Stratigraphy is the study of rock layers, or strata, and it is one of the main tools paleontologists use to place dinosaur fossils in time. Sedimentary layers form as mud, sand, ash, and other materials settle and harden, often preserving bones, shells, footprints, ripple marks, and plant remains. By reading the order, texture, and contents of layers, scientists can reconstruct ancient environments and build a timeline of Earth history.
This matters because dinosaur fossils are rarely found with a date stamped on them, so their layer is often the first clue to their age.
Key Facts
- Law of superposition: in an undisturbed sedimentary sequence, lower layers are older than upper layers.
- Relative dating tells whether one rock or fossil is older or younger than another, but not its exact age.
- Absolute dating gives a numerical age, often using radioactive decay in minerals from volcanic ash beds.
- Half-life formula: remaining fraction = (1/2)^n, where n is the number of half-lives elapsed.
- Index fossils are widespread, easy to recognize, and limited to a short time range, making them useful for matching rock layers.
- Cross-cutting relationship: a fault, intrusion, or erosion surface is younger than the layers it cuts across.
Vocabulary
- Stratum
- A stratum is a single layer of sedimentary rock or sediment with distinct characteristics.
- Stratigraphic column
- A stratigraphic column is a vertical diagram that shows the order, thickness, and features of rock layers at a location.
- Unconformity
- An unconformity is a gap in the rock record caused by erosion or a time interval when no sediment was deposited.
- Index fossil
- An index fossil is a fossil used to match rock layers because the organism lived over a wide area for a relatively short time.
- Trace fossil
- A trace fossil is evidence of an organism's activity, such as footprints, burrows, nests, or feeding marks.
Common Mistakes to Avoid
- Assuming every lower fossil is older in all cases is wrong because folding, faulting, or overturning can disturb the original order of layers.
- Treating relative dating as an exact age is wrong because relative dating only gives an order, such as older than or younger than.
- Dating dinosaur bone directly with carbon-14 is usually wrong because most dinosaur fossils are far older than the useful range of carbon-14 dating.
- Ignoring ash beds is a mistake because volcanic ash can contain datable minerals that provide numerical ages for nearby fossil layers.
Practice Questions
- 1 A cliff has five undisturbed layers labeled A at the bottom through E at the top. A dinosaur footprint is in layer B and a dinosaur bone is in layer D. Which fossil is older, and why?
- 2 A volcanic ash bed above a fossil layer is dated at 72 million years old, and an ash bed below it is dated at 75 million years old. What is the possible age range of the fossil layer?
- 3 A rock column contains ripple marks, mud cracks, dinosaur footprints, and a thin ash bed. Explain what these features could tell a paleontologist about the environment, timing, and preservation of the dinosaur evidence.