Meander refers to the reach that bends like a snake, which usually can be found in the middle and lower reaches of rivers. As the gradient slackens, the erosion effect of the river decreases, while the lateral erosion effect become more obvious. As a result, the river constantly erodes the river bank and expands the riverbed, causing the river to bend and forming meanders.
Polje, also known as karst basin or karst plain, refers to the wide and flat valley in karst areas. The two sides of the valley are often imbedded by the peak forest, with steep brae , flat valley and sulciform cross section . There is often a passing river running through the valley from one side to the other side and dive into the underground. Under the action of rivers, the valley expands rapidly, accumulating thicker alluvial deposits. In addition, there also reserved residual limestone eluvial laterite and low limestone peaks or residual hills.
Simeng, in Zhuan language, means a place that is beautiful in four seasons. There are precipitous peaks, scattered fields, with rays of sunlight passing through the arms of the mountains to embrace this beautiful field, it looks like old lovers' sweet snuggle who have been apart for a long time. The beautiful countryside scenery is bright; the landscape is pleasant. Entering the Simeng field, you can experience the beauty and affection of utopia.
The famous Porui Tiankeng group locates less than two kilometers to the east of Simeng field, among which the most spectacular ones are Porui Tiankeng and Nongkun Tiankeng. The rural scenery of Simeng is a combination of peak cluster and solution depression, forming a landscape of peak cluster--depression. Monadnock is also the representative geosite. The base of monadnock is independent and surrounded by terraces, the residual peak stands alone gracefully and steadily.
Tiankeng refers to the oversize karst negative landform, with huge volume, steep but trapped palisades. It is developed in a tremendous thick soluble stratum with extremely deep underground water. The average width and depth are more than 100 meters from the underground to the ground, with the bottom connected to the underground river. The famous Tiankeng in Donglan are Porui Tiankeng and Xianledong Tiankeng, which are magnificent and wonderful.
Porui Tiankeng group has unique characteristics of large scale and dense distribution. The most representative one is Porui Tiankeng with a depth of about 422 meters, which is rarely seen in China.
Donglan National Geopark is located in the west of Guangxi province, northwest of Youjiang basin. As for its evolution history, it has undergone Caledonian movement, Variscan orogeny, Indosinian movement, Yanshan movement, Himalayan orogeny and Neotectonic movement.
In the late period of Caledonian movement, early and middle period of Variscan orogeny and Indosinian movement, which are 222 to 405 million years ago, this region kept the process of marine deposition. The Triassic and upper Paleozoic strata are strongly folded and uplifted because of the late Indosinian movement, then forming the land and suffered denudation.
In the early period of Yanshan movement, the early characteristics in the region were generally maintained or inherited. The crust kept subsiding and thick sub-flysch formation was deposited. By the middle period, about 208 million years ago, the history of Marine facies deposition in the region was ended, and this region rose to become the eroded area, thus the Jurassic - Tertiary strata is missing. Since the Himalayan orogeny and Neotectonic movement, the strata has been dominated by uplifting movement, so that the general characteristics of karst morphology in the region are mainly vertical form. The stratum formed during the subsidence period are mainly composed by limestone, dolomite, silicalite, clasolite and glutenite. Under the effect of erosion, the main landscape is valley landscape accompanied by peak cluster, the stratification characteristics of karst caves are obvious, a typical karst landform is eventually formed.
In the long geological age, countless creatures lived on the earth. After their death, the remains of these creatures or the traces left are called fossils. By researching fossils, we can get to know the biomorph, structure and category of the creatures lived long ago. And then we can speculate the origin, evolution and development of creatures during billions years, and to infer the ecological environment of the earth in various stages of the long period of geological history. Therefore, fossils are faithful recorders of life on earth.
After the death of those creatures which have hard part, such as shell, bone or woody tissue, instead of being crushed, decaying or weathering, they were quickly buried by some materials that prevent decomposition . After a long period of burial and lithification, there eventually became the fossils. There are precious fossils in Donglan National Geopark, such as Lanmu fossil group, among which ammonite is the most representative one, which lived from Devonian to Cretaceous and is a kind of extinct Marine invertebrate fossil.
东兰国家地质公园兰木化石群主要以菊石为主，品种繁多，其中Protrachyceras、Joannites两属频繁出现，是世界各地海相拉丁期地层中的常见属。我国拉丁期菊石的分布较局限，产出的Protrachyceras ladinum Mojs、Joannites kossmati Diener现尚见于我国西藏阿里地区南部的拉丁期地层。兰木化石群不仅丰富了我国拉丁期菊石群的内容，同时对于区域地层的划分、对比及古环境、古地理的研究均有重要的意义。
There are many types of fossils in Mulan fossil group of Donglan National Geopark, among which ammonite is the most typical type. The most common category are Protrachyceras and Joannites, which are common in Marine facies Latin strata formation around the world.
When the crust is stable, under the effect of external geologic agent, the rolling surface of the earth gradually become the peneplain. With the uplift of the crust, the peneplain is cut and damaged, those which are left on the top of the mountain or on the hillside, are known as the planation surface. If the mountain has experienced several intermittent uplifts, several planation surfaces will be formed at different heights. This multilayered terrain is an important sign to judge regional intermittent uplift and drop movement.
In early stage, with the rising of crust and the development of cracks, the underground water eroded and hollowed the mountain, forming the underground karst cave. Under a long time of water erosion, the mountain where the cave lies collapsed and eventually connected the anterior and posterior cave. Later, it is gradually exposed to the surface because the crust continues to lift and the underground water level drops gradually.