1. How Old is Old?
Geologic Time Scales

2. Relative vs. Numerical Age
Geologists strive to establish both the sequence of events that produced geologic features AND the date on which each event occurred….
Relative Age: the age of one geologic feature with respect to another

3. Numerical Age (absolute age): the age of a geologic feature given in years
Geologists learned how to determine relative age long before they could determine numerical age!

4. Unconformities: Gaps in the Record!
James Hutton ( ) a Scottish farmer and doctor. One of the fathers of geological dating!
He was puzzled by this outcrop at Siccar Point on the coast of Scotland.
Do you see any “gaps”?

5. Devonian “Old Red sandstone”
Unconformity
Ordovician sandstone and shale
The gray sandstone had been deposited, turned into rock, and tilted BEFORE the red sandstone had been deposited!

6. Hutton deduced that the surface between the gray and red rock sequences represented a time interval during which new strata had not been deposited at Siccar Point and the older strata had been eroded away.

7. A surface – representing a period of non-deposition and possibly erosion, an unconformity!
Rocks below the unconformity were tilted or folded before the unconformity developed.

8. Does the Grand Canyon strata represent all Earth’s history?
BUT! By correlating rocks from locality to locality at millions of places around the world, geologists have pieced together…..
Because of unconformities, no single location on Earth contains a complete record of Earth history!

9. The Geologic Column
Geologists have pieced together a composite stratigraphic column – the geologic column.
Remember: Strata – a succession of several layers or beds together

10. Each segment represents a specific interval of time
The largest subdivisions break Earth history into the Hadean, Archean, Proterozoic, and Phanerozoic Eons.
The Hadean eon is rarely shown because rocks do not preserve a record of it.

11. The suffix –zoic means life!
Phanerozoic means visible life and Proterozoic means first life.
It wasn’t until after the eons had been named that geologists determined that the earliest life, cells of bacteria and archaea, appeared in the Archean Eon.

12. The Phanerozoic Eon is subdivided into eras.
1. Paleozoic – ancient life
2. Mesozoic – middle life
3. Cenozoic – recent life
Each era is divided into periods and each period into epochs.

13. Naming the Column – between 1760 and 1845
Sometimes by location of fairly complete strata
Example: Devonian Period crop out near Devon, England
Sometimes by characteristic
Example: Carboniferous – contain a lot of carbon!

14. Using Fossils! Back in the Industrial Revolution – coal was in demand!
Excavations of canals for coal and iron were done and these excavations exposed bedrock, which previously had been covered by vegetation.

15. Engineers learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage
Fossil assemblage: a group of fossil species found in a specific sequence of sedimentary rock

16. Other observations included:
Principle of fossil succession: In a stratigraphic sequence, different species of fossil organisms appear in a definite order
Once a fossil species disappears in a sequence of strata, it never reappears higher in the sequence

17. This provided the geologic underpinning for the theory of evolution!

18. Fossils are also used to determine the relative age of the beds!
Index fossils (guide fossils): used by geologists to associate the strata with the specific time interval

19. Simple bacteria and archaea appeared during the Archean Eon, but complex shell-less invertebrates did not evolve until the late Proterozoic.

20. The appearance of invertebrates with shells defines the Precambrian-Cambrian boundary!
Sudden diversity of life over a relatively short period of time – the Cambrian explosion!

21. Paleozoic – first fish, land plants (Silurian), amphibians (Devonian), reptiles (Carboniferous)
Mesozoic – first dinosaurs and small mammals (Triassic), birds (Cretaceous)
Cenozoic – diversification of mammals!