Sedimentary Rock Layers Were Originally Deposited in Horizontal Layers

Principle of Uniformitarianism – To understand geologic processes of the past, study geologic processes of today, such as volcanism and erosion.

Law of Original Horizontality

Sedimentary rock layers were originally deposited in horizontal layers

Sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed.

Unconformities

- Indicates where layers of rocks are missing in the strata sequence.

- Strata may be missing because they were never deposited or deposited and later removed (erosion).

There are three types of unconformities

1. Disconformity - the boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface.

2. Nonconformity - An unconformity in which stratified (layers of rock) rest upon unstratified rock

3. Angular unconformity - the boundary between a set of tilted layers and a set of horizontal layers.

Law of Superposition

Oldest rock layers are on the bottom

-An exception to the Law of Superposition is when tectonic forces are at work

Law of Continuity of the Layers

If sedimentary rock is missing layers in its profile it may be due to such action as erosion.

Relative Age

Indicates that one layer is older or younger than another layer but does not indicate the rock’s age in years.

b) Index Fossils - If a particular species lived only in a certain time period then you know that whatever layer you find its fossil, that layer is the same age as the fossil

Geologic Structures

•A fracture is a crack in rock.

•A dike is a sheet of rock that formed in a fracture in a pre-existing rock body. Dikes can be either magmatic (igneous intrusion) or sedimentary in origin.

•A fault is a fracture along which movement has occurred.

•Fold is when sedimentary rock is bent or curved

Law of Cross-Cutting

Any geologic event that affects a rock layer must be younger than the rock layer it affects

Based on what you now know about the Law of Superposition, the Principle of Original Horizontality, unconformities, and the Law of Crosscutting Relationships can you place the layers indicated in the diagram in the correct order, starting from the oldest layer?