<P> At the edge of the Bohai Gulf is Shanhai Pass, considered the traditional end of the Great Wall and the "First Pass Under Heaven". The part of the wall inside Shanhai Pass that meets the sea is named the "Old Dragon Head". 3 km (2 mi) north of Shanhai Pass is Jiaoshan Great Wall (焦 山 長城), the site of the first mountain of the Great Wall . 15 km (9 mi) northeast from Shanhaiguan is Jiumenkou (t 九 門口, s 九 门口, Jiǔménkǒu), which is the only portion of the wall that was built as a bridge . Beyond Jiumenkou, an offshoot known as the Liaodong Wall continues through Liaoning province and terminates at the Hushan Great Wall, in the city of Dandong near the North Korean border . </P> <P> In 2009, 180 km of previously unknown sections of the wall concealed by hills, trenches and rivers were discovered with the help of infrared range finders and GPS devices . In March and April 2015 nine sections with a total length of more than 10 km (6 mi), believed to be part of the Great Wall, were discovered along the border of Ningxia autonomous region and Gansu province . </P> <P> Before the use of bricks, the Great Wall was mainly built from rammed earth, stones, and wood . During the Ming, 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 30 cm (12 in) tall, and about 23 cm (9.1 in) 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 . </P> <P> While portions north of Beijing and near tourist centers have been preserved and even extensively renovated, in many other locations the Wall is in disrepair . Those parts might serve as a village playground or a source of stones to rebuild houses and roads . Sections of the Wall are also prone to graffiti and vandalism, while inscribed bricks were pilfered and sold on the market for up to 50 renminbi . Parts have been destroyed because the Wall is in the way of construction . A 2012 report by the State Administration of Cultural Heritage states that 22% of the Ming Great Wall has disappeared, while 1,961 km (1,219 mi) of wall have vanished . More than 60 km (37 mi) of the wall in Gansu province may disappear in the next 20 years, due to erosion from sandstorms . In some places, the height of the wall has been reduced from more than 5 m (16 ft 5 in) to less than 2 m (6 ft 7 in). Various square lookout towers that characterize the most famous images of the wall have disappeared . Many western sections of the wall are constructed from mud, rather than brick and stone, and thus are more susceptible to erosion . In 2014 a portion of the wall near the border of Liaoning and Hebei province was repaired with concrete . The work has been much criticized . </P>

What did they use to build the great wall of china