1. Chapter 17.1 Principle of Uniformitarianism
“Current geological processes are
the same as those at work in the past”
James Hutton ( )
Father of modern geology
Men of Rock – Deep Time (Stop at 39 min)
Hutton’s Uncomformity (BBC 3:48)
Sir Charles Lyell ( )
Author “Principles of Geology”
Foremost geologist of his day
James Hutton- The Discovery of deep time 6:17

2. Determining Relative Age
(What is older, or younger... not how old chronologically)
1. Law of Superposition – layers of sedimentary rock are older than the layers above it, and younger than the layers below it.

3. 2. Unconformities Movements of the earth lift up rock layers.
Layers are exposed to erosion.
Sediments are deposited, forming new rock layers
Stratified rock rests
upon unstratified rock
Stratified rock rests
upon tilted stratified rock
Stratified rock rests
upon stratified rock, with a large time gap

4. 3. Crosscutting Relationships – a fault or an intrusion
is always younger than the rock layers it cuts through
A – oldest; deepest layer
B – next oldest, layer above
C – next oldest, layer above
D – dike cuts through existing layers
E – crack filled through existing layers; youngest
Identify the
oldest to the
youngest layers 
Turn & Talk

5. 17.2 Determining Absolute Age
(How old? ,000 yrs old? A million years old?)
1. Rates of Erosion and Deposition
Erosion rates are accurate, only w/in the last thousand years.
Over long periods of time, erosion rates can vary greatly.
The edge of Niagara Falls is eroding at 1.3 meters/year. Estimate the falls formed 9,900 years ago.
Only useful w/in ~10-20 thousand years. Less dependable for older features. (i.e. Grand Canyon) because erosion rates vary greatly.
~30 cm of sedimentary rock is deposited every 1000 yrs.
But, floods can deposit many meters of sediment in a single day

6. 2. Varve Count May be inconsistent. Why?
Like counting tree rings, Varve counts are based on annual sedimentary deposits
Layers (varves) of alternating light and dark colored bands
Light bands – course particles
Dark bands – fine particles
Varves usually formed in glacial lakes. During the summer, snow and ice melt rapidly; water can carry large amounts of sediment into a lake.
Most course particles settle quickly to form layer on the bottom of the lake. With the coming of winter, the surface of the lake begins to freeze.
Fine cay particles, still suspended in the water settle slowly to form a thin layer on top. The winter and summer layers form one varve.
May be inconsistent. Why?

7. 3. Radioactive Decay
More accurate method for finding the absolute age of rocks
Unstable isotopes lose particles and energy at a constant rate.
This rate of change is measureable
and is not affected by external factors
U-238  emits 2+ & 2N...
mass now 234
element now 90 (Thorium)
Review from Chapter 8:
The atomic number = # protons.
The atomic mass = the sum of protons + neutrons
U-238 has 92 protons & 146 neutrons (mass = 238)
This cycle continues until a stable form (Pb-206) is reached SEE TABLE p. 239

8. 3. Radioactive Decay (con’t)
Half-Life –
time it takes for half the mass of
a given element to decay into its
daughter elements
Uranium-238 has a very long half life (4.5 billion years)
useful for rocks 10 million to 4.5 billion years old
Carbon-14 has a very short half life (5730 years)
useful for rocks between 100 to 30,000 years
only used for organic materials (once living)

9. The Fossil Record
Fossils – remains or traces of animals or plants from a previous geologic time
Paleontology – the study of fossils
Fossils provide clues to past geologic events, climates, and evolution of species.
Fossils are generally found in sedimentary rock.
Igneous rock forms from magma; remains burn up
Metamorphic rock forms from heat and pressure, destroying any fossils inside

10. Kinds of Fossils
Preservation of Organisms - Usually only the hard parts of
organisms fossilize; in rare cases, the whole organism is preserved...
a. Mummification - drying
c. Tar Beds – large variety
b. Amber – tree sap
Otzi – 5300 frozen human in Russia. Had tattoos, 2 wounds (1 fatal)... attacked? Had just eaten dinner.
Freezing keeps an animals skin and muscles intact.
d. Freezing

11. 2. Petrification – minerals in ground water replace the original
organic material; slow process, often resulting in perfect replica
Florissant fossil beds National Monument

12. 3. Traces of Organisms – no part of the original tissue survives
Coprolites
Trace fossils – example: footprints, burrows
Gastroliths
Imprints
Molds
Casts

13. Interpreting the Fossil Record
Index Fossils – fossils found exclusively in
rock layers of a particular geologic age.
Trilobites ( mya) & Ammonites ( mya)
are good index fossils