Introduction to Historical Geology

Historical Geology is the study of changes to Earth and life in time and space.

I. Time

A. How much time?

B. How do we know?

    Radioactive materials serve as geologic clocks.

C. What happened on the Earth during this long period of time?

    Many natural events:

    All of these natural events are still going on today -

    We see evidence in the rock record that these events have been occurring for a long time.

Geologists call this uniformitarianism.

The physical laws governing the universe operate uniformly through time.

See Levin, 6th edition, p. 6-7.

James Hutton (1726-1797) believed that "the past history of our globe must be explained by what can be seen to be happening now."

Some events which occurred in the past, and left a record in the rocks, ARE NOT OCCURRING TODAY, or have not occurred in the human lifespan:

Other events occur so slowly that they are difficult to measure:

Still other events are short lived but very catastrophic:

Evidence of all of these events is preserved in the geologic record.

D. What do you mean by geologic record?

The geologic record consists of rock units, each of which records some event or series of events that occurred in the past.

  1. What are rocks?
    1. Rocks are defined as aggregates of one or more minerals.

  2. What are minerals?
    1. naturally occurring
    2. inorganic
    3. solids
    4. definite chemical composition
    5. characteristic internal crystal structure (arrangement of atoms in lattice)

E. How do rocks form?

II. Fundamental principles of Geology

Most sedimentary rocks occur in the form of layers called beds or strata.

Each layer is the result of the deposition of sediment during some natural event (such as a flood or storm).

A. Steno's Laws Named for Nicholaus Steno, a Danish physician living in Florence, Italy in the 1600's.

Levin, 6th edition, p. 3-4.

  1. Principle of Superposition
    1. Oldest rocks on the bottom
    2. Younger rocks on top

  2. Principle of Original Horizontality
    1. Sediments are deposited in flat layers

  3. Principle of Original Lateral Continuity
    1. Sediments are deposited over a large area in a continuous sheet

B. Other basic principles of Geology which we can use for relative dating (or determining which rocks are older or younger)

  1. Principle of intrusive relationships

    2. The intrusion is younger than the rocks it cuts.

  2. Principle of cross-cutting relationships

    3. See Levin, 6th edition, p. 10

    The fault is younger than the rocks it cuts.

  3. Principle of components or inclusions

    4. See Levin, 6th edition, p. 10-11

    Note the irregular erosional surface. This is an unconformity.

    The clasts (in the bed above the unconformity) are derived from the underlying (older) bed.

    The gravel clasts are older than the layer which contains them.

    The layer containing the gravel must be younger than the layer from which the clasts originate.

    The principle of components or inclusions also applies to xenoliths.

    A xenolith is a fragment of country rocks which has been broken off during an intrusion, and has become surrounded by magma. The xenolith is older than the igneous rock which contains it.

    Compare illustrations in Levin, 6th edition, p. 11

  4. Principle of fossil succession

    Fossils occur in a consistent vertical order in sedimentary rocks all over the world.
    (William"Strata Bill" Smith, late 1700's, England).

    This principle is valid and does not depend on any pre-existing ideas of evolution. (In fact, Charles Darwin's ideas on evolution did not appear until 50 years later - 1858).

    Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.

    See Levin, 6th edition, p. 7-8.

We can use these 7 principles to establish the relative ages of rocks.

Unconformities

Unconformities are buried surfaces of erosion or non-deposition.

See illustrations in Levin, 6th edition, p. 8, 11, 17, 91 and 92

  1. Angular unconformities

    Implies tectonic deformation and erosion of underlying strata.

  2. Nonconformity

    Sedimentary strata overlying igneous or metamorphic rocks (in an erosional - not intrusive- contact)

  3. Disconformity

    An irregular surface of erosion betwen two units of parallel strata

  4. Paraconformity

    A planar surface between two parallel units of sedimentary rock, representing a period of non-deposition, but no erosion.

Criteria for recognizing unconformities:
  1. Sedimentary criteria
    1. Basal conglomerate - Many unconformities are overlain by a layer of conglomerate or gravel. The clasts are commonly fragments eroded from the underlying rock.

    2. Buried soil profiles.

    3. Layers of phosphatized pebbles, glauconite (greensand), or manganese-rich beds.

  2. Paleontological criteria
    1. Abrupt changes in fossil assemblages
    2. Presence of bone or tooth conglomerates
    3. Abrupt change from marine to continental fossils.

  3. Structural criteria
    1. Discordance of dip above and below a contact (angular unconformity).
    2. Irregular or undulatory contact that cuts across bedding planes in the underlying unit.
    3. Truncation of dikes or faults at a contact.

Return to Online Historical Web page

Return to GSAMS Historical Geology Web page

Return to Georgia Geoscience On-line

Document copyright: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

Created September 1995
Modified April 4, 1997
Modified February 5, 1999
Image link updated August 15, 2009