Famous Places

Grand Canyon

The Grand Canyon is part of the Colorado River basin which has developed over the past 70 million years,[13] in part based on apatite (U-Th)/He thermochronometry showing that Grand Canyon reached a depth near to the modern depth by 20 Ma.[14] A recent study examining caves near Grand Canyon places their origins beginning about 17 million years ago. Previous estimates had placed the age of the canyon at 5–6 million years.[15] The study, which was published in the journal Science in 2008, used uranium-lead dating to analyze calcite deposits found on the walls of nine caves throughout the canyon.[16] There is a substantial amount of controversy because this research suggests such a substantial departure from prior widely supported scientific consensus.[17] In December 2012, a study published in the journal Science claimed new tests had suggested the Grand Canyon could be as old as 70 million years.[18] However, this study has been criticized by those who support the "young canyon" age of around six million years as "[an] attempt to push the interpretation of their new data to their limits without consideration of the whole range of other geologic data sets."[15] The canyon is the result of erosion which exposes one of the most complete geologic columns on the planet. The major geologic exposures in the Grand Canyon range in age from the 2-billion-year-old Vishnu Schist at the bottom of the Inner Gorge to the 230-million-year-old Kaibab Limestone on the Rim. There is a gap of about a billion years between the 500-million-year-old stratum and the level below it, which dates to about 1.5 billion years ago. This large unconformity indicates a long period for which no deposits are present. Many of the formations were deposited in warm shallow seas, near-shore environments (such as beaches), and swamps as the seashore repeatedly advanced and retreated over the edge of a proto-North America. Major exceptions include the Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition. Several parts of the Supai Group also were deposited in non–marine environments. The great depth of the Grand Canyon and especially the height of its strata (most of which formed below sea level) can be attributed to 5–10 thousand feet (1,500 to 3,000 m) of uplift of the Colorado Plateau, starting about 65 million years ago (during the Laramide Orogeny). This uplift has steepened the stream gradient of the Colorado River and its tributaries, which in turn has increased their speed and thus their ability to cut through rock (see the elevation summary of the Colorado River for present conditions). Weather conditions during the ice ages also increased the amount of water in the Colorado River drainage system. The ancestral Colorado River responded by cutting its channel faster and deeper. The base level and course of the Colorado River (or its ancestral equivalent) changed 5.3 million years ago when the Gulf of California opened and lowered the river's base level (its lowest point). This increased the rate of erosion and cut nearly all of the Grand Canyon's current depth by 1.2 million years ago. The terraced walls of the canyon were created by differential erosion.[19] Between 100,000 and 3 million years ago, volcanic activity deposited ash and lava over the area which at times completely obstructed the river. These volcanic rocks are the youngest in the canyon.


Source: https://en.wikipedia.org/wiki/Grand_Canyon

Eiffel Tower

The Eiffel Tower (/ˈaɪfəl/ EYE-fəl; French: Tour Eiffel [tuʁ‿ɛfɛl] (About this soundlisten)) is a wrought-iron lattice tower on the Champ de Mars in Paris, France. It is named after the engineer Gustave Eiffel, whose company designed and built the tower. Constructed from 1887–1889 as the entrance to the 1889 World's Fair, it was initially criticised by some of France's leading artists and intellectuals for its design, but it has become a global cultural icon of France and one of the most recognisable structures in the world.[3] The Eiffel Tower is the most-visited paid monument in the world; 6.91 million people ascended it in 2015. The tower is 324 metres (1,063 ft) tall, about the same height as an 81-storey building, and the tallest structure in Paris. Its base is square, measuring 125 metres (410 ft) on each side. During its construction, the Eiffel Tower surpassed the Washington Monument to become the tallest man-made structure in the world, a title it held for 41 years until the Chrysler Building in New York City was finished in 1930. Due to the addition of a broadcasting aerial at the top of the tower in 1957, it is now taller than the Chrysler Building by 5.2 metres (17 ft). Excluding transmitters, the Eiffel Tower is the second tallest free-standing structure in France after the Millau Viaduct. The tower has three levels for visitors, with restaurants on the first and second levels. The top level's upper platform is 276 m (906 ft) above the ground – the highest observation deck accessible to the public in the European Union. Tickets can be purchased to ascend by stairs or lift to the first and second levels. The climb from ground level to the first level is over 300 steps, as is the climb from the first level to the second. Although there is a staircase to the top level, it is usually accessible only by lift.


Source: https://en.wikipedia.org/wiki/Eiffel_Tower

Golden Gate Bridge

Although the idea of a bridge spanning the Golden Gate was not new, the proposal that eventually took hold was made in a 1916 San Francisco Bulletin article by former engineering student James Wilkins.[16] San Francisco's City Engineer estimated the cost at $100 million (equivalent to $2.3 billion today), and impractical for the time. He asked bridge engineers whether it could be built for less.[10] One who responded, Joseph Strauss, was an ambitious engineer and poet who had, for his graduate thesis, designed a 55-mile-long (89 km) railroad bridge across the Bering Strait.[17] At the time, Strauss had completed some 400 drawbridges—most of which were inland—and nothing on the scale of the new project.[3] Strauss's initial drawings[16] were for a massive cantilever on each side of the strait, connected by a central suspension segment, which Strauss promised could be built for $17 million (equivalent to $391 million today).[10] Local authorities agreed to proceed only on the assurance that Strauss would alter the design and accept input from several consulting project experts.[citation needed] A suspension-bridge design was considered the most practical, because of recent advances in metallurgy.[10] Strauss spent more than a decade drumming up support in Northern California.[18] The bridge faced opposition, including litigation, from many sources. The Department of War was concerned that the bridge would interfere with ship traffic. The navy feared that a ship collision or sabotage to the bridge could block the entrance to one of its main harbors. Unions demanded guarantees that local workers would be favored for construction jobs. Southern Pacific Railroad, one of the most powerful business interests in California, opposed the bridge as competition to its ferry fleet and filed a lawsuit against the project, leading to a mass boycott of the ferry service.[10] In May 1924, Colonel Herbert Deakyne held the second hearing on the Bridge on behalf of the Secretary of War in a request to use federal land for construction. Deakyne, on behalf of the Secretary of War, approved the transfer of land needed for the bridge structure and leading roads to the "Bridging the Golden Gate Association" and both San Francisco County and Marin County, pending further bridge plans by Strauss.[19] Another ally was the fledgling automobile industry, which supported the development of roads and bridges to increase demand for automobiles.[13] The bridge's name was first used when the project was initially discussed in 1917 by M.M. O'Shaughnessy, city engineer of San Francisco, and Strauss. The name became official with the passage of the Golden Gate Bridge and Highway District Act by the state legislature in 1923, creating a special district to design, build and finance the bridge.[20] San Francisco and most of the counties along the North Coast of California joined the Golden Gate Bridge District, with the exception being Humboldt County, whose residents opposed the bridge's construction and the traffic it would generate.[21]


Source: https://en.wikipedia.org/wiki/Golden_Gate_Bridge