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History of Geology The way the earth changes: Fossils, erosion & modification in the landscape 1. Shells on the top of mountains 2. Changing landscape after rain, snow, tempest, volcanic eruption, etc. 3. Presence of strata with clear boundaries and superposition 4. Change in the climate Earth Changing : fossils & erosion ● ● Fossils has been used as jewels for thousands of years They were found as decoration in Neanderthal man tombs Earth Changing : fossils & erosion ● ● Fossil: From Latin fossus meaning “having been dug” Fossils were discovered and described during the Greek and Roman periods ➔ ➔ How to explain that rocks can have the shape of shells? How to explain the presence of shells far from ocean? Earth Changing : fossils & erosion ● ● Xenophanes (540 BCE) concluded from his examination of fossils that water once must have covered all of the Earth's surface He supposed a world with alternating two extremes: earth and water Earth Changing : fossils & erosion ● ● ● The ancient alchemy tried to transform things into others (metal into gold) Aristotle though that living thing could arise spontaneously from rocks or mud Fossils would be the failure of this process Earth Changing : fossils & erosion ● ● ● Theophrastus said that fossils had never been living organisms They would be only unusually shaped rocks A “shaping force” within the earth would form stones in various shapes Earth Changing : fossils & erosion Eratosthenes (276 BCE) though that the strait of Gibraltar was closed and the level of the Mediterranean sea higher than now Earth Changing : fossils & erosion ● ● The opening of the “pillar of Hercules” would have lower the sea level The presence of fossil is explained by ancient high sea level Earth Changing : fossils & erosion ● ● Strabo (24 CE) said that the level of land would change according to earthquakes, eruptions, tsumanis, landslips, etc. Fossils would have been deposited after such disasters Earth Changing : fossils & erosion ● ● Big fossil bones were attributed to an ancient races of giant Giants were supposed to populate the world before some disaster that would have been their end Earth Changing : fossils & erosion ● ● Greeks and Romans could explain the fossilisation of still existing and complete organisms They could not imagine that nummulites, Glossopetræ, ammonites could be also fossils Earth Changing : fossils & erosion ● Pliny supposed that the glossopetræ fall from the sky during lunar eclipse Earth Changing : fossils & erosion ● ● ● Pliny supposed that the glossopetræ fall from the sky during lunar eclipse Nummulites are abundant in some limestones, especially in egypt Pyramids are built with nummulite limestones Earth Changing : fossils & erosion ● ● ● Nummulites are small lenticular fossils (1-10 cm) that was generated by unicellular organism They are common in tertiary limestones They are often found around the Mediterranean sea Earth Changing : fossils & erosion ● ● Guides in the Egyptian pyramids used to say that nummulites in the stones were the fossilised food of the slaves employed for building Most of the Greek and Roman philosophers did not believe this story but had no explanation Earth Changing : fossils & erosion ● ● ● ● Ammonites are an extinct group of mollusc They were numerous during the Secondary Era Some of them may be more than two metre wide Ammonites are good index fossil and allow precise dating Earth Changing : fossils & erosion ● ● ● The name “ammonite” comes from the common name that was given to these fossil until th the 18 century: horns of Ammon They were not supposed to be “fossils” but some natural curious rocks. No explanation for them Earth Changing : fossils & erosion Amon (Râ) was a god in Egyptian mythology who in the form of Amun-Ra became the focus of the most complex system of theology in Ancient Egypt He created himself, without father nor mother and is the god of sun Earth Changing : fossils & erosion ● ● ● Amon was often represented with the head or as a ram The spiral shape of the ammonites resembles ram's horn Even now, names of ammonites end with -ceras, which mean “horn” in greek Earth Changing : fossils & erosion In China, fossil bones and teeth were considered as “dragon bones”. They were used, as powder, as medicine to cure anything. In some areas of China, they are still sold as such today. Earth Changing : fossils & erosion ● ● ● Greeks and Roman noticed the existence of fossils They tried several hypothesis to explain their presence: ➔ The sea once covered the whole earth ➔ Increase of the sea level ➔ Shaping force of the Earth Nummulites, ammonites and glossopetræ were interpreted not as fossils but “precious stones” with magical properties Earth Changing : fossils & erosion What was the significance of fossil shells for Greek and Roman philosophers? Earth Changing : fossils & erosion What did they think about ammonites? Earth Changing : fossils & erosion What did they think about ammonites? And bout nummulites? Earth Changing : fossils & erosion How did they explain giant fossil bones? Earth Changing : fossils & erosion ● The silt carried along by rivers to the sea is the sign of the erosion of the continents Earth Changing : fossils & erosion ● ● The silt carried along by rivers to the sea is the sign of the erosion of the continents Floods, especially the Nile floods, were known to erode river banks and to leave more or less thick silt deposit Earth Changing : fossils & erosion ● ● ● Representations of floods are ancient Greeks and Romans supposed that continents should become progressively flat under the action of erosion Earth Changing : fossils & erosion ● ● ● Representations of floods are ancient Greeks and Romans supposed that continents should become progressively flat under the action of erosion The sea should, in contrast, fill up with silt Earth Changing : fossils & erosion ● ● Erosion happen at human time scale near to the seashore Roman and Greek were navigator people who noticed this phenomenon and tried to explain it Earth Changing : fossils & erosion ● Landslides and their relations with rain and water were known and described by early philosophers Earth Changing : fossils & erosion ● ● Landslides and their relations with rain and water were known and described by early philosophers The phenomenon of erosion, landslide, sedimentation were integrated in some global vision of the world Earth Changing : fossils & erosion ● ● ● Ancient text about erosion, landslide, floods, sedimentation are missing Description of these phenomenon are rather precise in the Greek and Roman world Explanations and interpretation are missing Earth Changing : fossils & erosion ● Theophrastus and Aristotle though that the world were eternal, without end or beginning ➔ ➔ ● They had to explain how the earth is not flat Their theory of the world invoked a flatteningrenewing cycle with disaster like eruption and earthquake to make new mountains Zeno of Citium (~300 BCE) argue that with no beginning the earth should be flat: for him the world was created recently with valley and mountains