Question: What materials were used in the construction of the Great Wall of China?
About the article "Great Wall of China", section "Abstract".

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Before the use of bricks, the Great Wall was mainly built from rammed earth, stones, and wood.
During the Ming Dynasty, however, bricks were heavily used in many areas of the wall, as were materials such as tiles, lime, and stone.
The size and weight of the bricks made them easier to work with than earth and stone, so construction quickened.
Additionally, bricks could bear more weight and endure better than rammed earth.
Stone can hold under its own weight better than brick, but is more difficult to use.
Consequently, stones cut in rectangular shapes were used for the foundation, inner and outer brims, and gateways of the wall.
Battlements line the uppermost portion of the vast majority of the wall, with defensive gaps a little over tall, and about wide.
From the parapets, guards could survey the surrounding land.
Communication between the army units along the length of the Great Wall, including the ability to call reinforcements and warn garrisons of enemy movements, was of high importance.
Signal towers were built upon hill tops or other high points along the wall for their visibility.
Wooden gates could be used as a trap against those going through.
Barracks, stables, and armories were built near the wall's inner surface. 
The Xinhai Revolution in 1911 forced the abdication of the last Qing Emperor Puyi and ended China's last imperial dynasty.
The revolutionaries, headed by Sun Yat-sen, were concerned with creating a modern sense of national identity in the chaotic post-imperial era.
In contrast to Chinese academics such as Liang Qichao, who tried to counter the West's fantastic version of the Great Wall, Sun Yat-sen held the view that Qin Shi Huang's wall preserved the Chinese race, and without it Chinese culture would not have developed enough to expand to the south and assimilate foreign conquerors.
Such an endorsement from the "Father of Modern China" started to transform the Great Wall into a national symbol in the Chinese consciousness, though this transformation was hampered by conflicting views of nationalism with regard to the nascent "new China.
"The failure of the new Republic of China fanned disillusionment with traditional Chinese culture and ushered in the New Culture Movement and the May Fourth Movement of the mid-1910s and 1920s that aimed to dislodge China's future trajectory from its past.
Naturally, the Great Wall of China came under attack as a symbol of the past.
An example, an influential writer of this period, Lu Xun, harshly criticized the "mighty and accursed Great Wall" in a short essay: "In reality, it has never served any purpose than to make countless workers labour to death in vain... surrounds everyone.
"The Great Wall itself suffered battle damage in the tumultuous Warlord Era of Republican China (191628).
The stretches near Shanhai Pass featured in both the First (1922) and Second Zhili–Fengtian War (1924), when the Manchuria-based Fengtian clique vied to control the civilian Beiyang Government against the Hebei-based Zhili clique.
While Fengtian forces overran the Shanhai Pass early in the First Zhili–Fengtian War, the Second saw the Zhili clique hold the pass until the Beijing Coup behind Zhili lines caused the defenders to waver and brought a swift conclusion to the war.
Sino-Japanese conflict (193145) gave the Great Wall a new lease of life in the eyes of the Chinese.
During the 1933 defence of the Great Wall, inadequately-equipped Chinese soldiers held off double their number of Japanese troop for several months.
Using the cover of the Great Wall, the Chinese – who were at times only armed with broadswords – were able to beat off a Japanese advance that had the support of aerial bombardment.
With the Chinese forces eventually overrun, the subsequent Tanggu Truce stipulated that the Great Wall was to become a demilitarized zone separating China and the newly created Japanese puppet state of Manchukuo.
Even so, the determined defence of the Great Wall made it a symbol of Chinese patriotism and the resoluteness of the Chinese people.
The Chinese Communist leader Mao Zedong picked up this symbol in his poetry during his "Long March" escaping from Kuomintang prosecution.
Near the end of the trek in 1935, Mao wrote the poem "Mount Liupan" that contains the well-known line that would be carved in stone along the Great Wall in the present day: "Those who fail to reach the Great Wall are not true men" ().
Another noteworthy reference to the Great Wall is in the song "March of the Volunteers" from the 1935 film "Sons and Daughters in a Time of Storm".
The song enjoyed continued popularity in China, and was selected as the national anthem of the newly established People's Republic of China (PRC) in 1949 when the communists emerged victorious from the Chinese Civil War.
In 1952, the scholar-turned-bureaucrat Guo Moruo laid out the first modern proposal to repair the Great Wall.
Five years later, the renovated Badaling became the first section to be opened to the public since the establishment of the PRC.
The Badaling Great Wall has since become a staple stop for foreign dignitaries who come to China, beginning with Nepali prime minister Bishweshwar Prasad Koirala in 1960, and most notably the American president Richard Nixon in his historic 1972 visit to China.
To date, Badaling is still the most visited stretch of the Great Wall.
Other stretches did not fare so well.
During the Cultural Revolution (196676), hundreds of kilometres of the Great Wall — already eroded by wind and rain — were deliberately destroyed by fervent Red Guards who regarded it as part of the Four Olds to be eradicated in the new China.
Quarrying machines and even dynamite were used to dismantle the Wall, and the pilfered materials were used for construction.
As China opened up in the 1980s, reformist leader Deng Xiaoping initiated the "Love our China and restore our Great Wall" campaign () to repair and preserve the Great Wall.
The Great Wall was designated as a UNESCO World Heritage Site in 1987.
However, while tourism boomed over the years, slipshod restoration methods have left sections of the Great Wall near Beijing "looking like a Hollywood set".
The less prominent stretches of the Great Wall did not get as much attention.
In 2002 the New York-based World Monuments Fund put the Great Wall on its list of the World's 100 Most Endangered Sites.
In 2003 the Chinese government began to enact laws to protect the Great Wall. 
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The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China in part to protect the Chinese Empire or its prototypical states against intrusions by various nomadic groups or military incursions by various warlike peoples or forces.
Several walls were being built as early as the 7th century BC; these, later joined together and made bigger and stronger, are now collectively referred to as the Great Wall.
Especially famous is the wall built between 220–206 BC by the first Emperor of China, Qin Shi Huang.
Little of that wall remains.
Since then, the Great Wall has on and off been rebuilt, maintained, and enhanced; the majority of the existing wall are from the Ming Dynasty.
Other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration.
Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor.
The main Great Wall line stretches from Shanhaiguan in the east, to Lop Lake in the west, along an arc that roughly delineates the southern edge of Inner Mongolia.
A comprehensive archaeological survey, using advanced technologies, has concluded that the Ming walls measure .
This is made up of sections of actual wall, of trenches and of natural defensive barriers such as hills and rivers.
Another archaeological survey found that the entire wall with all of its branches measure out to be .
 The Great Wall of China, massive series of fortifications in northern China built to defend against nomadic peoples living to the north of the wall.
Chinese city wall, defensive systems used to protect towns and cities in China in pre-modern times