How do we work out the order of events in the rock record without knowing exact ages?
Apply the principles of relative dating (superposition, original horizontality, cross-cutting relationships, inclusions and unconformities) to order events in a sequence of rock layers.
A Regents answer on relative dating: the law of superposition, original horizontality, cross-cutting relationships, inclusions, and how unconformities record missing time, used to put events in order in a cross-section, plus how faults, intrusions and contact metamorphism fit the sequence, with worked exam questions.
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What this topic is asking
The Regents wants you to order events in the rock record using the principles of relative dating: superposition, original horizontality, cross-cutting relationships, inclusions and unconformities. Almost every question is a cross-section you must put in order from oldest to youngest.
The principles of relative dating
Cross-cutting, inclusions and contact metamorphism
These three together let you order intrusions and faults relative to layers:
- Cross-cutting relationships: a fault or an igneous intrusion that cuts through layers is younger than every layer it cuts. If an intrusion cuts layers A, B and C but not D, the intrusion is younger than C and older than D.
- Inclusions: if layer B contains fragments (inclusions) of layer A, then A must already have existed to be broken up, so A is older than B.
- Contact metamorphism: an intrusion bakes the rock it touches, leaving a metamorphosed zone along the contact. The baked zone confirms the intrusion is younger than the rock it altered.
Unconformities: the missing time
Reading a cross-section
To order a cross-section from oldest to youngest:
- Lay down the sedimentary layers from the bottom up (superposition), assuming they started horizontal.
- Add deformation (folds, tilts) after the layers they affect.
- Add intrusions and faults after the rock they cut (cross-cutting), using contact metamorphism and inclusions to confirm order.
- Mark unconformities as erosion events at the gaps.
Try this
Q1. State the law of superposition. [1 point]
- Cue. In an undisturbed sequence of sedimentary layers, each layer is younger than the one below it (oldest at the bottom).
Q2. Explain why an igneous intrusion that cuts a layer must be younger than that layer. [2 points]
- Cue. By cross-cutting relationships, the layer had to already exist for the intrusion to cut through it, so the intrusion formed afterward.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (style)1 marksPart A. In an undisturbed sequence of sedimentary rock layers, the oldest layer is normally found (1) at the top (2) at the bottom (3) in the middle (4) it cannot be determined. Justify your choice.Show worked answer →
A 1-point multiple-choice question. The answer is (2).
By the law of superposition, in an undisturbed sequence each layer is younger than the one below it, so the oldest layer is at the bottom and the youngest at the top. (1) is the youngest layer; (3) is intermediate; (4) is wrong because superposition does determine the order in an undisturbed sequence. The trap is forgetting the "undisturbed" condition; if the layers were overturned by folding, the order could be reversed.
Regents (style)3 marksPart C. A cross-section shows four horizontal sedimentary layers (A oldest at the bottom up to D), an igneous intrusion that cuts through layers A, B and C but not D, and a fault that cuts the intrusion. (a) State which is older, the intrusion or layer C, and why. (b) State which is older, the intrusion or layer D, and why. (c) Place the fault in the sequence.Show worked answer →
A 3-point extended-response question.
(a) 1 point: layer C is older than the intrusion, because the intrusion cuts across (and therefore postdates) layer C (cross-cutting relationships).
(b) 1 point: layer D is younger than the intrusion, because the intrusion does not cut D; D was deposited after the intrusion was already in place.
(c) 1 point: the fault cuts the intrusion, so the fault is younger than the intrusion (and younger than layers A, B, C); it is among the most recent events, after the intrusion.
Markers reward applying cross-cutting relationships in both directions and placing the fault after the feature it cuts.
Related dot points
- Explain radioactive decay and half-life and use the Reference Tables Radioactive Decay Data to calculate the absolute age of a sample from the ratio of remaining radioactive isotope to its decay product.
A Regents answer on radioactive dating: what radioactive decay and half-life mean, the Reference Tables Radioactive Decay Data (Carbon-14 half-life 5700 years, Uranium-238 4.5 billion years), how to count half-lives from the ratio of parent to daughter, why Carbon-14 dates recent material and Uranium-238 dates ancient rock, with worked half-life calculations.
- Explain how fossils form, what index fossils are, and how fossils are used to correlate rock layers between distant locations and to infer past environments, using the Reference Tables.
A Regents answer on fossils and correlation: how fossils form, the features of a good index fossil (widespread, short-lived, easily recognized), how index fossils and matching rock match (correlate) layers between distant outcrops, what fossils reveal about past environments and evolution, and how to read the Geologic History of New York State chart, with worked exam questions.
- Describe how the geologic time scale is divided (eons, eras, periods, epochs), how its boundaries mark major changes in life, and use the Reference Tables geologic time scale to read ages and events.
A Regents answer on the geologic time scale: the divisions (eons, eras, periods, epochs), Precambrian time and the Paleozoic, Mesozoic and Cenozoic eras, how mass extinctions mark era boundaries, Earth's age of about 4.6 billion years, and how to read ages and events off the Reference Tables Geologic History of New York State chart, with worked exam questions.
- Use the Reference Tables Geologic History of New York State and the bedrock map to read New York's tectonic and environmental history, including ancient mountain-building, shallow seas and the most recent glaciation.
A Regents answer on New York's geologic history: how to read the Geologic History of New York State chart and the bedrock map together, the ancient mountain-building (orogenies), the shallow seas that left marine fossils and sedimentary rock, the oldest Precambrian rock of the Adirondacks, and the last ice age that shaped today's landscape, with worked exam questions.
- Explain where and why volcanoes form (boundaries and hot spots), describe how crustal rock is deformed by folding, faulting and tilting, and interpret evidence of crustal movement such as displaced rock layers and marine fossils on mountains.
A Regents answer on volcanoes and crustal deformation: why volcanoes form at subduction zones, divergent boundaries and hot spots, the Ring of Fire, how rock is folded, faulted and tilted, and the evidence that the crust has moved (displaced strata, tilted layers, marine fossils and rounded sediments now on mountains), with worked exam questions.
Sources & how we know this
- Reference Tables for Physical Setting/Earth Science (2011 edition) — New York State Education Department (2011)
- Regents Examination in Physical Setting/Earth Science — New York State Education Department (2026)