Cave cave is one of the most important architectural forms on the Loess Plateau for thousands of years. Relevant studies have pointed out that this architectural form was created in the late Neolithic Yangshao culture and has spread throughout the main areas of the Loess Plateau by the Longshan culture period [1]. However, previous studies have focused on identifying and tracing the shape, structure and evolution of caves, and other aspects have not been covered much and are not in-depth enough. On the basis of previous research and the newly discovered relevant data, this paper will discuss the layout, characteristics and evolution of the settlement form with the cave as the main body of the building in the settlement landscape, further sort out the specific process of the cave building form from the beginning to the gradual diffusion, and then combine the changes of other cultures and societies to make a preliminary discussion on the reasons for the emergence and development of the cave building settlement in the prehistoric period.
First, the form of cave building settlement
The so-called cave building settlements in this article refer to the settlements with caves as the main building form or ordinary dwellings. It should be noted that even after the caves spread all over the Loess Plateau during the Longshan Cultural Period, the settlements in this area were not all composed of caves, and the main buildings of the settlements would still be semi-crypts or other forms in the sandy land and the edge of the Gobi without deep loess, such as 52 semi-crypt houses were excavated and cleaned in 17,000 square meters of Shenmu wooden pillars, pillars and beams in the Mu Us Desert [2]. In areas where caves became the main body of settlement buildings, there were other forms of ground-based buildings in the settlements, such as the Kangjia houses in Lintong [3] and Mijiaya [4] in Xi'an, although it is unclear whether such buildings were ordinary dwellings in the Kangjia and Mijiaya settlements. On some sites with a clear layout of settlements, it can be seen that these ground-based rammed earth, adobe and even stone-built ground-style houses were relatively high-level or functional architectural forms in the settlements, and were an important part of the cave-type settlements at that time.
At present, the cave settlements are different in layout due to different cave excavation methods. According to the relevant research, the excavation methods of the cave are divided into two types: cliff type and sinking type [5], the former is excavated according to the vertical wall surface of the hillside, and the latter digs a pit or trench and excavates the cave at the bottom of the pit. In the current data, the most clear settlement layout is the settlement composed of cliff-like caves located on the hillside, which is widely distributed in northern Shaanxi, central and southern Inner Mongolia and northwest Shanxi. In the case of large-scale excavations, 32 cave-style houses (Fig. 1) excavated at the Yangjiesha site in Hengshan in the second phase of Yangshao Cultural Quanhu Phase II (Fig. 1) are distributed on the southwest slopes of the first and second beams (Fig. 2) [6]. Caves were found on the slopes of the three mountain beams, covering an area of 210,000 square meters, and the two lower mountain beams were separated by stone walls [7]. During the Longshan culture period, the Yulin Zhailiang site covers an area of more than 30,000 square meters, and a stone wall was found, and the 109 caves cleaned up were distributed in the saddle part of the mountain on the south side of the site and the slope on the west side, and the east, north, and west sides of the Zhailiang mountain were arranged in an arc shape along the contour line, and the height was staggered, which could be roughly divided into four to five rows up and down, and the distribution law of the group was obvious, and the doorways were all open to the bottom of the hillside [8]. The cultural settlement of Laohushan on the slope of Manhan Mountain on the north shore of Daihai is very regular, the excavated Laohushan [9], Xibaiyu [10] and Bancheng ruins [11] are all 100,000 square meters, and the landform of the ruins is "two slopes and one ditch" open to the southeast, and there are stone walls along the ridges of the two slopes. The stone walls of the Laohushan site converge on the northwest summit, and a large number of caves are distributed on the slopes on both sides of the gully within the wall (Fig. 3), and 70 caves have been cleared in different areas (Fig. 4). The Yuanzigou site is larger, with no stone walls, and there are three settlements on the slopes facing south on three sides, each with an area of 100,000 square meters, and 45, 47, and 40 caves have been found, each forming a courtyard, group, or row [12].
As seen in the Laohushan settlement, many of the cliff-type cave settlements have stone walls built on the ridge above the cave settlement. It is generally believed that the earliest settlement stone wall was found in the first phase of the Baicao Pagoda site in the Jungger Banner [13], which is equivalent to the second phase of Quanhu in northern Shaanxi and Guanzhong [14], and there are too few examples of stone walls in this period, and there are also two and three settlements in Baicao Pagoda, which are equivalent to the second phase of Miaodigou and the Longshan period, so the stone wall is not necessarily from the late Yangshao period. A large number of examples that can be identified by stone walls to defend cave settlements are found in the second phase of Miaodigou, such as a series of "stone cities" [15] in the southern foothills of Daqing Mountain and the southern flow of the Yellow River in Inner Mongolia [15], most of these settlements are very ordinary, not necessarily large-scale settlements, such as Laohushan, Xibaiyu, Bancheng and Yuanzigou, which cover an area of 100,000 square meters, are also ordinary settlements in terms of the various relics and settlement layouts that have been exposed in a large area at present. Some large settlements, such as Lushan Pass, did not have stone walls, but most of the large settlements in the Longshan period had stone walls, especially in northern Shaanxi and northwestern Shanxi [16].
In addition, cliff-side cave settlements also built stone buildings on the top of the hillside cave complex. This is true even for ordinary settlements, such as Tiger Mountain, where a 40-meter-long square wall was built on the top of a hill where stone walls converge, and a stone house was found inside. On the northwest top of the Bancheng ruins near Tiger Mountain, there are 5 "stone square altars" lined up in a row with basically the same structure. The second phase of the stone city at the south foot of Daqing Mountain is also in a similar situation. It's just that the scale of the mountaintop stone buildings in these settlements in central and southern Inner Mongolia is not large, and the use has not yet been determined, and it is not necessarily a daily residential house. Some of the settlements recently discovered in northern Shaanxi and western Shanxi are relatively clear, which is a settlement mode in which caves are dug on the hillside and rammed earth, adobe or stone courtyards are built on the top of the mountain. For example, the site of the Shijiajiashan City, which is basically the same as Laohushan, is surrounded by an outer city, and there is an "inner city" of about 3,000 square meters on the top of the hill in the city, which is estimated to be a courtyard, and caves have been found on the slopes of the city [17]. Another example is Bicun, Xingxian County, with an area of about 750,000 square meters, which mainly includes four platforms from west to east, including Zhailiangshang, Xiaoyuliang, Dianleliang, and City Wall Battlements, and a city wall was built on the edge of the city wall battlements in the east of the site, and four large stone houses were excavated on Xiaoyuliang [18]. Combined with the following situation of Yan'an Lushan Pass and Shenmu Stone Pass, this settlement pattern should be the normal layout of large settlements in the second phase of Miaodigou and Longshan period.
The newly excavated Miaodigou Phase II—Lushan Ridge Courtyard of the Longshan Period [19] (Fig. 5). With an area of more than 2 million square meters, Lushan Pass is located on the north-south watershed between Yanhe River and Nianzhuang Ditch - "Dashan Liang", at the top of "Dashan Liang", drilling has confirmed four large rammed earth platform foundations, from north to south for Zhaizi Pass, Xiaoying Panliang, Erying Panliang and Daying Panliang. Each pedestal is topped by well-planned walled courtyards and building complexes. On the slope of the mountain beam, there are more than 300 relics found by exploration and white-ash ground house sites - caves, ash pits, tombs, etc. Daying Panliang has been excavated, there is a pedestal of about 160 meters long from north to south and about 100 meters wide from east to west, and there are three courtyards surrounded by rammed earth walls at the top of the pedestal, and a large courtyard (No. 1 courtyard) in the north and two small courtyards in the south constitute a "product" character layout. One of the courtyards is two-in, sitting in the north and facing south, the middle of the courtyard is north of three main buildings, a single construction area is more than 200 square meters, the main building divides the courtyard into the front yard and the back yard, there are wings in the courtyard walls on the east and west sides, and the door direction is towards the center of the courtyard. Outside the south wall of Courtyard No. 1, there are two independent small courtyards, each with an area of 700 square meters. There are several small rammed-earth houses scattered throughout each small courtyard, which may be the gate school area responsible for security and security. At the southernmost end of Dayingpanliang is a small square, where roads from inside and outside the courtyard converge to the southern part of the site, such as Majiaqi. In the accumulation near the large building site, a certain number of cylinder tiles and trough slab tiles were also found, with more than 100 individual pieces.
At present, the largest cliff-type cave settlement found is the site of Shiyun City in the Longshan period [20], which is backed by the cliff bank of the Balwei River in the northwest, with a total area of more than 4 million square meters, and is composed of three parts: the imperial city platform, the inner city and the outer city. The inner city was built earlier than the outer city, which means that the outer city is an expanded part. The city wall is built along the ridge, and there are horse faces at intervals, and the city gate is composed of the inner and outer urn castles, the piers on both sides, and the gate and other buildings, and the defensive nature can be seen at a glance. Caves have been excavated on the slopes of Houyangwan, Hujiawa, and Hanjiaqidan in the inner city, and more than 40 have been excavated in Hanjiaqidan (Fig. 6), indicating that the most common dwellings in the city are caves [21]. The highest point in the northwest of the inner city is a platform - the Imperial City Terrace. The Imperial City Terrace is magnificent, with an area of 240,000 at the bottom and 80,000 square meters at the top, with a stone slope that shrinks upwards layer by layer, up to 11 levels of 70 meters. The only exit from the Imperial City Terrace is located on the south-east side, and there is a city gate here, which is exactly the same structure as the east gate of the outer city. On the top of the Imperial City Terrace, a large-scale stone-clad rammed-earth platform building and a so-called "pond garden" (Fig. 7) were built, and stone figures were embedded in the walls [22]. Compared with the smaller settlements such as Lushan Pass and Bicun, Huangchengtai should be the same as the Dayingpanliang of Lushan Pass and Bicun Xiaoyuliang in their respective settlements, and it is a building complex on the top of the mountain in the cave settlement. However, the scale of the Imperial City Gate is exactly the same as that of the Inner City Gate and the Inner City Gate, and it has complete defense facilities and structures, and the Imperial City Gate has the posture of "beggar-thy-neighbor", which can be called a "city within a city", and it is the only city site structure seen on the Loess Plateau so far.
At present, the sunken caves are more found in the middle and south of Jin, such as Wutai Yangbai, Taigu Baiyan, Xiaxian Dongxia Feng and Xiangfen Tao Temple. Among them, the layout of the settlement can be roughly seen in the Dongxia Feng settlement in the Erlitou period [23]. The Dongxiafeng site covers an area of 250,000 square meters, but it lasts for a long time, with both Neolithic relics (mainly in the excavation of the western area, but also in the central area), and the remains of Erlitou and the Shang Dynasty (central, eastern, and northern areas). Among them, the third phase of Dongxiafeng, located in the central area, has been excavated with a "hollow-shaped" trench, the inner trench is roughly close to square, the side length is 120~150 meters, and the side length of the outer trench is nearly 200 meters. The width of the groove is 4~5, the width of the bottom is 2~3, and the residual depth is about 3 meters. At the fifth site uncovered by the continuous excavations (the southwest part of the settlement surrounded by the inner and outer trenches), rows of caves and storage rooms were dug on both sides of the bottom of the inner and outer trenches, and there were 11 caves and 6 storage rooms on both sides of the Xili ditch. Where there is a cave, there is road soil at the bottom of the ditch, and there are rammed earth walls in some places in the trench, probably to separate the courtyard. Large earth pits were found in the upper area surrounded by the inner trenches, and the four walls of the pits were also excavated with caves, storage rooms, and pottery kilns, such as F554 and F558 on the south wall, and F572 and S15 on the west wall. Since there are very few remains found in the first and second phases, and the remains in the inner and outer trenches and the excavated earthfills belong to the third phase, it can be seen that this settlement surrounded by two circles of trenches was planned and built in phase III, but it is not clear whether the inner and outer trenches were built at the same time or in sequence. The distance between the two circles of trenches on the east side is the largest, 12.5 meters, and the north side is the smallest, 5.5~8.5 meters. Therefore, regardless of whether the two adjacent trenches are in sequence at the same time, it is redundant if the priority is to defend, and the main purpose of the trench should be to dig a cave first, and it can also be used for defense.
Compared with the third phase of the Dongxia Feng settlement, the area of the Tao Temple site is much larger, close to 3 million square meters. In 2000~2002, 2.8 million square meters of the middle period of the Taosi culture and 560,000 square meters of the early town in the north of the city were found on the site [24], coupled with the subsequent identification of the "observatory" [25] in the southeast town outside the city and some signs such as "handicraft workshop area", "palace city" and "palace" [26], at least the middle Taosi settlement has been explained by the excavators according to a plan similar to that of the capital city of the Central Plains Dynasty. However, in 2013, after the discovery of the early to mid-term "Miyagi" of Taoji Temple in the location of the small town, the early small town was immediately denied. In 2017, it was finally confirmed that the direction of "Miyagi" was the same as that of Ayutthaya, with an area of 130,000 square meters, but it is disputed whether there is a ditch in the wall of this "Miyagi" [27]. The "foundation groove" under the "Miyagi City Wall" is actually a ring moat with a depth of 7~9 meters in the early Tao Temple, and an early cave was found at the bottom of the ring moat at the anatomical excavation site [28], so it can be concluded that the ring moat was open at least in the early period, which is very similar to the form of a trench cave in Dongxiafeng. Most of the houses found in the early and middle Taosi culture are caves (Fig. 9) and patio-style cave courtyards [29], such as F319 and F321 in 1978III (Fig. 10), and F407 and F402 in 1978 [30]. For example, 9 of the 10 houses excavated in Area II are such houses, which are not large, ranging from 7.7~16.7 square meters, and are obviously cave caves (Fig. 11) [31]. However, the early and middle caves are round, and the late ones are rounded and square, which has the same characteristics as the caves on the Loess Plateau in the late Longshan period. Large semi-crypt houses, rammed earth foundations, and stone pillar foundation flat-ground houses (IFJT3) have been found on the eastern flat ground of the so-called "palace area" in the early ring moats, as well as similar rammed earth structures in other parts of the site, such as the so-called "observatory" (IIFJT1), and large cemeteries (Fig. 12). Therefore, in a large settlement like Tao Temple, the general dwellings should also be "pit courtyard" or "trench" type caves, and there are high-grade buildings built on flat ground. The layout and form of the early, middle and late phases of the Tao Temple are not entirely clear. The conclusion of the paleoenvironmental study is that although the Nanhe River and Songcungou on the northeast and southwest sides of the Taosi site were not cut so deep during the Taosi culture period, they were already valley-type rivers [33], sandwiched between the two rivers, and even the city wall was on the river, whether it was necessary to exist in the middle of the Taosi period is worth considering, and the "observatory" (area of 1740 square meters) was enclosed in a narrow long small city in the southeast of the city, and the line of sight was not transparent and not easy to understand. It is probably problematic to conceive of the Taosi settlement according to the urban planning of the prehistoric and even historical "city sites" of the Central Plains.
It is not clear whether the gullies on the site of the Tao Temple were originally rivers with water for a long time, or whether they were dry ditches that could be used as roads, as they are now. As far as the landscape of the ruins is concerned, the landform of the Qinghai people and Lajia Qijia cultural sites is very similar to that of Tao Temple, and there are many gullies on the ruins. This site is located in a small basin by the Yellow River, backed by the terraces in front of the mountain, facing the Yellow River, covering an area of nearly 700,000 square meters. A row of caves were found on the wall of the ditch (Lujiagou) in the II area, numbered F1, F3, F4 (Fig. 14), F7, and F10, with the doors facing north, and the row of ditches excavated and cleaned on the wall of another ditch to the south had F13, F14, F15, F17, etc., and the doors were all facing west [34] It can be seen that the doors of these two rows of caves are opened in an unfavorable direction, which is obviously taking advantage of the existing terrain, and of course it may also be the location specified by the settlement plan. In 2005, the Ganggou in Area XI and Lujiagou in Area II were excavated, and the two houses F30 and F31 were excavated along the slope of the Ganggou facing east[35], in the 2014 excavation area, three houses F47, F52, and F56 shared a front site [36], and in the VIII.3 area excavated in 2015, a ditch and rows of caves were also discovered, with the doors facing south to the bottom of the ditch [37], and there were foot sockets on the walls of the ditch that could be used up and down [38]. In the 2016 excavation and cleaning of Qijia Culture G12, traces of tools from that time were found on the east and west walls, and although it is not possible to determine whether the trench was all excavated manually, the purpose of at least repairing the walls of the trench to facilitate the excavation of the cave is obvious [39]. In addition, excavations in 2004 and 2005 in Area VIII. in the western part of the site cleared a number of houses, among which a row of houses F23, F24, F26, and F27 faced east, opposite to F25, and F28 and F29 faced south to the east and west [40], indicating that there may also be sunken pit courtyards here. On the terrace above the gully in Area V, "small squares", "dry-column buildings" and tombs have been excavated [41], indicating that there may have been public activity places such as granaries and cemeteries (Fig. 13). The above discoveries all over the site of multiple excavation sites show that during the Qijia culture period, there were many east-west and north-south gullies or ditches on the Lajia Plateau, and the caves should be dug along the cliff faces on both sides of the ditch, otherwise the caves facing north and west are difficult to understand, the caves are completely distributed in rows according to the trend of the ditch, and the ditch is usually waterless, and the bottom of the ditch is the road at that time. The Lajia settlement seems to be a cave at the bottom of a ditch, and the terrace is a settlement layout with pit courtyards and public activity buildings. If the Lajia ditch is indeed manually excavated for the construction of the cave, then the Lajia cave can also be classified as a sunken type.
In summary, no matter how the cave is built, the basic idea of the settlement is the same, it is necessary to use the slope to cut out the vertical wall to dig the cave, or simply dig a ditch or pit downward, and use the wall of the ditch wall to dig the cave to form ordinary residential houses, and other special ground-level buildings are built on the high ground above the cave. The overall layout of the settlement is clearest in the cliff-type cave settlements around northern Shaanxi and the Hetao area of Inner Mongolia, using the slope of the hill to build the cave, the ridge on the slope to build a stone wall, the top of the mountain to build a platform, and to build rammed earth, adobe, stone wall large courtyards or large building complexes. Although the underground cave settlements such as Jinzhong and Jinnan can also be found in the bottom of the ditch cave, the courtyard of the patio cave and the buildings built on the flat ground, the overall structure is not clear, perhaps it should be like the settlement structure of Dongxia Feng in the Erlitou period, using the ring moat as a defense system, and at the same time, the bottom of the ring moat is also used to dig the bottom of the ditch cave, and there are flat buildings and patio cave courtyards in the ring moat. The ruins of the Longshan to Erlitou period in southern Jin are mostly surrounded by moats without city walls, such as Zhoujiazhuang [42] and Xiwubi [43] in Jiangxian County, and I don't know if there are caves in the ring moats. Then there is the form of settlement like Lajia, which uses natural gullies, walks at the bottom of the ditch, and digs caves on the wall of the ditch. Cliff-type cave settlements should be mostly found in the developed areas of Liangshan landform, while sunken cave settlements should be mostly found in the wide and flat terraces and loess plains of southern Jinnan, western Henan, Guanzhong and even Ganqing areas.
2. The initial appearance and diffusion of caves
According to the available data, the earliest cave appeared in the late Yangshao culture [44], but due to the relatively long duration of the late Yangshao period, further division can be made. According to the understanding of Wang Weilin and Di Nan [45], the late Yangshao period in the Weihe River Basin and its surrounding areas can be clearly divided into at least two stages: the fourth phase of Banpo and the second phase of Quanhu. In the fourth phase of Banpo, two kinds of cave-style buildings have appeared on the Loess Plateau. The first kind is found in Longdong, Guanzhong and Jinzhong areas, the plane is mostly round or gourd-shaped, there is a stove at the back of the room, the form is simple, and the area is less than 10 square meters. The best preserved is a 72-meter-long row of 13 cave-like buildings in the southern area of the Gaoling Yangguanzhai site (Fig. 15), each with a front and back chambers, and a back room (Fig. 16), but there are several pottery kilns attached to the doors, and it is unknown whether the age of the pottery kilns is completely at the same time as the caves [46]. It is also possible that many buildings with a very small area, such as F11 in the third phase of Fort Fulin, have a base diameter of only 1.4 meters [47]. In addition, one such oval-shaped "semi-crypt" building was also excavated at the Mijiaya site in Xi'an, with a length diameter of 2.6 meters and a short diameter of 1.7 meters, with a stove at the rear [48]. However, there are still many ground-level buildings of about 20 square meters found in the northern area of Yangguanzhai and Fulinbao, so even in the Weishui Valley, where small caves have been widely discovered, there is no conclusive evidence to prove that the caves were the most common dwellings in the settlement at that time. However, the Yangqi site in Longdongning County was only 4,500 square meters, and 33 houses were found, and 12 of them were all round and gourd-shaped caves, with an area of less than 10 square meters, which should have been normal ordinary residences at that time [49]. In addition, there is a small round house of about 10 square meters in the first phase of the first phase of the second, third, and fourth sites of the Baiyan site in Taigu, Jinzhong [50], and a similar 12-square-meter house found at the Yangbai site in Wutai, which may also be a cave and should be dated to this period [51]. Most of these small and rudimentary caves were found only during this period, and have been rare since then.
The second type is currently only found in Longzhong, where the remains of the fourth phase of Banpo are often referred to as the Shilingxia type. In the third section of the fourth phase of Dadiwan in Qin'an [52], the fourth phase of Tianshui Shizhaocun [53] and Fujiamen in Wushan [54], a kind of "semi-crypt" type building with round, square or short rectangular and short doorway generally appeared, the living surface is mostly white and gray surface, the circular stove is centered, most of them have no pillar holes, and the area is mostly about 10 square meters [55], such as F401, F402, F404 in Dadiwan, F28 in Shizhao Village, F4 et al., in the 7th phase of Shizhao Village and the 7th phase of Xishanping (early Qijia culture), it was found that the upper part of the cave wall was adducted in a relatively deep way, which should be judged to be a cave (Fig. 17), which is actually a typical cave of Qijia culture or Longshan period. The number of such houses found in Shilingxia is still very small, and it is not known whether they constitute the main body of the settlement. At the turn of the middle and middle and late Yangshao periods (the first and second members of the fourth phase of Dadiwan Plateau) before the Banpo Phase IV-Shilingxia type, the Yangshao settlements on the Loess Plateau were composed of semi-crypt houses with large areas, and five houses have been found in the Qinghaimin and Yangwapo sites (Dadiwan Phase IV) in the far west, all of which are semi-crypt houses with pillar holes [56]. In western Henan, Guanzhong, Shanxi, and northern Hebei, most of the houses are pentagonal semi-crypt houses [57]. At the same time, there are ground-level buildings like Dadiwan F901 in the Guanzhong and Longzhong areas of Dadiwan Phase IV. Obviously, the two cave-style buildings in Guanzhong, Longdong, Jinzhong and Longzhong areas of the fourth phase of Banpo are new architectural forms, but the caves that became popular later are the second cave styles that first appeared in Longzhong.
By the second phase of Quanhu, there is a lot of conclusive evidence that the caves have become the main architectural form of the settlement houses in most areas of the Loess Plateau. In northern Shaanxi in particular, several such sites have been excavated. Such as Hengshan Yangjiesha, Jingbian Miaoliang [58], Wuzhuang Guoyan [59] and other sites, the area is thousands to tens of thousands of square meters, all found on the hillside in a row of caves to form a settlement, the plane of the cave is mostly "convex" shape, a few are "Lu" shape, the shape has an oval, round and rectangular variety, the main room area is generally between a few square meters and more than ten square meters. Caves from this period were discovered in the second phase of the first phase of the third and fourth sites of the Baiyan site in Taigu in Jinzhong [60]. The "Miaodigou Phase II" (the report says that the age is between the third phase of Xiwangcun and the second phase of Miaodigou, which is roughly the second phase of Quanhu) [61] and the seven caves of Quandi Gou Phase II excavated by Dongxiafeng [62] are round and square in plan, without white ash surface, and there are niches or cellars at the back or side. The remains of the Xiangfen Tao Temple site, which were reported as the "second phase of the temple ditch" by the excavation, also belong to the second phase of Quanhu, which are distributed in the third area of the residential site, and the 17 houses cleared are all circular caves and form patio-style courtyards [63]. This period in Longzhong area is the Majiayao period of Majiayao culture, and there are round, square, rectangular and other forms of "convex" and "Luzi" houses in the fifth phase of Shizhao Village [64], Wushan Fujiamen [65] and Dongxiang Linjia ruins [66]. In the Hetao area of central and southern Inner Mongolia, the first phase of the Zhukaigou site in the fourth phase of Banpo [67], the second phase of the Baotou Ashan site [68], the first phase of the Baicaota site in the Zhunger Banner [69] and the first and second phases of the Xiyuan site [70] are all semi-crypt houses, and the sites of Miaozigou, Dabagou [71] and on the slope of Wang's tomb [72] near Daihai and Huangqihai are also Yangshao-style semi-crypt houses. However, the Dongtan site and the Hongtaipo site, which are relatively late, should be dated to the second phase of Quanhu, and the form of the houses seen is already a cave-like building [73].
In the second phase of Miaodigou, the Wubaohou Zhaizi Pass in northern Shaanxi, Hengshanzhai Mountain [74], the first phase of Fugu Zheng Zeyuan [75], the Lushan Pass in Yan'an, and the Weiluo Ruins in Xunyi [76] are still settlements with caves as the main building. To the east of the Yellow River, the site of Shilouchagou in Shanxi Province has found buildings with cave caves and rammed earth walls [77]. Although there are few discoveries in this period in Guanzhong and western Henan, the houses of the third phase of the Fufeng Caseboard Site [78], the second phase of the house of the bottom ditch of the temple in Wugong Huxizhuang [79], the house F801 [80] excavated in the third phase of Quanhu Village in Huaxian County, and the second phase of the Miaodigou site in Shaanxi County F551 [81] should all be caves. The second phase of the temple ditch at the Taosi site in Jinnan (the early stage of the Tao Temple culture) still sees the patio-style cave courtyards (F321, F319) that have appeared before, that is, the pit courtyard, in addition to the simple caves of the second phase of the temple ditch found in the Yuanqu Dongguan [82] and the Ningjiapo site [83]. The caves cleaned up at the North Loess Ditch site in Fengzhen in the Hetao area of south-central Inner Mongolia seem to be dated between the second phase of Quanhu and the second phase of Miaodigou [84], and in the following third cultural period of Ashan, the second phase of Chengzui in the Qingshui River [85], the second phase of Baini kiln [86], the second phase of Baicaota in Zhungerqi [87], the third phase of Guandi site [88] and the third phase of Lujiapo [89] are all caves, and the third phase (late section) of Baotou Ashan site [90] and the third phase of Xiyuan site are generally found to have "convex" shaped caves [91]. This period in Ningxia is the so-called lower culture of Changshan or vegetable garden culture, and it is obvious that the lower layer of the Changshan site in Zhenyuan [92] and the Linziliang [93] of Haiyuan are settlements with caves as the main body. However, in the Hehuang area west of Lanzhou, the Lanzhou Qinggang fork F1 is a semi-crypt house in the mid-mountain period equivalent to the second phase of the temple ditch, with an area of 48 square meters and a pillar hole around it [94], and the Yongjing Majiawan site excavated 4 square and 3 round semi-crypt houses, with a total of 5 pillar holes in the center and four corners, which can be seen to be not a cave [95], the age of the Majiawan site is roughly between the mid-mountain period and the early Machang period, and the cave has obviously not been used in this period in the Hehuang area.
During the Longshan Culture Period, although no clear house foundation sites were excavated in the Machang period (also known as the Machang type) in the Hehuang area, cave tombs suddenly became popular in this area from the Machang period, which is generally believed to imitate the style of cave-style dwelling houses. Corresponding to the early Qijia culture in the late Longshan period, the houses seen in the excavation of the Hehuang area and the Lajia ruins are all typical cave-style buildings with a front site, a doorway, a white ash surface and a circular stove site in the center. In the easternmost region of the Loess Plateau, in northern Hebei, the newly excavated Chongli Denggou Gouliang site has also excavated caves [96]. So far, the cave-style buildings are all over the Loess Plateau, reaching the largest range of distribution, from the westernmost Qinghai to the easternmost Zhangjiakou in northern Hebei, before the Yangshao-style semi-crypt houses were completely gone. Moreover, the caves that can be seen everywhere on the Loess Plateau during this period have almost only one standard cave style, which is the shape of the white ash surface and the ground stove in the center and circle. During the Longshan period, a large number of caves were discovered and excavated on the Loess Plateau, including Jingbian Miaoliang [97], Fugu Zheng Zeyuan Phase II [98], Shenmu Xinhua [99], Shiyuan Ruins Hanjia Qidan and other sites and Xunyi Xia Weiluo [100], in Guanzhong Ke Province Zhuang Phase II, Lintong Kangjia Phase 5 [101], Xi'an Ke Province Zhuang Phase II [102], Gaoling Majiawan [103], Dongying [104], Wugong Zhaojia Laike Province Zhuang Phase II [105], Qishan Shuang'an [106], Shizuitou [107] in Baoji; Hequpingtou [108], Biguanlougou [109], Baiyagou [110] and Bicun in Xingxian County in the northwest of Shanxi Province; Qiaojiawan [111], Yaozipo [112], Yuanping Xinzhang [113] in Jinzhong, Taigu Baiyan, Shilouchagou [114], Xinzhou Youyun [115], and Bailongshan in Wutaiyang; Fenglongshan period in Xia County in the east, Xiangfending Village [116], Quyutou [117], Taosi middle and late period, and Zhoujiazhuang in Jiangxian County [118] in southern Jin In the Hetao area of Inner Mongolia, there are Liangcheng Yuanzigou, Laohushan, Xibaiyu, Mianpo [119], Bancheng, Erliban [120] and Yongxingdian [121] in Zhungerqi, and in Ningxia, there are Longde Shatang [122] and Yehezi "Kexingzhuang Culture" [123], and in Longzhong, there are Wushan Fujiamen Qijia Culture, Tianshui Shizhao Village and Xishanping Phase 7 [124], Yongjing Dahezhuang [125] and so on. There are many other sporadic discoveries, which will not be repeated here.
3. The re-flourishing of cave settlements and the prehistoric Loess Plateau
There should be no doubt that the cave building form generally began to appear on the Loess Plateau in the fourth phase of Banpo. The first place to appear is currently in Guanzhong, Longdong, the eastern part of Longzhong to the area of Jinzhong, and it should also include Jinnan and western Henan, but there is no information on houses in this period in Jinnan and western Henan. Due to the lack of relevant information, it is difficult to accurately assess the extent to which the early caves played a role in the settlements at that time, but at least as seen by Yangguanzhai and Yangqi, the caves have become an important part of the settlements at that time. To the second phase of Quanhu, the settlements in northern Shaanxi, central Jinzhong, and southern Shanxi have all been built with caves as the main building, and should also include the Jingwei area, central Gansu, and Ningxia. The second phase of Miaodigou then expanded to the entire south-central part of Inner Mongolia, northwestern Shanxi Province, and western Henan, but it did not include the Ganqing region west of Lanzhou. In the Longshan era, from Zhangjiakou in the east to Qinghai in the west, all the Loess Plateau was already the world of caves. On the Loess Plateau, there are no large-scale settlements in the late Yangshao period (Banpo Phase IV and Quanhu Phase II), and the scale of settlements with caves as the main buildings is also very small. To the second phase of Miaodigou, the cliff-type cave settlements in the Hetao area, including northern Shaanxi, built stone walls on the ridges on both sides of the hillside, even if it is a small settlement, and the large settlements such as Lushan Pass have formed the structure of the hilltop courtyard and the hillside cave. In southern Jin and western Henan, it is a sunken "pit courtyard" cave settlement like Tao Temple, and there should also be a ring ditch (ring moat) cave as seen in the Dongxia Feng site in the Erlitou period, and there should also be rammed earth and adobe buildings on the flat ground in the ring ditch or ring moat, which is roughly the same as the design concept of the above-mentioned cliff-type cave settlement. The Yongsan period continued the basic form of this cave settlement, except that more advanced settlements such as stone ridges strengthened the scale of the hilltop buildings and created the style of a "city within a city". The residential form of cave dwelling has gradually developed into a settlement model with stable form and can accommodate a variety of social groups and social organizations.
The emergence of cave-type dwelling is not the result of the gradual evolution of prehistoric society and culture on the Loess Plateau, but the time node when caves began to appear in the late Yangshao culture, but the period of decline of Yangshao culture. Previously, the Yangshao culture on the Loess Plateau had undergone 3,000 years of development, and began to flourish in the middle of Yangshao, that is, in the Miaodigou period, and reached its most prosperous peak at the turn of the middle and late Yangshao culture [126]. During the prosperous period of Yangshao, the eastern part of the Loess Plateau was densely populated, and there were many large-scale ring moat settlements of hundreds of thousands or even millions of square meters in Guanzhong, southern Jin, and western Henan in the neighboring areas of Shaanxi, Jin, and Henan, such as 105 sites in the middle Yangshao period in the Lingbao Basin, of which there were many places with an area of 400,000 to 1 million square meters, such as Beiyangping, Xipo, Wudi, Yaotou-Renmazhai, etc., and the social differentiation was also the most serious in this period, as seen in the Xipo sites [127]. However, in the subsequent Banpo Phase IV, the number of settlements decreased, and large settlements disappeared completely, and only 44 sites of the late Yangshao period were discovered in the Lingbao Basin survey, and there were no large sites [128]. Even the iconic artifacts of Yangshao culture, faience pottery, were the first to disappear from this area. Due to the limitations of the current materials, we do not know what happened in the eastern part of Guanzhong, Jinnan and western Henan in the fourth phase of Banpo, but the earliest caves were actually found in the periphery of this area, such as western Guanzhong, Longdong and Jinzhong. It is worth noting that these earliest caves are very small, with an area of less than 10 square meters, compared with the large semi-crypt houses of hundreds of square meters in the previous Miaodigou period, which are in any case the product of cultural and economic decline. Moreover, the decline of the late Yangshao period actually occurred mainly in the Shaanxi, Jin, and Henan neighboring areas where the culture of the middle Yangshao period was the most prosperous, and there was no sign of decline in the late Yangshao period in the surrounding areas such as Jinzhong, northern Shaanxi, Hetao and Ganqing areas. Subsequently, these areas gradually abandoned the semi-crypt houses from the second phase of Quanhu and turned to the use of cave as a form of construction, and even the Hehuang area on the edge of the Loess Plateau, where the tradition of semi-crypt houses lasted for the longest time, also became cave settlements in the Longshan period (Machang type - Qijia culture), which is enough to show that the Yangshao-style semi-crypt and ground-level buildings with a large amount of wood as the building material are unsustainable, and the more prosperous the area, the sooner the semi-crypt buildings will disappear. The advantage of cave architecture is not only that it does not require a large amount of wood in the building structure, but also that it is particularly suitable for the geographical conditions of deep sediment accumulation of loess on the Loess Plateau, which is developed vertically jointed [129].
The appearance of the cave is not only a change in the architectural form, but also a change in the layout and location of the entire settlement due to the characteristics of this architectural form. Since the Neolithic Age, agricultural villages have entered the Loess Plateau from the Laoguantai culture to the middle Yangshao period, and the semi-crypt houses have always been the main architectural form of the settlements, and the semi-crypt buildings need to be built in relatively flat terrain conditions, so the location of the settlements is generally located on the riverside terraces. The Dadiwan site is a good example, there are settlement remains of multiple periods on this site, among which the settlements of Dadiwan Phase I, Yangshao Early Period (Dadiwan Phase II) and Yangshao Middle Period (Dadiwan Phase III) are all located on the riverside terraces of Qingshui River, and it was not until the middle and late Yangshao period that the Dadiwan Phase IV (Section 1 and 2) began to migrate to a higher position on the southern slope, and in the third section of Dadiwan Phase IV, that is, the period of Banpo Phase IV or Shilingxia type, cave-like buildings appeared. The migration of the settlements of the first and second sections of the fourth phase of Dadiwan to the mountains is a common trend in the evolution of settlements on the Loess Plateau since the late Yangshao period, and the reason for the migration is unknown, but the problem of water use in the mountains should have been solved at that time. In addition to digging caves on the hillside, large settlements also built stone walls on the ridges and special flat buildings on the top of the mountain, forming a fixed settlement form of the hillside courtyard - the hillside cave. The new form of settlement broke through the limitation of the river terrace, so that the original Liangshan landform of the Loess Plateau was more fully utilized, and the cave settlements could penetrate deep into the hinterland of the Loess Plateau, which should be one of the reasons why the Yangshao culture and its many variants continued to expand westward and northward in the area after the late Yangshao period, and then finally occupied the whole territory of the Loess Plateau in the subsequent Longshan period.
The cave is difficult to dig very large because of its architectural structure, and the general indoor area rarely exceeds 20 square meters, only the area of Lin Ziliang LF13 and Lougou F2 has reached 30~40 square meters, however, even these two large caves can not see the difference between function and social differentiation with ordinary caves. In large settlements such as Lushan Pass and Stone Pass, it is not the caves that stand out, but the courtyards, Taicheng and large city defenses built on the top of the mountain, which are huge in size, fully reflecting the tense community relations at that time, the intensification of social hierarchy within the community, and the ability to mobilize labor resources. In the cliff and sunken cave settlements seen above, the situation of the settlement defense system and the large-scale buildings on the ground in the latter is not very clear, but a large site such as Taosi should be comparable to the Lushan Pass and Shiyuan of the same period in terms of community scale and the degree of social differentiation seen in large tombs. The stone walls and mountaintop stone buildings of cliff-like settlements are mainly found in northern Shaanxi, northwest Shanxi and eastern Hetao in central and southern Inner Mongolia. The earliest stone buildings appeared in the second phase of Quanhu in the late Yangshao period, such as the second phase of stone houses in Zhukaigou VII area of Yijin Huoluo Banner, which should also be a substitute for semi-crypt houses. The stone "city wall" and the stone buildings on the top of the mountain are found a little later, and are found in five sites at the southern foot of Daqing Mountain, including Ashan, Xiyuan, Shamujia, Heimaban, and Weijun, as well as the sites of Baicao Pagoda, Xiaoshawan, Zhaizi Tower, and Malu Tower along the Yellow River in the south of the Qingshui River [130], all of which are small-scale ordinary settlements, and most of the buildings in the settlements should be caves. The scale of these stone cities or stone walls is not very clear, and some of the stone buildings on the top of the hill are considered "altars" and are not necessarily related to social division. Although some researchers speculate that the origin of this stone building technique may be related to the Eurasian steppe Afannashevo-Okunev cultural system, there is no tangible evidence. Considering that the Hongshan culture in southeastern Inner Mongolia had a tradition of stone building very early, and the Hongshan culture was distributed in the Zhangjiakou Bashang area of northern Hebei Province in the late period [131], which was quite close to the eastern part of the Hetao, the last stage of the late Yangshao period was closely related to the Xiaoheyan culture, and it is more reasonable that the stone building originated from the tradition of the Hongshan culture. Until the late stage of the second phase of the temple ditch and the Longshan period, a number of larger-scale stone cities and mountaintop buildings appeared in northern Shaanxi and northwest Shanxi, the structure of the stone city is as seen in the stone pass, it is a complete defense system with a horse face on the wall, a pier platform at the city gate, a gate school, and an inner and outer urn city. On the top of the hill, there are rammed earth buildings with walls and even rammed earth and stone walls on the stone ridges. This settlement landscape, the architectural design of the city defense, and even the stone masonry construction technology on the top of the hill were later inherited by the surrounding areas such as the lower culture of Xiajiadian. The systematic defense planning of Shiyuan Castle influenced the concept of city building in East Asia for thousands of years [132].
Of course, the emergence of complex society on the Loess Plateau after the second phase of Miaodigou is not caused by caves, but by a variety of economic and social reasons. According to the survey in the Yulin area of northern Shaanxi, the number of settlements gradually increased from the late Yangshao period to the Longshan period, reaching a peak in the Longshan period, and the number and scale of large settlements gradually increased [133]. The reason for this change should also be related to the change in the industrial economy. One of the most important factors is the introduction of herbivorous livestock such as sheep, goats, and cattle, which has greatly increased the proportion of livestock in meat sources. Taking the most common settlement wooden pillars and beams as an example, the proportion of sheep and cattle in the excavated animal bones is 77.32%, and that of pigs is 11.34%[134], presumably the proportion of large settlements such as Shiyuan should be higher. In addition, with the steady growth of these herbivores, the subsistence economy should also develop secondary product resources such as wool, milk and animal power. This has formed a new economic form of semi-agriculture and semi-animal husbandry, which has enriched the content of the previous dryland agriculture and improved the utilization capacity of environmental resources on the Loess Plateau. However, the formation of this economic form is still only the materials of the Longshan period, and it will not exceed the second phase of Miaodigou at the earliest[135], and the evidence from multiple sites shows that the livelihood economy in the late Yangshao period in northern Shaanxi was still the dry farming tradition of the previous Yangshao culture, not only did there be no herbivorous livestock such as cattle and sheep, but the proportion of domestic pigs was also very low, and the source of meat was mostly dependent on hunting [136]. That is to say, when the late Yangshao culture and its variants, such as the Majiayao culture and the Haishengbulang culture, penetrated into the hinterland of the Loess Plateau to the west and north, and the trend of continuously expanding the development space increased, the change of the industrial economy had not yet occurred. In the late Yangshao period, there was no large social group represented by particularly large settlements for hundreds of years, which shows that the expansion of settlements and groups of people did not cause particularly large changes in society. The appearance of caves in the late Yangshao period was only an alternative strategy to save resources when the socially developed areas of Yangshao were in decline, but it was also the emergence of this form of settlement that enabled the culture and society to continue after the decline of the prosperous area in the middle Yangshao period in the southeast of the Loess Plateau, and with the advantages of this alternative architectural form, the people of the late Yangshao period were able to continue to expand to the west and north to the whole territory of the Loess Plateau, laying the foundation for the cultural, economic and social transformation thereafter. The cave architecture and the subsequent cave settlement model have continued to this day, which lasted longer than the previous Laoguantai-Yangshao culture semi-crypt building model, opened up a wider terrain and landform, and accommodated more social forms, so it can be said that the cave conquered the Loess Plateau.
IV. Conclusion
The Yangshao culture has continued on the Loess Plateau for 3,000 years, and has been composed of large and small ring moat settlements with semi-crypt-style houses, distributed on the river terraces. In the heyday of the middle and mid-to-late Yangshao periods, dense settlements and large communities with an area of hundreds of thousands or even millions of square meters developed in the neighboring areas of Shaanxi, Shanxi and Henan, and obvious social differentiation had emerged in the large communities. However, in the following four phases of Banpo, there was a sudden decline, large settlements disappeared, and the number of small settlements also decreased. At the same time, there were settlements in Guanzhong, Longdong and Jinzhong with simple small caves as ordinary dwellings, and there were concentrated and contiguous caves in Yangguanzhai in Guanzhong and Yangqi in Longdong area, indicating that the main building of the settlement was already a cave-type house. However, during this period, the type of caves under the stone ridge found in the sites of Dadiwan, Shizhao Village, Fujiamen and other sites in Longzhong were relatively large, characterized by white ash surface and centered round ground stoves, which were the standard shape of the caves on the Loess Plateau after that. The caves should also appear in the eastern part of Guanzhong, southern Jinnan and western Henan in the fourth phase of Banpo, but there is little archaeological work at present, and there is not enough evidence. In the second phase of Quanhu and the second phase of Miaodigou, cave-type settlements were generally found in northern Shaanxi, Hetao, northern Shanxi, central Jin, southern Jin, western Henan, Guanzhong, Longdong and Longzhong regions, and expanded to eastern Qinghai and northern Hebei during the Longshan cultural period, completely occupying the entire Loess Plateau. In this process, the Loess Plateau gradually began a new round of prosperity, and reached the peak of prehistoric culture and society during the Longshan culture period. Under the situation of continuous social tension and increasing social differentiation, starting from the second phase of the Miaodigou culture, the cave settlements on the hillside were built with stone walls on the ridges, and rammed earth and stone courtyards were built on the top of the mountains, such as the newly excavated Lushan Pass site. A more extreme example is the "city-within-a-city" model of the Longshan period, in which a platform was built on the top of the highest hill in the city. This led to the formation of cave settlements that could accommodate large social groups and exhibit social differentiation. And in the south of Jin, western Henan and even the eastern Qinghai river terraces and loess plain, there should be another form of sunken cave settlement, Dongxia Feng is a typical case known so far, the settlement is composed of two trench type caves and trenches enclosed on the flat ground of the pit courtyard, or as in the Lajia ruins along the two sides of the ditch dug into a row of caves, the bottom of the ditch is the road, and there are pit courtyards, public buildings and cemeteries on the flat ground of the ditch, although the Tao Temple is not clear enough, but it should also belong to such a settlement. Cave architecture was supposed to be an alternative and simple form of living in response to the decline of social, cultural and resource resources in the middle of Yangshao, but after that, with its advantages of saving wood and being suitable for the original Liangshan landform in the hinterland of the Loess Plateau, it penetrated deep into the hinterland of the Loess Plateau, opened up a broader space than the semi-crypt building form in the early and middle Yangshao period, and developed a new settlement style, which influenced the living form of the area for thousands of years.
Source: Archaeology and Antiquities, Issue 2, 2022