Geology
Geology (from Greek - (ge-, "the earth") and  (logos, "word", "reason")) is the science and study of the Earth, its make-up, structure, physical properties, history, and the processes that shape it.
Geologists have helped establish the age of the Earth at about 4.5 billion (4.5x109) years, and that tectonic plates move over a viscous mantle to form the earth's crust. Geologists help locate and manage the earth's natural resources, such as oil.
Astrogeology refers to the application of geologic principles to other bodies of the solar system. However, specialised terms such as selenology (studies of the Moon), areology (of Mars), etc., are also in use.
The word was first used by Jean-Andre Deluc (1727 - 1817) in the year 1778 and introduced by Horace-Benedict de Saussure (1740 - 1799) in the year 1779 as a fixed term. 

# History
Georg Agricola (1494-1555) wrote the first systematic treatise about mining and smelting works, De re metallica libri XII, with an appendix Buch von den Lebewesen unter Tage (book of the creatures beneath the earth). He covered subjects like wind energy, hydrodynamic power, melting cookers, transport of ores, extraction of soda, sulfur and alum, and administrative issues.  The book was published in 1556.
James Hutton is often viewed as the first modern geologist.  In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh.  In his paper, he explained his theory that the Earth must be much older than had previously been supposed, in order to allow enough time for mountains to be eroded, and for the sediment to form new rocks at the bottom of the sea, which were then raised up to dry land.
Followers of Hutton were known as plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the neptunists, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.
William Smith (1769-1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.
Sir Charles Lyell first published his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875.  He successfully promoted the doctrine of uniformitarianism.  This theory states that slow geological processes occurred throughout the earth's history, and are still occurring today.  In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter.  (Hutton believed in uniformitarianism, but the idea was not widely accepted at the time.)
The theory of continental drift was proposed by Alfred Wegener in 1912 and by Arthur Holmes, but wasn't broadly accepted until the 1960s when the theory of plate tectonics was developed.
See also: Timeline of geology

# Fields or Sub-disciplines
- economic geology
- environmental geology
- geochemistry
- geochronology
- geodetics
- geomicrobiology
- geomorphology
- geophysics
- glaciology
- hydrogeology or geohydrology
- marine geology
- mineralogy
- paleoclimatology
- paleontology
- pedology (soil study)
- petroleum geology
- petrology
- sedimentology
- seismology
- stratigraphy
- structural geology
- tectonics
- volcanology
There is also engineering geology, which supports civil engineering, especially geotechnical engineering, and geological engineering.  The difference between geological engineering and engineering geology is real: geological engineers are licensed as engineers,  engineering geologists are licensed as geologists.

# Regional Geology
- Geology of the Alps
- Genesee River: Glacial Geology (New York, Pennsylvania)

# National Geology
- Geology of Australia
Geology of Victoria
- Geology of Victoria
- Geology of the United States of America
Geology of California
Geology of the Grand Canyon area
- Geology of California
- Geology of the Grand Canyon area
- Geology of the United Kingdom