC: what contour lines and the contour interval mean, how to find elevation and total relief, how contour spacing shows slope steepness, how the rule of Vs gives stream direction, and how to calculate gradient, with worked exam questions.
Sources & how we know this
- 2018 Science Standards of Learning (Earth Science) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)