What are the principles of relative dating for the NY Regents?

Superposition (in an undisturbed sequence the bottom layer is oldest), original horizontality (layers are deposited flat, so tilted or folded layers were deformed later), cross-cutting relationships (a fault or intrusion is younger than anything it cuts across), inclusions (fragments inside a rock are older than the rock containing them), and unconformities (buried erosion surfaces that record missing time). To order a cross-section, lay down the layers from the bottom up, then slot in each fold, intrusion, fault and unconformity by what it cuts or deforms.

How do you calculate the absolute age of a sample using half-life?

Count how many half-lives have passed from the ratio of remaining radioactive isotope. After one half-life half remains, after two a quarter, after three an eighth. Then multiply the number of half-lives by the half-life from the Reference Tables Radioactive Decay Data. For example, a sample with one-quarter of its Carbon-14 has gone through two half-lives, so its age is 2 times 5700 years, which is 11,400 years. A 50:50 ratio of parent to daughter is exactly one half-life.

Which radioactive isotopes does the NY Regents use, and for what?

Carbon-14, with a half-life of 5700 years, dates recent once-living material such as wood, bone and charcoal, up to a few tens of thousands of years. Uranium-238, with a half-life of 4.5 billion years, dates ancient rocks and minerals, including the oldest rocks on Earth. Carbon-14 cannot date a billion-year-old rock because its short half-life means almost all of it has decayed after tens of thousands of years; you would use Uranium-238 instead.

What makes a good index fossil?

A good index (guide) fossil lived for only a short span of geologic time, so it marks a narrow, specific age, but was geographically widespread and easily recognized, so it appears in many places. Finding the same index fossil in two distant rock layers means those layers are about the same age, which lets geologists correlate (match) layers between locations. A long-lived organism cannot pin down a precise age, and one found in only one region cannot correlate distant outcrops.

What does the rock record show about the geologic history of New York State?

The oldest bedrock (Precambrian, over a billion years old) is in the Adirondack region. Through much of the Paleozoic, shallow seas covered New York and laid down marine sedimentary rock and fossils (corals, brachiopods, trilobites). The state went through episodes of mountain-building (orogenies) as plates collided, raising ancient mountains that have since eroded. Most recently, the last ice age covered New York in glaciers, leaving striations, U-shaped valleys (the Finger Lakes), unsorted till, erratics and moraines that shape today's landscape.

New YorkEarth and Environmental Science

Dating the rock record and reading geologic history: the geologic history unit for the NY Regents

A deep-dive guide to the geologic history unit for the NY Regents: the principles of relative dating for ordering a cross-section, half-life calculations with the Radioactive Decay Data, index fossils and correlation, the geologic time scale and mass extinctions, and reading the Geologic History of New York State chart, with worked half-life and cross-section problems.

Generated by Claude Opus 4.818 min readESCI-GEOHISTUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

Reading the rock record like a detective
Ordering a cross-section
Half-life calculations
Index fossils and correlation
The geologic time scale
The geologic history of New York
Check your knowledge

Reading the rock record like a detective

The geologic history unit asks you to read time from rock. Two skills carry it: ordering events with the principles of relative dating, and putting numbers on them with half-life calculations. Both appear every administration, and the New York chart ties the story to your own state. This guide ties the dot-point pages together: relative dating and the rock record, radioactive decay and absolute age, fossils and correlation, the geologic time scale and the geologic history of New York State.

Ordering a cross-section

Work it in this order: lay the layers from the bottom up (superposition), assume they began horizontal, add folds and tilts after the layers they affect, add intrusions and faults after the rock they cut (cross-cutting), use inclusions (fragments inside a rock are older) and contact metamorphism (an intrusion bakes what it touches) to confirm, and mark unconformities as erosion events at the gaps.

Half-life calculations

Index fossils and correlation

A good index fossil is short-lived but widespread. The same index fossil in two distant layers means they are the same age (correlation). Fossils also reveal past environments (shallow-marine shells mean a shallow sea) and the history of life.

The geologic time scale

From largest to smallest: eon, era, period, epoch. Earth is about 4.6 billion years old, and Precambrian time is most of it. The Phanerozoic has three eras: Paleozoic (ancient life), Mesozoic (Age of Reptiles, dinosaurs), Cenozoic (Age of Mammals). Era boundaries are mass extinctions, abrupt changes in the fossil record.

The geologic history of New York

Read the Geologic History of New York State chart with the bedrock map: oldest rock in the Adirondacks, repeated shallow seas (marine fossils) through the Paleozoic, episodes of mountain-building (orogenies), and the most recent glaciation (striations, the Finger Lakes, till, erratics, moraines) that shaped today's landscape.

Check your knowledge

Attempt these with the Reference Tables open, then check the solutions.

State the law of superposition and the inclusions principle. (2 marks)
A sample has one-eighth of its Carbon-14 left. Find its age (half-life 5700 years). (2 marks)
A rock has a 50:50 ratio of Uranium-238 to lead-206. Find its age (half-life 4.5 billion years). (2 marks)
State the two features of a good index fossil. (2 marks)
Name the three Phanerozoic eras in order and what defines their boundaries. (2 marks)
State what marine fossils across New York indicate about its past. (2 marks)
State two pieces of evidence that glaciers once covered New York. (2 marks)

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)