Basic Geologic Concepts and Principles

 

 

Uniformitarianism—“The present is the key to the past”.  This concept allows scientist to describe environments of the past by looking at similar rock-forming conditions of today.  It is also used to describe geologic events of the past.

 

Stratigraphic Superposition—Younger rock units are deposited on top of older rock units.  Provided that noting has happened, older rocks are encountered with greater depth.  When rock units do not follow this principle, it must be determined what the sequence of events were that made them otherwise. 

 

Original Horizontality—All sedimentary and extrusive igneous rocks are deposited at or near horizontal due to gravity.  If they are not found this way, it must be determined what the sequence of events were that made them otherwise.

 

Intrusion and Deformation—Faults, folds and intrusions (dikes, stocks, batholiths, sills, etc.) are younger than the rock units (country rock) they deform, fault or intrude.  This allows us to make statements about relative ages of rocks and/or events

 

Faunal Succession—This principle recognizes that plant and animal fossils follow an ordered sequence throughout geologic time.  The principle allows geologists to correlate distant rock units, describe environments of the past and place age dates on rock units.

 

Unconformity—Describes rock units that are missing in a given stratigraphic column.  Rocks may be missing because they were eroded away or were never deposited in the first place.  Since rocks represent time, missing rock units equate to missing time in a stratigraphic column.  The unconformity is represented by an erosional surface between two rock units.

 

Note:  at any given time in geologic history rocks (or sediment) in any given area are being deposited or eroded.  Time is being “erased” by erosion (unconformities) and being recorded by deposition.

 

 

 

 

 

 

 

 

 

 

 

Sept ‘04