Evidence of the Past

Correlations

• Definition: Matching rock layers (strata) or geologic phenomena of similar ages across different regions to develop a geologic time scale.

• Key Methods:

  1. Stratigraphic Correlation: Identifies sedimentary layers of the same age by their sequence, thickness, and features. Uses stratigraphic columns to detail layers.

  2. Lithostratigraphic Correlation: Matches rock layers based on their physical properties (e.g., color, mineralogy, grain size).

  3. Chronostratigraphic Correlation: Matches rock layers of the same age, even if they differ in composition or form in varied locations.

  4. Biostratigraphic Correlation: Uses index fossils to determine the age of strata. Index fossils represent species that existed during specific time intervals.

Fossils

• Definition: Remains or traces of ancient plants or animals, offering insights into evolution and Earth's history.

• Key Types:

  1. Fossilized Body Parts: Bones, teeth, claws, skin, etc.

  2. Fossilized Traces (Ichnofossils): Footprints, nests, tooth marks, or feces.

• Four Fossil Categories:

  1. True Form Fossil: Entire organisms preserved (e.g., in amber, ice).

  2. Mold Fossil: Hollow impressions left by organisms.

  3. Cast Fossil: Minerals fill molds, forming replicas of the original organism.

  4. Trace Fossil: Tracks or imprints from organism activities.

Principle of Fossil Succession

• States that fossils in sedimentary rock layers are organized chronologically, with older fossils found in deeper layers.

• Developed by William Smith (late 1700s), who noted unique fossil contents in sedimentary strata and used them for correlation.

Index Fossils

• Fossils used as time indicators to correlate rock layers of similar ages across different regions.

• Help define specific spans of geologic time.