Geologic Time: The science of Geology didn't take shape until the discovery of geologic time (deep time).
- James Ussher, Bishop of Armagh (1581-1656) Calculated biblical Creation of Earth to have been in 4004 BC.
- Nevertheless, by Ussher's time, significant steps had been taken:
Hutton and Uniformitarianism:
- 15th and 16th century astronomers had shown that actual obsrvations of nature could yield important new information.
- Nicholas Steno: (Danish) Critically examined the current belief among intellectuals that fossils were not the remains of organisms but, rather, "sports of nature" whose resemblance to living things was coincidental. Looking at "tongue stones" which suspiciously resembled sharks' teeth, he noted that they not only looked exactly like sharks' teeth, but that he could even say what species of shark and what part of the mouth. Steno concluded that fossils were, indeed, the remains of organisms. But this led to a problem:
Fossils are found in rocks. What was a shark tooth doing inside a rock?
This led Steno to study how the rocks (which we today recognize as sedimentary) formed. He recognized them to be composed of lithified remains of sediment deposited in layers (or strata) and proposed a set of principles of stratigraphy by which one coud distinguish younger and older sediments. His results were published in Prodromus, In 1668. Two of these principles are:
- Original horizontality. Sediments originally deposited in horizontal layers
- Superposition. In undisturbed strata, older layers lie beneath younger ones.
Remember, Steno still thought that all of this deposition took place during Noah's flood. In the next stage, natural historians took up the question of whether the Earth has a long or short history.
During the 18th century, the naturalists observed that in the modern world, sediments were still being deposited in separate layers by separate depositional events, even without a world-wide deluge.
On a more prictical level, Hutton provided a third principle of stratigraphy, based on his observation of the orientation of rock layers at Siccar Point.
- Cross-cutting relations. If one structure cuts across another, then the one that is being cut must be oldest
Relative Dating: Thus the notion of Geological time was born. Using Steno's and Hutton's priciples of stratigraphy, it began to be possible to say what order the separate rock layers had formed in, provided they could be seen in association. I.e., Rocks could be dated relative to one another.
The rock record tells us more than relative age. It also tells us about the environment i which the rock formed. In fact, we say that:
A rock is a record of the environment in which it formed.
So, rocks tell us about ancient environments and how they have changed. Let's use sedimentary rocks as an example.
Depositional environment: Places where sediments are deposited. The physical characteristics and geographical location determines the type of sediment that will normally be deposited. In broad strokes, we classify depositional environments as:
Sedimentary structures: The keys to the identification of sedimentary environments are small scale physical features they display. Here are a few common or familiar examples.
- Continental: Deposited on land or in fresh water. Examples:
- Transitional: Deposited in an environment showing influence of both fresh water or air and marine water.
- Deltas: Deposits at the mouths of large rivers.
- Estuaries: Deposits in river mouths occupying valleys drowned by rising sea level.
- Shoreline: Deposits in shallowest marine water (influenced by waves).
- Marine: Only influenced by sea water.
- Stratification - layering / bedding
-individual layers are called strata (sing. stratum) or beds
beds are lain down parallel, with younger beds on top of older ones (superposition):
- Cross bedding:- beds (parallel to each other) aligned at an angle to the surface upon which they accumulated. These result from the progression of a dune or ripple:
- Big cross beds form in the dune fields of deserts
- Small crossbeds form in "subaqueous dunes" - piles of sand on the bottom of a flowingstream.
- Ripples: Formed by movement of a current over loose sand to clay sized sediment. Ripples formed by waves have a symmetrical cross section. Those formed by a stream current are asymmetrical.
- Mud cracks: Polygons formed by the contraction of wet mud exposed to the air.
Modern mud cracks.
Ancient mud cracks.
VERY COOL mud cracks.