Relative Time: Ages of events are placed in order of occurrence. No exact date is identified. Ex. WWI and WWII Ex. I am the second child in my family.

Absolute Time: Identifies the exact date of an event. Ex. 65 Million Years Ago Ex. 1990

Let’s practice with a sequencing example…

A B C D I E H

Finding age with relative time: Principles used to determine relative order of geological events: 1. The Law of Superposition In a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom. Picture the pile of clothes in your room. Where is the first thing that you put on the floor now?

2. The law of cross-cutting relationships – anything that cuts across layers of rock is younger than the rocks that it has intruded into. This applies to faults and igneous intrusions. Igneous rock cuts earlier layering Layered rock formed first

Which statement is true? D crosscuts B 2. D crosscuts C 3. C crosscuts D 4. B was deposited before E A B C E D

3. The law of included fragments - the fragments that make up a rock are older than the entire sample.

4. Original Horizontality- Rocks are usually deposited flat and level. Picture I took of Grand Canyon

5. The Law of Folds or Tilts: tilts in rocks are younger than the rocks themselves.

Picture I took at Red Rocks Amphitheater (Colorado)

Other Guidelines for figuring out a sequence: Sedimentary rocks are usually formed under water. Weathering and erosion usually happen above water (on dry land). Contact metamorphism (“igneous intrusion”) shows that the rock that was changed was there first when the intrusion happened.

Now sequence that stratigraphy!

Sequence 1: Uplift & Erosion First limestone is deposited Then sandstone is deposited Then shale is deposited Then the layers are uplifted Lastly, erosion takes place

Sequence 2

Sequence 2: Faulting First, limestone is deposited Then sandstone is deposited Next shale is deposited Then faulting occurs

Sequence 3

Sequence 3: Folding First limestone is deposited Then sandstone is deposited Next shale is deposited Lastly folding occurs

Sequence 4

Sequence 4: Igneous Intrusion Limestone deposited Sandstone deposited Shale deposited Igneous Intrusion Note: contact metamorphism is the same event as the intrusion.

What is an Unconformity? A buried erosional surface. Look for a “wavy” line that is buried beneath the surface (indicates that at one time this layer was at the surface and weathering and erosion took place).

Formation of an unconformity Layers are formed according to superposition.

Something happens to uplift the area (folding, faulting, etc.)

Erosion wears away the uppermost layers

Area submerges and deposition begins again. Here’s the unconformity