1. Geologic Time Scale
Historical Geology

2. Geologic Time Scale

3. Geologic Time Scale Used relative age dating
Developed in 19th and 20th Century
Used relative age dating
Principle of Superposition
Faunal succession
Angular unconformities
X-cutting relationships
Absolute ages came later

4. Piecing Together Time Scale
Examined outcrops around the world
Each time interval = set of rocks and fossils
Time scale is piecemeal
Unconformities played major role
Unit boundaries placed at unconformities
Thought unconformities had widespread correlation
Unconformities can be local

5. Geologic Time Scale Scale is divided by Eons Eras, Periods and Epochs
Eon is the largest division
Proterozoic and PreCambrian Eons
Before life became abundant
7/8 of all time
Phanerozoic Eon
visible life
Represents 1/8 of time

6. Precambrian Eon Eons subdivided into Eras
Hadean Era (beneath the earth) Ga
Archean Era (ancient) Ga
Proterozoic Era (former life)
2.5 Ga-543 Ma

7. Phanerozoic Eon Phanerozoic =Visible life
Boundaries defined by massive extinction events
Paleozoic Era (ancient life)
Age of Fishes
Mesozoic Era (middle life)
Age of Dinosaurs
Cenozoic Era (recent life)
Age of Mammals

8. Geologic Time Scale Jurassic Eras divided into Periods
Boundaries due to less drastic faunal and floral changes
Different Periods named after localities or rock types
Triassic
Three-part division of rock types in Germany
Cretaceous
“Creta” - Latin for chalk
Jurassic
For Jura Mountains

9. Geologic Time Scale Periods subdivided into Epochs
Based on % of recent fossils
Eocene
1.5% fossils = recent species
Miocene
20-40% fossils
Pliocene
50-100% fossils
Pleistocene
Later introduced

10. Naming Time Scale

11. Naming Time Scale Sedgwick & Murchison
Mapped from bottom of greywackes (Northern Wales)
Identified Cambrian
Latin for Wales
Murchison
Mapped from top of greywackes
Identified Silurian
Ancient Welsh tribe

12. Sedgwick & Murchison Controversy
Sedgwick used rocks; Murchison used fossils
Discovered both systems overlapped
Dispute resolved after both died
Broke out Ordovician
Ancient Celtic tribe

13. Geologic Time Scale Memorize

14. Resources for Learning Times Scale
mnemonic for learning time scale
practice naming time scale, insert names in blank fields and check answers
USGS online publication on geologic time
comprehending geologic time in days, meters, kilometers, and years
UC Berkeley clickable geologic time scale—learn about the different periods