Principles of Relative Dating
* Superposition: The oldest layers of sedimentary rock are at the bottom, and the youngest are at the top.
* Original Horizontality: Sedimentary layers are originally deposited horizontally. If they are tilted or folded, it means something has happened since deposition.
* Lateral Continuity: Sedimentary layers extend laterally in all directions until they thin out or pinch out.
* Cross-Cutting Relationships: Any feature that cuts across a rock layer is younger than the layer it cuts.
* Inclusions: Fragments of one rock type found within another are older than the rock containing them.
Conclusions from Relative Dating
* Sequence of Events: The scientist can determine the order in which the sedimentary layers were deposited. This tells us the sequence of events in the hillside's history.
* Past Environments: Sedimentary rocks often tell us about the environment in which they were formed. For example, sandstone might indicate a desert, limestone might indicate a marine environment, and coal might indicate a swamp. By analyzing the types of sedimentary rocks and their sequence, the scientist can reconstruct the environmental history of the hillside.
* Geologic Processes: By looking at the relationships between layers, the scientist can identify geological processes that have acted upon the hillside.
* Folding and Tilting: Indicates compressional forces.
* Faults: Indicate displacement of the earth's crust.
* Erosion: Indicates the removal of rock material.
* Relative Age of Features: The scientist can compare the age of different features within the hillside, such as faults, intrusions, or erosional surfaces, to understand their relative timing.
Limitations
* Absolute Age: Relative dating only provides a sequence of events, not specific numerical ages. To get absolute ages, scientists use methods like radiometric dating.
* Incomplete Record: Erosion or other processes can remove parts of the sedimentary record, making the sequence incomplete.
In Summary
Relative dating of sedimentary rocks is a powerful tool for understanding the geological history of a hillside. It provides a framework for reconstructing past environments, identifying geological processes, and understanding the relative timing of events.
