The Heaven Pit and Ground Seam Scenic Spot is a part of Qiyao Mountain range and the geodetic structure is located in the extension part of the upward and folded zone of Sichuan, Hubei, Hunan and Guizhou at the west edge of Wuyi Mountain-Xuefengshan Mountain folded zone in Neocathaysian structural system. The direction of structural line is NE-SW. It is a typical landform of shallow-medium cut platform peak-cluster depression. The emerged strata are the medium-thick strata in Jialingjiang formation of the lower series in Triassic system, with little thin limestone interbedding, and gradually ascended in three steps from southwest to northeast.
The karst hydrogeological system of Xiaozhai Heaven Pit-Tianjingxia Ground Seamis composed of the wider valleys on its upper reaches, the gorges, water discharging holes and ground seam on its middle reaches and the underground river and heaven-window funnel on its lower reaches, the drainage area of the system is 230 km2, the height difference between the head and the tail of the system is 1700m, and the whole length from the head to the tail of the system is about 37km. The wider valley section of the upper reaches is 7.5 km long. Tongshuoba-Maochaoba Basin which was developed along the anticlinal axis is located on the peak -cluster depression in elevation of 1900-2000m, with the elevation at the valley bottom of 1710m and slope descent of 0.02. The section on the middle reaches is featured by gorges and is 13km long, the gorge is 100m-500m wide, with the cut depth of 200m-500m. This section is a section strongly affected by Shuoyuan erosion in the Quaternary period, with the slope descent of 0.04. The ground water flows into underground from several places one-odd kilometer from its lower section, water mainly flows into the black holes at the valley's bottom and always ends at these places in dry and normal-flow seasons. The compound valley section (its lower part is a screen-like valley) of Tianjingxia ground seam is 5.5km long and is a box-type valley with the width of its upper part of 200-400m and depth of 300m-400m, extending towards Chigucao dry valley with its bottom elevation of 1000m. The lower part is the "Tiangjingxia Ground Seam" with the width of top opening of 10m-30m and the width of seam bottom of 1m-15m. The depth is gradually added to 150m-229m and the width gradually gets to be narrower from its upper reaches to its lower reaches, finally, it becomes a "seam" and thus called Tianjingxia Ground Seam. The slope descent of the valley section of the ground seam is 0.07, it is the largest in this hydrogealogical system. The Tianjingxia valley, starting from the south end of Chigucao dry valley, changes into an underground river, which flows through the Xiaozhai heaven pit and ends at the outlet of Migonghe River, with total length of 7.026 km and slope descent of 0.046, this is the section of lower reaches of this hydrogeological system. The width of the underground river cavern is mostly less than 15m, but its height is over 100m. The temperature in the cavern is lower than 10?, annual average flow rate is 8.77 m3/s and the maximum flowrate is 174m3/s. The outlet of the underground river is located on the precipice of Migonghe River, with the waterfall drop of 46m.
Xiaozhai Heaven Pit is actually a "heaven window" of the underground river on the lower reaches of this karst hydrogeological system, and is called karst tunnel. Xiaozhai Heaven Pit is close to the edge of the precipitous cliff on the right bank of deep-cut Daxi River. The elevation of the pit mouth is 1330m, and the elevation of the river bed in a shortest distance is 300m, the relative height difference between them exceeds 1000m. The measured data obtained by Wanxian hydrogeological team in 1984 indicate that the elevation of the highest and lowest points on the precipitous cliff at the heaven pit mouth is respectively 1331m and 1188m and there is an underground river flowing through the bottom of the heaven pit, with the elevation of 669m. It can be seen from this that the maximum and minimum depth of the Xiaozhai Heaven Pit is respectively 662m and 511m. In the vertical depth, Xiaozhai Heaven Pit is in double-layer structure, its upper part is a large elliptic pit, with the diameter of 537m -626m and depth of 320m, and its lower part is a vertical rectangle-like well, with the depth of 342m and mouth diameter of 357m in north-south and 268m in west-east, and at the pit bottom, there is a developed inclined slope. The total volume of the heaven pit is 119.348 mm3. Precipices stand on its all sides, just like they are cut by axe or knife. The pit is completely enclosed, if you stand at the pit bottom, you seem to look at the sky from the bottom of a well.
This scenic spot is in middle subtropical warm and moist southeast monsoon climate zone, and since it is affected by the terrain rise and the general configuration of the earth's surface, the vertical climate changes greatly, the main climate of mountains is distinct and the rainfall is abundant, which make the animals and plants in middle and north subtropical zones and middle temperate zone able to live and breed. It has beenproved that in this scenic spot there are 224plant families with 1285 species, including the national Class I protected plant dawn redwood, 13 species of the national Class ? protected plants such as gingko, walnut, cucommia and so on ,and 8 species of Class ? protected plants. The coverage rate of vegetation is up to 80% or more. There are the national Class I protected animal golden monky, 15 species of Class II Protected animals such as giant salamander, otter, zibet, clounded leopard, golden pheasant, Chinese pangolin and musk deer, and 14 species of Class III protected animals. Owing to the particular geography and land form, here live many unique wild animals, including the Yang fish and glass fish in the underground river, the white tadpoles in the hidden river of the ground seam, the white crickets in deep caverns and the rare flying squirrels in the caverns, etc.