|Karst Landforms And Caves Around The Western Hills, Beijing|
Weng Jintao, Zhang Cheng, Xie Yunqiu
(Karst Dynamics Laboratory, MLRC)
|The karst landforms and caves
around the Western Hills of Beijing are typical of the semiarid and semihumid
karst. There are not only the karst form complex matched with the modern
climate, but also the karst features formed under the particular environmental
conditions. The latter, including the subterranean river system with large
caves and the marvelous pseudocone karst landforms (photo 2, inner back
cover), will be the subject of this excursion. Our field trip will mainly
investigate the relationship between karst features and geological, climatic
and hydrological conditions, and discuss processes and mechanisms of the
special karst landscapes.
Though the fundamental geological researches in the Western Hills of Beijing started early and got along well, the results on researches of the karst landforms and caves in this area were rather poor. The important results are mainly on the caves around Zhoukoudian,Yunshui Cave, Shihua Cave and the karst hydrogeology at Niukouyu. The subterranean river system on the south bank of the Dashi River was only investigated simply, and no determinate conclusions were drawn. The pseudocone karst along Juma River had been considered as paleocone karst before July, 1991 when IGCP 299 Project held a field trip in this area. Some karstologists from different countries opened debates on the geomorphologic features and their formation in that trip. Weng Jintao, Jiang Zhongcheng et al. studied the landforms over two years and recently put forward a viewpoint of pseudocone karst (Jiang et al., 1994). They still picked up the unique karst forms unmatched with the modern climate and divided them into two types. Type one includes the paleokarst such as the polje at Niukouyu, "Peking man" Cave and Yunshui Cave which were formed under paleo-environments. Type two is the marvellous landforms formed under particular geological and hydrogeological conditions that will be visited. By means of the detailed field surveys and the experimental analyses on the karst environments and histories, they raised the three subperiods of karst formations in the Western Hills of Beijing (Weng et al., 1995).
The karst tourism resources along this trip route are very abundant and colorful. Most of our stops are charming tourism spots. With the intensive exploitation of tourism and the limestone resources, the geological landscapes are being heavily destroyed, and the surrounding environments are facing rather serious pollution. Therefore, protection of the important karst geological landscapes is imperative.
The field trip will be held mainly in Fangshan District of Bejing City. Setting out by bus from Beijing in the morning, participants will pass through Mentougou, Lujiatan,Cijiawu, Fangshan, Shidu, Liangxiang etc.. The total length of the one-day's route is about 400 km.
The field trip areas are to the southwest of Beijing City, including Dashi River and Juma River, 40 km away from Beijing City. Their coordinates are 39¡ã40' N, 115¡ã55' E and 39¡ã35' N, 115¡ã35' E respectively. Fangshan is in the middle between the two areas.
The transportation facilities of the area are pretty well. There are railroads passing through Fangshan and Shidu. Highways, mostly asphalt road surface, are criss-cross. And there is a motorway between Beijing and Fangshan.
The excursion area, lying at the juncture between Taihang Mt. and North China Plain, is generally called as Xishan(i.e. the Western Hills). One of the highest peaks is Baicaopan located in the northwest, with an altitude of 2046 m a. s. 1.. The general relief is gradually lowered from northwest to southeast, i.e, from the middle mountains, low mountains, hills to plain in turn. The main landforms of the area are low mountains and hills, from 200 to 800 m in altitude. The surrounding plains are about 50 m a.s.1..
Both Dashi River and Juma River belong to the Daqing River system. Dashi River originated on the south foot of Baihua Mt. and flows from west to east and finally into the plain, with a length of more than 100 km. The water flow of the river is intermittent and its discharge varies greatly with the seasons. Juma River originated from Hebei Province and flows towards North China Plain from west to east. But the river is zigzag. The runoff of Juma River is abundant and has never dried up, and the water quality is good, so the river will be one of the clean water sources of Beijing City.
The Western Hills belong to the zone of temperate semiarid and semihumid monsoon climate. The summer is hot and rainy, but the winter is cold and dry. Both the highest temperature and the largest rainfall occur from June to August. The mean annual rainfall is 602. 8 mm, the mean annual temperature is 11.6¡æ , and the relative humidity is 55%. August is the last month of the hot and rainy periods, with a mean monthly temperature of about 25¡æ, a mean monthly rainfall of about 230 mm and a highest daily temperature up to 30¡æ. Though there may be heavy rain, most time in this month have fine weather.
The trip route is shown on Fig. 1.
|REGIONAL GEOLOGY AND KARST
Geologically, the Western Hills are at the center of the North China Massif and is located in the northeast end of NNE-trending Taihan Mountain Uplift, facing North China Plain on the east and neighboring E-W-trending Yanshan Mountains on the north. The Indosinian E-W-trending folds and the Yanshanian NE-trending folds, faults and magmatic intrusives are the fundamental tectonic pattern.
The trip starts in the lower reaches of Dashi River. It is located in the northern limb of Beiling Syncline and consists of the Precambrian, Cambrian, Ordovician, Carboniferous and Permian systems. The Nandazhai-Babaoshan Overthrust Fault passes through Cijiawu, where the Jixian System overlaps upon Carboniferous and Permian. A decollement fault is developed around the Fangshan Intrusive on the south. Shihua Cave, Yinhu Cave and Wanfotang Spring are developed in the Ordovician pure carbonate rocks which is more than 1000 m thick. Around the decollement fault the joints and fissures are very well developed, which laid foundation to the origination of karst features.
Along Juma River appear the thick siliceous dolomite and dolomitic limestone of Wumishan Group of Jixian System of Middle Proterozoic (Jxw), with a thickness of more than 1000 m. The structure around Shidu is simple and is characterized by gentle bedding structures and vertical joints, with occasional faults filled by dikes, and small-scale folds. The area has been uplifted greatly in the neotectonic movement.
The karst landforms in the Western Hills is characterized by the coexistence of karst of typical temperate semiarid and semihumid type, and karst of particular origin and paleokarsts, exhibiting marvellous karst form for the Western Hills. The semiarid and semihumid karsts can display a karst feature complex composed of normal shaped mountains, karst dry valleys, karst springs, solution fissures and carbonate rock scree, which reflects the weak karstification processes. "Peking Man" Cave at Zhoukoudian, Yunshui Cave in Shangfangshan and the polje at Zhoukoudian are the examples of paleokarsts that formed under the warm and humid paleoclimate in Neogene. The pseudocone karst along Juma River and the subterranean river system from Yinhu Cave, Shihua Cave to Wanfotang Spring are the marvellous karst forms developed under the particular geological and hydrological conditions. The former is mainly composed of high peaks, stone pillars and deep gorges, and the latter is characterized by wonderful solution forms like multi-layered big underground passages, big karst spring, large cave systems and the chemical deposits.
DESCRIPTION OF FIELD TRIP STOPS
Part 1, Karst area along Dashi River
Stop 1, The semiarid and semihumid karst landscape in Lujiatan
The stop is in the west of Lujiatan Basin. Standing on the bridge, participants can have a spectacular bird's eye view of the uninterrupted mountains, which are 400 - 924 m a.s.l., with the continuous ridge line. These are the normal shaped mountains and no typical positive or negative karst landforms owing to the weak solution rate of the semiarid and semihumid environments, though all the northern, western and southern mountains are composed of the Cambrian and Ordovician pure limestones or dolomites. On foot of the mountain there are a lot of limestone screes that resulted from the physical weathering.
The bedrock of the river below the bridge is Ordovician limestone. The river valley,180 m in altitude, is a karst dry valley and only drains flood in rainy season. There is a little spring water flowing out from the rock fissures along the valley in rainy season. Though there is a basin form around Lujiatan, it is not closed and was cut down by the river on the south into North China Plain. Without underground drainage network, it is not a real karst basin or polje. The basin landforms resulted from erosion of both water of the confluent rivers and paleo-glaciers (Shui, 1977). There are thick Pleistocene loessal deposits in the west of the basin, with a pale yellow upper layer and a brown red lower layer. The spore-pollen analysis shows that the deposits developed under the cold and semihumid to semiarid climate which is similar to that of the present day. Therefore, Lujiatan area has long been in the temperate semihumid and semiarid conditions, with the representative karst feature complex characterized by the normal shaped mountains, karst dry valleys, karst springs and limestone screes. This kind of complex is widespread in the karst mountains of the whole North China.
Stop 2, Heilongguan Spring
Heilongguan Spring, located at Heilongguan Village north of Dashi River, 140 m in altitude, is the biggest spring of Beijing karst area, with a mean discharge of 102, 000 m3 /d. It comes out from the dolomite of Tieling Formation of the proterozoic Jixian System (Jxt), and was formed from the obstruction of karst ground water by phyllites of Qingbaikou System (Qnx,also Proterozoic) (Fig.2). The spring is genetically related with the water of the upper reaches of Dashi River. When the water reaches the turning point of the river near Heilongguan, it flows towards Heilongguan Spring along a short cut and passes through the soluble rocks. The water quality of the spring is similar to that of the river water. As shown by the data of Oct, 12, 1992, the spring and river water of the upper reaches have the hardness of 10.50 and 10.46 German degree, and the total mineralization content of 304.94mg/l and 339.09 mg/l respectively.
|The karst fissures below the
surface of Dashi River valley are well-developed, so all the river water
can seep underground, as is obviously different from the upper reaches in
the sandstone and shale rocks where perennial waterflow is available. In
the middle and lower reaches of the river, the river is usually dry though
it can still get the recharge of some large karst springs such as Heilongguan,
Hebei and Wanfotang springs and other small springs. The leakage of the
river water is profitable to the development of the karst conduit and caves
on the southern bank of Dashi River valley, just like the formation of Heilongguan
Heilongguan Spring water is clean and available for exploitation as a drinking-water source.
Stop 3, Yinhu Cave
Yinhu Cave is a tourism cave, located in the valley at Xiayinshui Village. The entrance was originally for coal mining. The natural cave is below the middle of the mining tunnel, with a length of about 5 km. There are two branches in the cave. The east branch is for tourism. The west branch, with a low relief, has a subterranean stream where the alluvial deposits is abundant. The cave can be still divided into 160 m and 180 m a.s.l. layers which both are far lower than the 197 m altitude of the stream bed at the village.
Yinhu Cave was developed in pure limestone of Middle Ordovician Majiagou Formation (O2m), which is underlying the Middle Carboniferous sandstone on the south. There is a decollement fault between the Lower and Upper Paleozoic. Under the influence of the fault, the limestone became broken and deformed, and the fissures in the limestone are well-developed, which are favourable for the development of karst caves (Fig.3).
The quartz sand and gravel
deposits of the subterranean river in Yinhu Cave reflect the recharge
of allogenic water. The mineralization degree (314.28 mg/l) and total
hardness (10.66 German degree) of the water are similar to that of the
Dashi River water but different from that of non-karst stream water on
the south. The high Mg2+ content (13.53mg/l) suggests that it flowed through
dolomite area. There is a large amount of leakage from the middle reaches
of Dashi River water into the southern aquifers, so the Dashi River, with
a very long section of its middle reaches becoming dry on the surface,
is probably the most important source of the Yinhu Cave water.
A contrast between the radius of crook and the height of cliff in the
|There are three big stone pillars on the west of Yingzuiya, with a relative height ranging between 30 to 100 m. They stand side by side and are separated by karst fissures. One pillar has a cone-shape top and a steep body. The slopes are different in different directions, e.g., the north slope (facing the river) of the east stone column is 86¡ã; the west is 61¡ã; the south slope is 68¡ã; and the east is 75¡ã.Besides, the pillars are rectangular but not circular. All these features suggest that the pillars are evidently different from the cone karst in South China. These stone pillars were formed not only by solution and erosion along vertical joints in rocks, but also by river cutting and physical weathering. The siliceous dolomite in this area is hard but fragile, having low dissolution and corrosion ratios but high mechanical failure (Table.1.). The strata are gently dipping (less than 15¡ã), and the vertical joints are well developed, with a fissure density of more than 5 fissures per meter and a fissure width of 0.03-0.05mm. So the rock is easily eroded along the vertical fissures on the one hand, but it is able to be tall and be kept for longtime in shaping of steep peaks on the other hand. Consequently, a series of steep cliffs and stone pillars were finally formed.|
|Stop 7, Sidu terrace
and pseudocone karst
At Sidu, Juma River has the smallest-radius crook, where there was not only developed a 250 m high cliff on the southeast bank but also resulting in lots of river deposits on the north bank. Going up the northern mountain slope from the Sidu bridge, three terraces can be clearly seen (Fig.5), in which the third was formed in the Middle Pleistocene, and the first and second were formed in the Late Pleistocene (Xie et al,. 1985). The terraces are a result of tectonic uplift and cutting down of the river. The height of Sidu terraces can tell that, since the Middle Pleistocene, Juma River has cut down at least 10 m.
Looking south from the Sidu terraces, the mountains on the south bank of Juma River apparently consist of peak-clusters with steps of cliffs and seem to be similar to the scenery of Lijiang River in Guilin. However, if we go into the mountains, we can not find the negative karst landforms such as closed depression, and only find the linear stream drainage system. Though there is a karst spring at the mouth of valley, the water flow is small and mainly comes from the deposits of stream but not the bedrocks, as can be proved by karst geochemical methods. In fact, there are few real cones or tower peaks in the mountains, and some cone-like shapes are due to that we can only see the cross sections of a continuous mountain bodies which were cut down or eroded along the vertical fissures.
The stream system of Juma River is controlled by the joints. According to statistics, the E-NE and NNW-trending joints are dominant and formed many parallel grooves. Even some big streams, like tributaries of Juma River, have their trends also corresponding to strikes of the joints (Table.3). Therefore, in some positions such as Sidu, once facing one group of dense vertical joints, we can see dense and parallel deep grooves or streams and the peak-cluster-like landforms.
comparison between numbers of 1 st-order tributaries
|Stop 8, The karst
landscape around the southern bank at Jiudu
Jiudu is the center of Shidu scenery area. Along a tributory on the southern bank of Juma River, we may not only investigate pseudocone karst landforms in detail but also find some illuminating special karst features.
Tongtian Cave, located near the top of Panshe Mt., about 10 m in length, is a through tunnel and is very simple, with two trumpet-shape entrances and a little physical weathering scree inside the cave, which recorded the strong physical weathering processes.
Arriving at Xincun, firstly we can see two intersecting karst dry valleys. At the intersecting point, a stone pillar, about 3 m high, looks apart from the hill body and tends to collapse. There are a few dozens of centimeters wide karst fissures between the stone pillar and the hill body, which were mainly formed by solution of rain water along the vertical joint and tension of gravity of the stone. The stone pillar is rectangular and has split open along joints, with the upper part tending to collapse. These features of the stone pillar not only clearly show geomorphologic significance of the vertical fissures, but also suggest the strong regional physical weathering. In the west valley of Xincun Village there are "debris-flow-like" deposits, with lots of sharp angular boulders, which are another index of the physical weathering.
There is a typical solution cap-like rock feature on the south of Xincun Village, that is a several meter high stone pillar on the top of hill. The form is a residual part of the hill top after erosion and denudation, which has been reformed by the physical weathering processes.
Stop 9, Bianfu Mountain
The high and great Bianfu Mt. is situated on the north of Shidu Village. It is a representative form of the pseudocone karst landscape and also a symbol of Shidu scenery. The mountain is composed of two connecting peaks, forming a 500 m high concave cliff that looks like a big bat with spread wings (Bianfu means bat in Chinese). There is an arete-like ridge with some horns on the mountain. Shidu River, an important tributary of Juma River, had passed by the front foot of the mountain, as can be proved by the cemented gravel sediments of the alluvial facies. Therefore, cutting down by the paleo-crook formed the special structure of the mountain. The sharp forms on the top of the mountain are a result of the long period wind erosion and frost weathering.
A big slopewash apron on the foot of Bianfu Mt. can still be seen. It has a height of 90 m and a slope of 30 degrees and forms a sharp contrast with the steep cliff. The apron is composed of collapse deposits, containing clastic and boulder rocks, sand and clay. Lots of aprons and big rock collapse masses along the banks of Juma River can be found which suggest not only a strong regional physical weathering, but also a weak solution, which is different from the humid and warm karst areas in South China where limestone scree can be dissolved rapidly. Therefore, the karst landforms along Juma River is quite different from the peak-cluster karst in South China (Table 4).
Table 4 A
contrast on forms between the pseudocone karst around Juma River and
The Pianshangling terraces of Juma River
The terraces lie in the east of Pianshangling Village and on the north bank of the river, with a shape of flat-top hill. There are evident three terraces. The first terrace is erosion terrace. The second and third ones are accumulation terraces (Fig.6). There is a loess layer on the top of the third terrace. The second terrace consists of the weathered alluvial gravel layer of Pliocene (Xie et al, 1985). And between the first and second terraces there are intercalated with red sand beds. So we can reckon that Juma River had formed before Pliocene.
The thick second and third terrace deposits and the red sand and clay materials suggest that the terraces developed in rather humid and warm environments. Barbus fossils, and spore-pollen of evergreen arbors in Pliocene sediments of surrounding area are also indices of the climate environments. In a word, water and heat were enough to support a stronger karst process, and it was possible for development of some typical solution forms such as caves and karst fissures. Meanwhile, the humid paleo-climate led to a basic pattern of the modern river drainage system.
The first terrace which has been cut down at 10m deep suggests an active neotectonic movement. Because this location is near North China Plain, the uplift scale of the massif is far less than that at the center of the mountain areas (Li et al., 1990). Strong tectonic uplift is an important dynamics for cutting down of Juma River and development of both the steep cliffs and gorges.
|Stop 11, Leiyin
Cave in Shijing Hill
Shijing Hill, more than 300 m in altitude, has nine small caves near the top with green pines. The caves can be grouped into two layers. There are seven caves on the upper layer and two caves on the lower one. Because the nine caves were hidden with stone Buddhist Scriptures, they were named as Cangjing (hiding place for Buddhist Scripture) Cave. Engraving Buddhist words on stones is to avoid its destruction, especially the destruction in Buddhism-extermination events.
The Scripture-carving activity had gone about a thousand years from Sui Dynasty to Qing Dynasty. 14,000 slabstones had been engraved, including more than 1,000 Buddha classics, about 3,000 volumes, so here is the oldest and greatest library on old stone Buddha scriptures. The stone scriptures include two versions in different size, the bigger one amounts to 4195 pieces, 2 m high and 0.66 m wide each, with about 8000 words, and the smaller one 10,082 pieces, 0.66 m in both length and width, over 0.66 m thick, with more than 1800 words on both sides of a slabstone.
Besides Leiyin Cave being opened, the other eight caves are closed and preserved. Leiyin Cave, known as the biggest one of the nine caves in Shijing Hill, has four octagonal pillars at its center with 1056 Buddha relief sculptures on the surface of pillars. Some old stone scriptures are seen on the cave wall and there are five steps of stone niches. The top niche is made of marble; the second, of limestone; the third, of silver; the fourth covered with silver, with one colourful wood bead and eleven natural pearls; and the fifth is made of jade, with two Buddha pearls and two natural pearls.