paragraph_text: Two recent discoveries indicate probable very early settlements near the Thames in the London area. In 1999, the remains of a Bronze Age bridge were found on the foreshore north of Vauxhall Bridge. This bridge either crossed the Thames, or went to a now lost island in the river. Dendrology dated the timbers to 1500 BC. In 2010 the foundations of a large timber structure, dated to 4500 BC, were found on the Thames foreshore, south of Vauxhall Bridge. The function of the mesolithic structure is not known. Both structures are on South Bank, at a natural crossing point where the River Effra flows into the River Thames.

paragraph_text: The Ponte dell'Accademia is one of only four bridges to span the Grand Canal in Venice, Italy. It crosses near the southern end of the canal, and is named for the Accademia di Belle Arti di Venezia, which from 1807 to 2004 was housed in the Scuola della Carità together with the Gallerie dell'Accademia, which is still there. The bridge links the sestieri of Dorsoduro and San Marco.

paragraph_text: At 1.6 million acres (650,000 ha) the Kaibab National Forest borders both the north and south rims of the Grand Canyon, in north-central Arizona. It is divided into three major sections: the "North Kaibab Ranger District" (offices in Fredonia) and the South Kaibab and are managed by the United States Forest Service. The South Kaibab is further divided into two districts, the "Tusayan Ranger District" (offices in the Grand Canyon), and the "Williams Ranger District" (offices in Williams). The Grand Canyon is a natural boundary between the North Kaibab and the South Kaibab. The South Kaibab covers and the North Kaibab stretches over . Elevations vary on the forest from 5,500 feet (1,676 m) in the southwest corner to 10,418 feet (3,175 m) at the summit of Kendrick Peak on the Williams Ranger District. The forest as a whole is headquartered in Williams.

paragraph_text: Aldford Iron Bridge is a bridge crossing the River Dee north of the village of Aldford, Cheshire, England, linking the village with Eaton Hall, forming part of the Buerton Approach to the hall (). It is recorded in the National Heritage List for England as a designated Grade I listed building.

paragraph_text: Hagwilget Canyon Bridge is a suspension bridge over the Hagwilget Canyon on the Bulkley River, at the Wet'suwet'en village of Hagwilget, British Columbia. The current bridge was constructed in 1931, and later reinforced in 1990.

paragraph_text: This is a complete list of current bridges and other crossings of the Monongahela River starting from Pittsburgh, Pennsylvania, where the river helps to form the headwaters of the Ohio River, and ending in Fairmont, West Virginia, where the West Fork River and Tygart Valley River combine to form the Monongahela.

paragraph_text: The Grand Canyon (Hopi: Ongtupqa; Yavapai: Wi: kaʼi: la, Navajo: Tsékooh Hatsoh, Spanish: Gran Cañón) is a steep - sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is 277 miles (446 km) long, up to 18 miles (29 km) wide and attains a depth of over a mile (6,093 feet or 1,857 meters).

paragraph_text: The Jamsu Bridge crosses the Han River in South Korea and connects the districts of Yongsan-gu and Seocho-gu. The bridge was completed in 1976, and lies just meters above the waterline, allowing the bridge to submerge during periods of high rainfall. In 1982, Banpo Bridge was built on top of the Jamsu Bridge, creating a two-deck bridge.

paragraph_text: Hopi House is located on the South Rim of the Grand Canyon, within Grand Canyon National Park in the U.S. state of Arizona. Built in 1904 as concessioner facilities at the South Rim were being developed, it is the first of eight projects at the Grand Canyon that were designed by architect Mary Colter, along with Bright Angel Lodge, Hermit's Rest, Lookout Studio, Phantom Ranch, Desert View Watchtower, Colter Hall and Victor Hall, (the latter two being employee dormitories). Hopi House was built by the Fred Harvey Company as a market for Native American crafts, made by artisans on the site. The Hopi, as the historic inhabitants of the area, were chosen as the featured artisans, and the building was designed to closely resemble a traditional Hopi pueblo. Hopi House opened on January 1, 1905, two weeks before the El Tovar Hotel, located just to the west, was opened.

paragraph_text: The North Grand Island Bridge is a pair of twin two-lane truss arch bridges spanning the Niagara River between Grand Island and Niagara Falls in New York, United States. Each bridge carries one direction of Interstate 190 (I-190). Both crossings are operated by the New York State Thruway Authority as part of the Niagara Thruway. The northbound span opened in 1935; the southbound span was finished in 1965.

paragraph_text: The Grand Canyon North Rim Headquarters is a historic district on the North Rim of the Grand Canyon in Grand Canyon National Park, Arizona. Established from 1926 through the 1930s, the district includes examples of National Park Service Rustic architecture as applied to employee residences, administrative facilities and service structures.

paragraph_text: Thistle is a ghost town in Spanish Fork Canyon in southeastern Utah County, Utah, United States. During the era of steam locomotives, the town's primary industry was servicing trains for the Denver and Rio Grande Western Railroad (often shortened to D&RG, D&RGW, or Rio Grande). The fortunes of the town were closely linked with those of the railroad until the changeover to diesel locomotives, when the town started to decline.

paragraph_text: Hampton Court Bridge crosses the River Thames in England approximately north–south between Hampton, London and East Molesey, Surrey. It is the upper of two road bridges on the reach above Teddington Lock and downstream of Molesey Lock.

paragraph_text: The Burlington–Bristol Bridge is a truss bridge with a lift span crossing the Delaware River from Burlington, New Jersey to Bristol Township, Pennsylvania in the United States. Construction of the bridge started on April 1, 1930, and the bridge opened to traffic on May 2, 1931. The bridge carries NJ 413 and PA 413.

paragraph_text: The Diwei Bridge is a cable-stayed bridge which crosses the Yangtze River in Chongqing, China. Completed in 2008, it has a main span of . The bridge carries road of traffic between the Jiangjin District south of the Yangtze River and the Dadukou District to the north.

paragraph_text: Granite Canon (also Granite or Granite Canyon) is an unincorporated community in Laramie County, Wyoming, United States. Its elevation is 7,336 feet (2,236 m). The community of Granite Canon is unincorporated, but does have a postal designation (under the name of Granite Canon) with the ZIP code of 82059.

paragraph_text: Olympic Bridge or Grand Olympic Bridge () is a bridge over the Han River in Seoul, South Korea. The bridge links the Gwangjin and Songpa districts. Its construction started in 1985, but was not completed until 1990, after the Seoul Olympic Games of 1988, because the bridge under construction had collapsed.

paragraph_text: The South Canon Bridge is a truss footbridge spanning the Colorado River near Glenwood Springs, Colorado. It was built in 1915 by the Missouri Valley Bridge & Iron Co.. It was listed on the National Register of Historic Places (reference number 85000212) in 1985.).

paragraph_text: The Brady Street Bridge, also known as the South 22nd Street Bridge, was a steel bowstring arch bridge in Pittsburgh, Pennsylvania, which crossed over the Monongahela River at South 22nd Street. It was replaced by the Birmingham Bridge in 1976, and was demolished on May 29, 1978. The bridge's main span was a tied arch with a suspended road deck, with two through-truss side spans, carrying two traffic lanes between Brady Street on the Pittsburgh side and South 22nd Street on the south side. Approach viaducts were built at either end. The bridge was built by the Schultz Bridge and Iron Company.

paragraph_text: Nearly two billion years of Earth's geological history have been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted. While some aspects about the history of incision of the canyon are debated by geologists, several recent studies support the hypothesis that the Colorado River established its course through the area about 5 to 6 million years ago. Since that time, the Colorado River has driven the down - cutting of the tributaries and retreat of the cliffs, simultaneously deepening and widening the canyon.

Question: When did the river crossed by the South Canon Bridge start to form the Grand Canyon?
Answer: about 5 to 6 million years ago