The geologic time scale began to take shape in the 1700s. Geologists used fundamental concepts to understand the chronological order of rocks around the world. It wasn't until the advent of radiometric dating techniques in the middle 1900s that reliable dates could be assigned to the previously named geologic time divisions.
The National Park System contains a magnificent record of geologic time because rocks from each period of the geologic time scale are preserved in park landscapes.
Fundamental Geologic Concepts
- Uniformitarianism: Geologic processes operating on the Earth's crust have acted in the same manner and relative intensity throughout geologic time.
- Superposition: In an undisturbed sequence of sediments or rocks, the older layers occur at the bottom with successively younger layers on top.
- Original Horizontality: Layers of sediment are originally deposited horiztonal to the Earth's surface.
- Lateral Continuity: Layers of sediment start out as continuous beds spreading out in all directions.
- Cross-Cutting Relationships: A rock layer that is cross-cut by a fault, rock intrusion, or an erosion surface is older than the cross-cutting feature.
- Faunal Succession: Fossils often exhibit identifiable patterns or characteristics which progressively change over time. A rock containing a certain fossil must have been deposited during the time that creature existed on earth.