Definition of historical geology: the science of unraveling the earth's history.

Detectives, and mysteries as a metaphor for historical geology.

Decipherment of local histories:

• Nicolaus Steno in 1600s

-> natures sports versus fossils in hard rock.

-> law of superposition

-> law of original horizontality

• James Hutton in the late 1700s
  • cross cutting relationships.
  • unconformities - 3 types:
    • nonconformity: with an older underlying sequence composed of plutonic and metamorphic rocks (basement).
    • angular unconformity: with an older underlying sedimentary sequence that is at an angle to and truncated by the unconformity.
    • disconformity: with an older underlying sedimentary sequence that is concordant to the overlying younger sequence, but a significant time gap.
  • provided conceptual framework for construction of local histories.
• Grand Canyon as an example. Time for all this to happen.
• 

  ---

  To the left is a view from the Grand Canyon outer rim wall down into the inner gorge area. Note the dashed yellow line that separates flat lying sedimentary rocks above from much older metamorphic and plutonic rocks with almost vertical layering below. Some of the whitish layers in the lower sequence are sill intrusions of the Zoraster granite into the Vishnu schist. What type of unconformity is represented by the yellow line? If you carefully at the cliff forming layer of sandstones directly above the unconformity you note they vary in thickness, and indeed a small, filled, paleovalley is seen to the right. Also note that other sedimentary rocks (red beds) lie underneath this same basal unit above the unconformity in the background, so that different rock units lie underneath this unconformity in different places.

  ---

  In this view picture below of the Grand canyon wall you can see the layers in the lower portion are at angle to and truncated by the layers above. What is this type of unconformity known as? The cliff forming sandstone above the unconformity is the same one as in the photo above and is of Cambrian age. Click on either photo for an enlargened view.
• 

An example: Below is a schematic cross section showing the relationship between various rock units. What is the history of formation of the various rock units? What events can be inferred to have occurred between or during the formation of the various rock units. Sometimes, portions of the history are not well constrained; i.e. multiple interpretations are possible. Using the above concepts a fairly detailed history can be inferred for this cross section, which is loosely based on areas in the SW part of the U.S..

Regional correlation, the geologic time scale and global history

• Werner and Universal Formations: a failed model.
• William 'Strata' Smith and George Cuvier (late 1700s) and the law of faunal succession and the science of biostratigraphy.
• Lyell in the early 1800s utilized index fossils, biostratigraphic regional correlation, and initial development of time scale.
• The unique history of life, full of extinctions and evolution provides a time scale. While basalts and sandstones have been formed here and there throughout most of the history of the earth, most species have only survived for a brief period and their remains in the sediments can be used to constrain the time the sediments were deposited. Hence the age of the dinosaurs. Extinction is a critical marker and process in this science.
• What characteristics of a fossil species would make them a good index fossil?
• Time scale units: Eras -> Periods -> Epochs -> Stages and Ages (larger to smaller)
• major eras:
  • Cenozoic: recent life, age of the mammals, since 65 Ma.
  • Mesozoic: middle life, age of the dinosaurs, 248 - 65 Ma.
  • Paleozoic, old life, age of shelly critters, 545 -248 Ma
  • Proterozoic, early life, 2.5 billion to 545 Ma
  • Archean, simple unicellular life and colonies, stromatalites, before 2.5 billion.
  • Ma = millions of years.
  • based on reference sections scattered throughout the world. The boundaries between Eras often represent some major punctuation in earth history.
• Examples of index fossils:
  • trilobites for the Lower Paleozoic.
  • foraminifera for the Upper Paleozoic.
  • ammonites for the Mesozoic.
• Based on comparisons of many stratigraphic successions from many localities around the world and on many fossils species. The geologic time scale is still in the process of refinement - think of it as continuous quality improvement.

Good reference: Eicher, D. L., 1968, Geologic Time; Prentice Hall, 2nd ed., 150 p.

Harmon D. Maher Jr. reserves copyrights to the materials in this site. Material may be used for non-profit educational purposes as long as proper attribution is given. For permission for any other use please contact author. Thank you.

Return to Physical Geology index page.

Return to my home page index.