Principles of Geology

Each branch of science is based on fundamental principles. The periodic table in chemistry and conservation of matter and energy in physics are examples. The founding principles of geology are:

1. (Nicholas Steno, 1638-1686) Superposition. In an undisturbed sequence, a rock layer is younger than the layer upon which it rests and older than the rocks that rest on it. Like a brick wall, older rocks (like the first laid bricks) are at the base of a sequence and younger layers (later courses of brick) are on top.
   

2. (Steno) Original horizontality. Sedimentary rocks were originally deposited in nearly flat layers.

3. (Steno) Lateral continuity. Within the limits of a particular environment, similar sequences of rocks from one area can be matched to those in another area: across a valley, extending underground, and even across continents.

4. (James Hutton, 1726-1797) Cross-cutting relations. A fold, fault, or volcanic rock is younger than the rocks it effects.

5. (Hutton) Unconformities. Some boundaries between rock layers represent significant time during which either no rocks were deposited or pre-existing rocks were removed by erosion.

6. (Hutton) Uniformitarianism. The “present is the key to the past” idea. Environments and processes we see shaping the Earth around us today can be used as models for what happened in the past.

7. (William "Strata" Smith, 1769-1839) Faunal succession. The fossils in a rock are unique to that unit. This modern name implies change through time, but succession is a more recent idea than the original concept. Charles Darwin didn't publish Origin of the Species until 20 years after Smith died.

These principles form a set of guidelines for studying rocks. They are not unbreakable laws like the law of gravity, for example. They do, however, establish an orderly way to begin the study of rocks. When a sequence of rocks doesn't conform to these principles, geologists know to look closely and carefully to determine what the story is that the rocks are telling.

Apparent violations of Superposition, like the Lewis Overthrust, for example, are often cited as cause to reject geology and the fossil record. This criticism fails to admit that geologists (and many others): 1) have observed rocks moving in earthquakes; 2) have seen and investigated the structures and marks left in rocks by that movement; and 3) have found those same features in faults and overthrusts that may have moved millions of years ago. Geologists have been able to trace the movement of the rocks over time and match the rocks that moved to their original positions.

Before I leave this subject, a couple of terms should be defined:

1. Fault -- a break in a sequence of rocks along which movement has occurred, the San Andreas Fault for example
2. Fold -- an upward (anticline) or downward (syncline) bending of rocks (occurs after they were formed or deposited)
3. Overthrust -- a type of fault that is nearly flat and where the rocks above the fault have moved a long way relative to the rocks below
   
4. Thrust fault -- a type of fault implying a specific movement of one side of the fault relative to the other

As a geologist, I'm sorely tempted to define such minutiae as hanging wall, foot wall, normal fault, slickensides, recumbent folds, facies, and others, but I hear the patter of patter of dodos approaching.