Uniformitarianism is based on the idea of geology being explainable in terms of directly observable processes. Certainly, processes can take place at different paces. But this is still the same processes at different orders of magnitude. To some extent the laws of nature allow for such to happen. In contrast, they don’t allow something entirely different to happen all of a sudden. The latter consists the basis of pseudoscientific catastrophism.
A few examples of things observed in historical times:
1. Beach material is moved by waves and is accumulated were they are stopped.
2. Clay is accumulated on the bottom of lakes and seas.
3. Corals lose pieces which are accumulated on the sea floor.
4. Dust is moved by wind and is accumulated where the wind is stopped.
5. Earthquakes can make the ground locally rise or fall.
6. Glaciers melt and leave a mix of rocks, gravel and sand.
7. Land rises or sinks which is noticeable over generations.
8. Landslides take lose material with them down slopes.
9. Peat is formed below the water’s surface in nutrition-poor wetlands.
11. Saltpans form in salt lakes periodically drying up.
12. Sand is moved by the wind and form sand dunes.
13. Sea level has risen measurably over the decades.
14. Shells from various sea animals can be accumulated on the bottom.
15. Volcanoes throw up lava and ash when they have eruptions.
Even meteoritic impacts have been directly observed. Impacts large enough to cause local devastation happen once a century. Even larger impacts than these must cause larger damage. Effects can then be extrapolated to regional and global scale. This way, single disasters can be described in observable terms.
Early scientists described rock layers in terms of observed processes. The boundaries between different layers can be used to determine which was formed first. In some cases, one can see rock has protruded into another. Then the intruding rock must have formed later. In the early 19th century people saw fossil forming patterns. Specific fossils were only found in specific layers. (Usually, they are invertebrates having lived in water.) The order in which these rock layers formed was always the same too. This made it possible to correlate the ages of rock layers. First it was across
To keep track of all rock layers geological periods were defined. In the middle of the 19th century 11 such had been defined. However, it was still disputed if a certain group of rock layers were Cambrian or Silurian. This problem was solved in 1879 by classifying them separately. The Ordovician was then inserted between the Cambrian and Silurian. In modern times the Tertiary has been divided into Paleogene and the Neogene. Moreover, the Ediacaran has been added before the Cambrian. Each period’s beginning is defined by the age of a certain rock layer. This is marked by a gilded nail hammered into the rock. Everything younger until the next definition is seen as part of this period. It was not until the 20th century we could put numbers on their age. Thanks to this it become possible to find out more about the time before. Additional periods could be defined by an absolute date. But it is only the ones before the Ediacaran which are defined this way.
The Earth’s history is now divided into four eons (¤). All but the first are divided into ten eras (ф). The two last eons are divided into 22 periods (•). I use different symbols to make the list clearer. Their age is now dated like this:
¤ Hadean was 4,550 – 4,000 million years ago.
¤ Archean was 4,000 – 2,500 million years ago.
ф Eoarchean was 4,000 – 3,600 million years ago.
ф Paleoarchean was 3,600 – 3,200 million years ago.
ф Mesoarchean was 3,200 – 2,800 million years ago.
ф Neoarchean was 2,800 – 2,500 million years ago.
¤ Proterozoic was 2,500 – 539 million years ago.
ф Paleoproterozoic was 2,500 – 1,600 million years ago.
• Siderian was 2,500 – 2,300 million years ago.
• Rhyacian was 2,300 – 2,050 million years ago.
• Orosirian was 2,050 – 1,800 million years ago.
• Statherian was 1,800 – 1,600 million years ago.
ф Mesoproterozoic was 1,600 – 1,000 million years ago.
• Calymmian was 1,600 – 1,400 million years ago.
• Ectasian was 1,400 – 1,200 million years ago.
• Stenian was 1,200 – 1,000 million years ago.
ф Neoproterozoic was 1,000 – 539 million years ago.
• Tonian was 1,000 – 720 million years ago.
• Cryogenian was 720 – 635 million years ago.
• Ediacaran was 635 – 539 million years ago.
¤ Phanerozoic is the last 539 million years.
ф Palaeozoic was 539 – 252 million years ago.
• Cambrian was 539 – 485 million years ago.
• Ordovician was 485 – 444 million years ago.
• Silurian was 444 – 419 million years ago.
• Devonian was 419 – 359 million years ago.
• Carboniferous was 359 – 299 million years ago.
• Permian was 299 – 252 million years ago.
ф Mesozoic was 252 – 66 million years ago.
• Triassic was 252 – 201 million years ago.
• Jurassic was 201 – 145 million years ago.
• Cretaceous 145 – 66 million years ago.
ф Cenozoic is the last 66 million years.
• Paleogene was 66 – 23 million years ago.
• Neogene was 23 – 2,6 million years ago.
• Quaternary is the last 2.6 million years.
Please note the human age is contained entirely within the Quaternary. If one does not count Homo habilis, which is 0.2 million years older.
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