Čtvrtek, 28. března 2024

Cairo geology (Zajímavosti z geologie Káhiry)

Cairo geology (Zajímavosti z geologie Káhiry)

The city of Cairo is built on millions-of-years-old fossil limestone. You have to travel to the outskirts, though, to get a sense of the Cairo's terra firma -- at Moqattam, where the limestone outcrops from the necropolis; on Ain Sokhna Road, where the cuttings hold petrified tree trunks; or at Wadi Degla, a natural valley on the outskirts of Maadi.

The story of Cairo's geology: The first geologic map was completed in 1983. Since then, the science has improved, but the city has spread quickly outwards.

At Wadi Degla, industrial development from the 1990s has laid asphalt across limestone and shale crust. Nearby quarries mine limestone that is used to make cement.

The thousands of apartment blocks visible to the northwest, are made up of rocks from the surrounding valleys.

On Cairo's surface over the years, pyramids have been built, wars waged and presidents toppled. But below, the last big change happened around the same time mammals evolved. The rock surrounding Wadi Degla was probably formed in the Eocene epoch. The valley itself was probably cut during the Pliocene epoch..

The line about Egypt's Grand Canyon turns out to be just a bit of marketing spin from the mind of Ahmed Seddik. There are bigger wadis than Wadi Degla, which reaches about 50 meters.But Cairo once had a deeper canyon than the famous one in Arizona. Around 6 million years ago, an earthquake closed the Straits of Gibraltar and the Mediterranean Sea, cut off from the Atlantic, dried up. The Nile cut a 2-km-deep course, chasing the retreating tide.

Egypt's last big earthquake, in 1992, measured 5.8 in magnitude at its epicenter in Dashur.

The Giza Pyramids, for that matter, showcase two kinds of Cairene rock. For the most part, it is limestone quarried from Giza. But the white Tura limestone casing, still visible on the top of the Pyramid of Khufu, comes from the Moqattam hills. It has since been returned, quarried from the pyramids to build the Citadel.

To expand, Cairo has quarried its edges for building stone and cement. The rubble of collapsed buildings, meanwhile, have been conveyed in the opposite direction -- out to the quarries and the open desert.

On Google satellite maps, the quarries appear fluorescent against the dun-colored earth -- white at the recently cut edges and sea blue along the central axis.

In the mid-19th century, Maadi was orange and guava groves. Just 20 years ago, says Bosworth, Wadi Degla had gazelle and a family of owls. Industry appeared in the 1990s, then the apartments. In 1999, the Wadi was named a protectorate and cleared of truckloads of rubbish.

These days, there is a large sandstone entranceway guarding the protectorate. A Tourism Ministry sign explains the geology like this:"A group of valleys flew into this valley." The day we went, a mountain bike race took place, and while people whizzed over the Middle Eocene, behind the ridge mechanical diggers loaded it into trucks.

Or maybe it is not the Middle Eocene. At mid-century, it was thought the rock was Late Eocene, a difference of about 15 million years. Maybe we are wrong again. For all the achievements of the science, it is still difficult to date a pebble.

Geology in Egypt began about 100 years ago. Teams spread out across Cairo and set out to stitch together a geologic map. But when the teams from Helwan and Moqattam met at Wadi Degla, they couldn't agree on how to combine their results. Helwan and Moqattam have different mixtures of limestone and shale, but halfway between the two, at Wadi Degla, the differences have met halfway. The rocks on either valley wall are exactly the same.

Fifty million years ago, when Cairo and the Nile Delta were covered in a warm, Caribbean-type sea, enormous populations of plankton and foraminifera, of clams and oysters, thrived and died. Settling on the sea floor, they became limestone, which eventually became Cairo.

ZDROJ:http://www.egyptindependent.com/, kráceno, upraveno

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