Chapter 8: THE ROCK RECORD
I. DETERMINING RELATIVE AGE
- geologists estimate that the Earth is 4.6 billion years old
- James Hutton
- 18 th century Scottish physician and farmer
- wrote about agriculture, weather, climate physics and philosophy
- drew conclusions based on geological events
A. Uniformitarianism
1. Hutton theorized that the same forces that changed the
Landscape presently changed the landscape in the past
2. studying the present, could learn about the past
3. Uniformitarianism a. current geologic processes are the same processes that were
at work in the past
1a. volcanoes
2a. erosion b. one of the basic foundations of the science of geology c. processes of the past and present are the same, the rate of
the processes may vary over time
4. Earth’s Age a. before Hutton, people thought the Earth was only
6000 yrs old b. Hutton observed that forces the operated slowly c. reasoned that millions of years must be needed to create the
complicated rock structures of Earth’s crust
B. Relative Age
1. strata a. layers of rock b. show the sequence of events that took place in the past c. relative age
1c. indicates that one layer is older or younger than another layer
2c. does not indicate age of rock in years
2. various types of rock form layers a. igneous rock form layers when successive lava flows stack on
top of each other b. metamorphic rock, marble, can show layers c. sedimentary rock is commonly studied by scientists

C. Law of Superposition
1. sedimentary rock forms when new sediments are deposited on
old sediments and solidify
2. rock layers are called beds
3. the boundary between beds is called a bedding plane
4. Law of Superposition a. used to determine the relative age of a layer of sedimentary
rock b. the law that a sedimentary rock layer is older than the layers
above it and younger than the layers below it if the layers
are not disturbed c. Fig. 3
D. Principle of Original Horizontality
1. sedimentary rocks left undisturbed will remain in horizontal
layers
2. layers that are not horizontal have been tilted or deformed by
crustal movements that happened after the layers formed
3. tectonic forces a. forces can push older layers on top of younger ones b. law of superposition cannot be easily applied
4. clues to apply the Law of Superposition a. graded bedding
1a. largest particles of sediment are deposited in the bottom of the layer
2a. graded bedding – arrangement of layers in which coarse and heavy particles are located in the bottom of the layer
3a. if larger particles are located at the top of the layer, the layer may have been overturned b. cross beds
1b. cross beds – sandy sediment forms curved beds at an angle to the bedding plane
2b. tops of layers commonly erode before new layers are deposited
3b. sediment appear to be curved at the bottom

c. ripple marks
1c. ripple marks – small waves that form on the surface of sand because of the action of wind and/or water
2c. ripple marks may be preserved in sandstone
3c. crests of ripple marks point upward
E. Unconformities
1. unconformity a. a break in the geologic record created when rock layers are
eroded or when sediment is not deposited for a long period
of time b. three types (table 1)
1b. nonconformity – stratified rock rests upon unstratified rock
2b. angular unconformity – boundary between a set of tilted layers and set of horizontal layers
3b. disconformity – boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface
2. all rocks beneath an unconformity are older than the rocks
above the unconformity

3. Crosscutting Relationships a. fault – a break or crack in Earth’s crust along which rocks
shift their position b. intrusion – a mass of igneous rock that forms when magma
is injected into rock and then cools and solidifies c. Law of Crosscutting Relationships – the principle that a
fault of body of rock (intrusion) is younger than any other
body of rock that it cuts through (Fig. 5)