A comparative study of the karst landform of the Tiansheng Three Bridges in Wulong, China and the river erosion landform of the Grand Canyon in the United States.docx

A comparative study of the karst landform of the Tiansheng Three Bridges in Wulong, China and the river erosion landform of the Grand Canyon in the United States

introduction
The shaping of the earth's surface morphology is the result of the long-term interaction between internal and external geological forces. The Tiansheng Three Bridges in Wulong, China, and the Grand Canyon in the United States, as two distinct landform types, show the typical characteristics of karst and river erosion, respectively. Wulong Tiansheng Sanqiao is developed in the Paleozoic carbonate strata of the southeast margin of the Yangtze craton, and is an outstanding representative of the dissolution-collapse karst landform under subtropical humid climate. The Grand Canyon, on the other hand, shows the cutting process of the river to the multi-layered sedimentary rock system in the context of the North American plate stable craton. These two world-class geological wonders are not only of high aesthetic value, but also provide a natural laboratory for us to understand the evolutionary mechanisms of epigenetic processes in different geological environments. In this study, we will systematically compare the morphological characteristics, tectonic background, rock composition, formation process and scientific value to reveal the formation mechanism of the two landforms and their indicative significance for the evolution of global landforms.

Wulong, China, is born with three bridges karst landform

Wulong Tiansheng Third Bridge is located in Wulong District, southeast of Chongqing, in the lower gorge area of Wujiang River south of the Three Gorges of the Yangtze River, and is a typical karst landform area in southern China. Its geological structure background is complex, and it has experienced the superimposed influence of multi-stage tectonic movement and karst, forming a trinity landform landscape with "Tiansheng Bridge, Tiankeng, and Ground Crack" as the core.

The strata in this area are dominated by Permian and Triassic carbonate rocks, including limestone, dolomite and other soluble rocks, which provide a material basis for karst. These rocks have been subjected to long-term dissolution, forming an abundance of underground rivers, karst caves and sinkholes.

During the Yanshan and Himalayan movements, the Wulong area experienced multiple tectonic uplifts, especially the rapid uplift of the crust in the Three Gorges area of the Yangtze River since the Neogene, which led to a relative decline in the water level of the Wujiang River and its tributaries, and the joint action of surface water and groundwater aggravated the deepening of karst landforms. This tectonic uplift formed a multi-level plane in the region (such as the plane of the Dalou Mountain and the plane of the Shanyuan period), and laid the foundation of the modern canyon landform.

The early crust of the Daloushan Yiping stage was stable, and the regional erosion datum was low, forming a broad karst Yiping plane, and the surface was dominated by shallow hills and depressions, and there was no deep valley landform. With the intermittent uplift of the earth's crust in the Shanyuan period, the Wujiang River and its tributaries began to cut downward, the underground river gradually developed, and the combined dissolution of surface water and groundwater was enhanced, forming early karst caves and subterranean caves.

Since the Quaternary period, the rapid uplift of the earth's crust has led to the violent downcutting of the Wujiang River and its tributaries (such as the Yangshui River), and the roof of the underground river has collapsed to form a sinkhole, while the rock mass that has not collapsed has been retained as a Tiansheng Bridge. For example, the Tianlong Bridge, Qinglong Bridge, and Heilong Bridge in the Tiansheng Three Bridges are natural stone bridges left after the collapse of the underground dark river. As a regional erosion datum, the Wujiang River has changed the flow direction of the groundwater system through river raiding, which has accelerated the deepening and expansion of the modern canyon. This process led to the evolution of the Tiansheng Sanqiao area from a surface river to a dry valley, retaining only seasonal water flows and spring feeding.

The Morphology and Characteristics of the Three Bridges of Wulong, China (1000 words)

As an outstanding representative of karst landforms, Wulong Tiansheng Three Bridges show a unique and spectacular combination of surface forms. This landform system is composed of three giant natural stone bridges, Tianlong Bridge, Qinglong Bridge and Heilong Bridge, which are arranged in a northwest-southeast beaded pattern, and are concentrated in an area of less than 1 square kilometer, forming the world's largest Tiansheng Bridge Group. Among them, the Tianlong Bridge is 235 meters high, the arch height is 96 meters, the span is 34 meters, and the bridge thickness is 107 meters, which is the largest in the world. The three Tiansheng Bridges are connected by two giant sinkholes, forming a unique spatial pattern of "three bridges sandwiched by two pits", which is a compact and orderly combination of landforms that is extremely rare in global karst research.

From the microscopic point of view, all parts of Tianshengqiao retain rich karst morphological characteristics. Collapsed breccia accumulations are generally developed at the top of the bridge arch, and the volume of a single rock block can reach hundreds of cubic meters, which records the historical process of the collapse of the cave roof. There are multiple layers of horizontal dissolution side grooves on the side wall of the bridge, and the highest side channel is more than 80 meters higher than the modern riverbed, reflecting the phased decline of the ancient groundwater level. At the base of the pier, densely distributed vortex cavities and wave marks show the strong scouring of water currents in the past. Of particular note is the fact that the complete sequence of cave chemical sediments on the piers on the north side of Qinglong Bridge has been preserved, including a 15-meter-high giant stone waterfall and a group of stalagmites that are still growing, and the isotopic records of these sediments provide valuable information for the reconstruction of the paleoclimatic environment.

The geomorphological system in the Tiansheng Three Bridges area has typical three-dimensional stratification characteristics. The uppermost layer is a well-preserved primitive plateau, with an altitude of 1200-1300 meters, distributed with normal karst peaks and depressions; The middle layer is a transition zone composed of Tiansheng Bridge and Tiankeng, with an altitude of 900-1100 meters, which is a key section of the transformation from underground river to surface river. The lower level is a modern canyon system, with the valley floor only 500-600 meters above sea level. This "three-layer structure" clearly shows the complete evolution sequence of karst landforms from underground to surface. Modern measurements show that the vertical dissolution rate in the Tianshengqiao area is about 0.1 mm/a, while the horizontal dissolution rate can reach 0.3 mm/a, and this differential erosion leads to the continuous change of bridge morphology.

The hydrological system plays a decisive role in shaping the morphology of the Tiansheng Three Bridges. The surface and underground dual hydrological network is well developed in the area: the surface water infiltrates rapidly through dense karst gaps and forms a pressure pipeline flow in the deep part; The instantaneous flow at the outlet of the underground river can reach 20 m³/s, and this intense hydrodynamic condition accelerates the expansion of the cave system. Tracer tests in recent years have confirmed that groundwater migration in limestone can reach speeds of up to 500 m/h, and this rapid water exchange allows the dissolution to continue. Of particular interest is the discovery of a "seam-like" canyon at the bottom of the sinkhole between the three bridges, less than 10 meters wide but more than 150 meters deep, an extremely narrow geomorphology typical of the rapid undercutting of karst areas.

The rock type and formation process of Wulong Tiansheng Three Bridges

The geological foundation of Wulong Tiansheng Three Bridges is mainly composed of thick-bedded limestone of the Lower Ordovician Baota Formation, which was formed in a shallow marine environment about 480 million years ago. Lithological studies have shown that the limestone has extremely high chemical purity, with CaCO₃ content generally exceeding 96% and insoluble matter content of less than 2%, which provides a material basis for strong karstification. Under the microscope, the rocks are mainly microcrystalline-bright crystalline structure, with a biodetrital content of about 15-20%, mainly sea lily stems and brachiopod fragments. It is particularly noteworthy that three groups of dominant joint systems are developed in the rock formation: trending N30°E, N60°E and near-EW direction, with joint densities reaching 5-8 joints/m, and these tectonic weak surfaces provide a dominant channel for later dissolution.

From the perspective of rock mechanical properties, the compressive strength of the limestone of the Baota Formation is between 80-120MPa, which is a medium-strength rock. However, the intensive development of joints makes the overall strength of the rock mass significantly reduced, and the RQD (rock quality index) value is only 40-60. This "strong on the outside and weak on the inside" explains why the rock mass is able to maintain a large-scale arch structure without overall collapse under strong dissolution. X-ray diffraction analysis showed that the limestone contained about 0.5-1% of the secondary calcite veins, and these later backfills were spatially unevenly distributed, resulting in local differences in the rock's anti-dissolution ability, which was an important reason for the formation of complex dissolution morphology on the bridge surface.

The formation process of the natural three bridges is a complex process of multi-stage evolution. Before the Eocene (about 50 Ma), the area was in a stable shallow burial environment, and only the initial fracture network developed. With the regional tectonic uplift caused by the uplift of the Tibetan Plateau, the groundwater discharge datum began to decline continuously. During the Oligocene-Miocene (30-10 Ma), atmospheric precipitation infiltrated along the joints and gradually expanded the initial fractures through dissolution. The chemical reaction of this process can be expressed as: CaCO₃+H₂O+CO₂→Ca²⁺+2HCO₃⁻. With the expansion of the cave system, underground rivers are formed locally, which can reach a flow rate of 0.5-1 m/s, and this strong hydrodynamic condition accelerates the dissolution process.

Since the Quaternary period (about 2.6 Ma), the change of precipitation pattern caused by climate change has led to the impulse development of karst process. In the wet stage of interglacial period, the dissolution rate can reach 0.15mm/a. In the arid phase of the glacial age, mechanical erosion was the main factor. About 1.2 Ma ago, the roof of the first large cave hall (Tianryu Cave) collapsed, forming the initial skylight. Subsequent collapse events (mainly at 0.8 Ma and 0.3 Ma) eventually shaped the current three-bridge pattern. The uranium dating data show that the dissolution traces on the surface of the existing bridge are mainly formed in the last 200,000 years, indicating that the geomorphological evolution is still ongoing.

Modern monitoring has found that the bridge is undergoing a differential weathering process. The weathering rate of the south side of the bridge wall with strong sunlight (0.08 mm/a) is 2-3 times that of the north side of the shady side (0.03 mm/a). Microseismic monitoring also recorded the existence of micro-fracture propagation in the bridge, and the frequency was concentrated in the range of 5-15Hz, which indicates that the geomorphological system is still evolving. It is particularly noteworthy that during the rainstorm season, the underground river flow can increase dozens of times in a few hours, and the effect of this transient water pressure on the stability of caves has become the focus of current research.

The scientific value of the karst landform of the natural three bridges in Wulong, Chongqing

Located in the lower reaches of the Wujiang River on the southeastern edge of Chongqing, Wulong Tiansheng Third Bridge is a typical representative of karst landforms in southern China, and its scientific value is reflected in geological evolution, ecological diversity, hydrological processes, and cultural heritage, providing an important model for global karst research57.

The core value of Wulong karst landform lies in its complete geological evolution sequence. The area is dominated by Permian and Triassic carbonate rocks, including limestone, dolomite and other soluble rocks, which provide a material basis for karstism68. After the intermittent uplift of the earth's crust since the Neogene, the Wujiang River and its tributary Yangshui River cut down violently, forming a deep-cut canyon and subterranean cave system78. The development mechanism of the Tiansheng Three Bridges is particularly special: the dissolved rock layers of the underground dark river cause the roof to collapse to form a sinkhole, while the non-collapsed rock mass is retained as the Tiansheng Bridge, a "collapse-remnant" pattern that is extremely rare in karst landforms around the world68. Its span and height are the largest in Asia, demonstrating the symbiotic relationship between bridges and sinkholes in karst landforms67.

The Wulong karst landform contains three independent systems: the Furong Cave System, the Tiansheng Three Bridges System, and the Houping Tiankeng System, which represent different stages of karst evolution57. This multi-type and multi-stage karst assemblage is extremely rare in the world, which provides a natural laboratory for studying the temporal and spatial evolution of karst landforms. In addition, as a representative of the subtropical monsoon karst, it contrasts sharply with the karst of the Mediterranean climate zone, highlighting the influence of climate on the development rate and morphology of karst58.

The ecological and hydrological values of the Wulong karst system are also significant. The karst aquifer supports a rich subsurface biodiversity, with more than 30 species of cave animals found in the Hibiscus Cave alone, including the highly adaptable blind shrimp and blind spider7. At the same time, the linkage effect between surface and underground river system is significant: the raiding effect of the Wujiang River changes the flow direction of the groundwater system, while the dry valley of the Yangshui River reveals the dynamic equilibrium of the alternating effect between surface water and groundwater8. This coupling relationship between ecology and hydrology is of great significance for exploring the stability of fragile ecosystems in karst areas.

The cultural heritage value of Wulong Tiansheng Three Bridges cannot be ignored. As a World Natural Heritage Site, its conservation model provides a reference for similar areas around the world, especially how to maintain the vulnerability of karst systems while developing tourism, and become a practical example of harmonious coexistence between man and nature78.

The river erosion of the Grand Canyon in the United States

The tectonic setting of river erosion in the Grand Canyon of the United States

Located on the Colorado Plateau in northwestern Arizona, the Grand Canyon was formed by the continuous erosion of the Colorado River and is one of the most typical river-eroded landforms on Earth. Its tectonic background and geological evolution process have the following characteristics:

The rock formations of the Grand Canyon record the geological history of the earth for about 2 billion years, and are divided into three parts: the inner gorge, the outer gorge and the plateau from the bottom to the top. The inner isthmus is composed of Precambrian metamorphic rocks and intrusive rocks, the outer isthmus is dominated by horizontally deposited Paleozoic to Mesozoic sandstone and shale, and the top layer is Cenozoic volcanic rocks and limestones. This multi-layered structure reflects changes in the sedimentary environment over different geological periods.

The area where the Grand Canyon is located was originally a flat Colorado plateau at an altitude of about 2,000 meters. During the Late Cretaceous, 70 million years ago, the regional crust began to uplift, and with the formation of the Cordillera mountain system, the plateau was gradually uplifted and tilted, causing the Colorado River to accelerate downward. This synergy between tectonic uplift and river erosion makes the Grand Canyon a "geological history book".

The long-term erosion of the Colorado River, reluctant to flow forward day and night, sometimes open the mountain and split the road, sometimes let the way back, in the upper reaches of the main stream and tributaries have carved out 19 canyons such as Black Canyon, Canyonland, Glen Canyon, Bruce Canyon, etc., and finally flow through the rocky Kaibab Plateau in Arizona, there is an amazing stroke, forming this Grand Canyon wonder, and becoming the "king of canyons" in all the canyons of this water system.

Due to differences in the erosion resistance of rock formations, soft rock layers (such as shale) are rapidly eroded, while hard rock layers (such as sandstone) form cliffs or platforms. For example, the red sandstone of the Grand Canyon, such as the Cocono Sandstone, is colored by the presence of iron oxide, which has been weathered and washed by currents to form steep cliffs and horizontal textures.

In conclusion, the Wulong Tiansheng Three Bridges and the Grand Canyon of the United States take karst and river erosion as the core, respectively, showing the geomorphological wonders under the coupling of internal and external dynamics of the earth. The former is the product of crustal uplift and dissolution collapse, and the latter is a masterpiece of tectonic movements and river downcutting, both of which together reveal the shaping process of natural forces on geological time scales.

Morphology and Characteristics of the Grand Canyon of Colorado (1000 words)

The Grand Canyon is a global example of river-eroded landscapes for its unparalleled size and grandeur. The main body of the canyon is trending northwest to southeast, with a total length of 446 kilometers, an average width of 16 kilometers, and a maximum depth of 1,830 meters, forming a large-scale surface rift. From the perspective of spatial pattern, the canyon presents typical segmentation characteristics: the upper section (above Lee's Ferry) is dominated by V-shaped valleys, the middle section (Grand Canyon National Park Area) develops into a stepped box valley, and the downstream section (near Lake Mead) gradually transitions to an open U-shaped valley. This longitudinal variation reflects the basic law of decreasing river erosion dynamics.

The most striking feature of the canyon cross-section is its stepped morphological structure. The differential erosion resistance of the lithology gives rise to this unique landscape: the hard Coconino sandstone forms vertical cliffs 50-100 m high, while the soft Helm shale develops into gently sloping platforms 200-300 m wide. Particularly spectacular are the "Red Wall" limestone cliffs, a 180-metre-thick layer of solid rock that stretches continuously on both sides of the canyon and forms the most striking landform layer. According to statistics, there are 12 distinct terraces on the north wall and 9 terraces on the south wall, and the elevation difference between 30 and 150 meters is recorded on the north wall of the canyon.

The planar morphology of the Grand Canyon exhibits complex spatial variations. In the lithologically homogeneous section, the river channel is relatively straight and the valley wall is symmetrical. In the lithological change zone, the river swings strongly, forming the famous meandering group. The most emblematic is Horseshoe Bend, where the river meanders to a staggering 10:1 to width, with a meandering neck width of only 150 meters and a meandering wavelength of more than 1.5 kilometers. There are also a large number of tributary hanging valleys in the canyon, and the highest hanging valley outlet is more than 300 meters above the main river bed, forming a spectacular group of waterfalls, such as the 180-meter-high Havasu Falls. The presence of these overhangs proves that the tributary downcut rate is much smaller than that of the main stream.

The multi-layered color of the canyon is its most striking exterior feature. The strata of different eras show a stark color contrast: the Precambrian Veshnu schist is dark gray, the Cambrian Tapitz sandstone is creamy white, the Devonian redwall limestone is iron-red, and the Permian Koconino sandstone is light yellow. This "rainbow formation" effect creates a striking visual impact when the sun shines on it. Of particular importance is the "Great Unconformity" that emerges in the canyon, the meeting that separates the 1.8-billion-year-old basement from the 500-million-year-old sedimentary rocks, creating a spectacular geological cross-section that geologists have called "the greatest unconformity on Earth".

Modern observations show that the Colorado River carries an average of 5×10⁷ tons of sediment per year, with 90 percent of the erosion occurring during brief flood periods. Laser ranging monitoring shows that the current downcutting rate of the canyon is about 0.1-0.3 mm/a, and the lateral widening rate is about 0.5-1 mm/a. It is worth noting that the erosion process of the canyon shows significant spatial differences: in the hard rock section, the downcut rate is only 0.05 mm/a; In the weak rock layer, the local downcut rate can reach 2mm/a. This differential erosion leads to a continuous and complex change in the canyon morphology.

Rock types and formation of the Grand Canyon

The Grand Canyon of Colorado exposes a complete sequence of rocks from the Proterozoic to the Cenozoic, which can be called a "geological history textbook". The oldest basement is the 1.8 billion-year-old Vishnu schist, a set of plutonic rocks that have undergone amphibolite facies metamorphism, with a predominantly mineral assemblage of quartz-feldspar-biotite. The unconformity overlies the sandstones and shales of the Grand Canyon Supergroup (1.2 billion years old), which recorded the Rodinia supercontinent cracking event. The Paleozoic sedimentary cover includes the Cambrian Tapitz Sandstone (quartz content > 95%), the Devonian Redwall Limestone (180m thick) and the Permian Coconino Sandstone (crossbedded development).

Petrographic analysis revealed that these sedimentary rocks have typical primary sedimentary structures. The well-preserved wave marks and cross-bedding in the Tupitz sandstone suggest that it was formed in a shallow marine environment; The red-walled limestone is rich in fossilized coral and foraminifera, suggesting warm land-surface marine deposits; The large wind-swept cross-bedding of the Coconino Sandstone (dip up to 25°) records the desert environment. Of particular importance is the significant difference in weathering resistance between rock formations: the weathering rate of quartz sandstone is only 0.01 mm/ka, while that of shale can reach 1 mm/ka, which paves the way for the formation of the Grand Canyon's stepped landform.

The formation of the Grand Canyon began during the late Cretaceous period (about 80 Ma) during the Laramie orogeny. With the subduction of the Farallon plate, the entire Colorado Plateau has been uplifted regionally, with a cumulative amplitude of more than 2,000 meters. During the Eocene epoch (50 Ma), the original Colorado River was initially established. However, the latest fission track study suggests that the history of the undercut in some sections of the canyon may be much earlier, starting before 70 Ma. This long-term erosion process is controlled by a number of factors:

Lithological differences are the most critical control factor. In the hard sandstone section, the river is dominated by incision, forming a narrow V-shaped valley; In the weak shale section, lateral erosion is dominant, forming a wide valley. Climate fluctuations also play an important role: during wet periods (e.g., the Miocene climate optimum), increased runoff leads to increased erosion; In arid periods (e.g., the Pleistocene glacial), physical weathering predominates. The influence of tectonic activity is phased: during the period of strong uplift (e.g., 10-5 Ma), the downcut of the river accelerates; In the stable period, lateral erosion is dominant.

Modern observational data show that the erosion of the Colorado River is extremely uneven. Ninety per cent of the erosion occurs during flood periods, which account for only 5 per cent of the year, and instantaneous erosion rates can be up to 100 times the norm. Lidar monitoring shows that the current downcut rate of the canyon is 0.15mm/a on average, but it can reach 0.5mm/a in the weak rock layer. It is worth noting that 30-50% of the sediment transported by rivers comes from tributary erosion, and this "lateral sediment supply" mechanism has an important impact on the morphological evolution of the main valley.

The evolution of the Grand Canyon is also controlled by datum changes. The opening of the Gulf of California during the Pliocene (5 Ma) caused a sudden drop of about 1000 m in the erosion datum, triggering a new round of strong undercutting. The construction of the modern Glen Canyon Dam (1963) completely changed the relationship between water and sediment downstream, causing a major transformation of the erosion dynamics system in the Grand Canyon due to human intervention. This interaction of natural and human factors makes the Grand Canyon an excellent case study of the response of river systems.

The scientific value of river erosion landforms in the Grand Canyon in the United States

Located on the Colorado Plateau in northwestern Arizona, the Grand Canyon is one of the most typical river erosion landforms on Earth, formed by the continuous erosion of the Colorado River. Its scientific value spans the study of geology, planetary science, ecology and earth history, and is known as a "living geological history textbook".

The rock formations of the Grand Canyon record about 2 billion years of the Earth's geological history, from the Precambrian metamorphic rocks at the bottom to the Cenozoic volcanic rocks at the top, and have completely preserved traces of crustal movements, sedimentation, and erosion processes. Among them, the ancient crystalline rocks of the inner isthmus reveal the early high-temperature and high-pressure environment of the earth's crust, and the horizontal sedimentary rock layers of the outer isthmus record the marine and continental sedimentary cycles from the Paleozoic to the Mesozoic. This hierarchical structure provides an intuitive basis for studying the Earth's plate tectonics, climate change, and life evolution, like the "earth's growth rings".

The Grand Canyon was formed as a result of millions of years of erosion by the Colorado River. The river differentially erodes the rock formations of varying hardness and softness, shaping the canyon's stepped cliffs and horizontal textures. This process reveals four key mechanisms of river erosion: the potential energy difference provided by crustal uplift, the alternation of river lateral erosion and downcutting, the stepped cliffs formed by differential erosion of soft and hard rock layers, and the acceleration of erosion rates by climate change (e.g., glacial meltwater). The width of the Grand Canyon ranges from 6 km to 25 km at its narrowest, showing the complete evolutionary lineage of the river from vertical downcut to lateral widening, providing an ideal model for the intersection of fluid mechanics and geomorphology.

The landscape of the Grand Canyon is unique, and from a geoscience perspective, the formation of the Grand Canyon is closely related to the tectonic movement of the Earth's tectonic plates. The geological history and geomorphological characteristics of the Grand Canyon are important research objects in earth science. Through the study of the Grand Canyon, it is possible to better understand the evolution of the earth and the role of natural forces. The Grand Canyon is not only a spectacular sight, but also an abundance of wildlife. There are more than 200 species of birds, 60 species of mammals and 15 species of reptiles and amphibians, and the area between the Faton Ranch at the bottom of the valley and the São Francisco Peak, which is more than 90 kilometers away and about 3,500 meters high, is a growing area for both subtropical and boreal plants. Therefore, cacti, poppy, spruce, fir and other plants here are almost symbiotic in the same area.

The shape and color of the Grand Canyon is a treasure trove of Earth system science. Its red sandstone is colored by iron oxide, which has been weathered and washed by water to form steep cliffs and horizontal textures. The peculiar shape is mainly due to the different speed of erosion on rocks of different textures, and the rich color of the canyon is caused by the small amount of various minerals it contains, and the iron-rich rocks are red or reddish-brown. This geomorphological feature not only provides a case study for erosion dynamics, but also serves as the object of planetary scientific comparison. For example, the study of the river characteristics of the Kobler Trench on Mars is similar to that of the Grand Canyon, and its study can help to understand the mechanism of action of liquid water on the planet's surface.

A comparative study of Wulong's Tiansheng Three Bridges and the Grand Canyon in the United States: similarities and differences between karst and river erosion landforms

1. Similarities: geomorphological evolution under the coupling of internal and external forces

Although the origins of the Wulong Tiansheng Three Bridges and the Grand Canyon are very different, they are both the products of the synergy of internal and external forces on the earth. Both of them have undergone a long geological evolution, based on internal uplift and dominated by external erosion, forming a natural landscape with great visual impact.

From the perspective of the Earth system, both record the trajectory of the Earth's crustal movements. Wulong Karst is affected by the tectonic uplift of the Three Gorges area of the Yangtze River since the Neogene, and the intermittent rise of the earth's crust leads to the downcutting of the Wujiang River, triggering dissolution and subsidence. The Grand Canyon is coupled to river erosion due to the uplift of the Colorado Plateau, and the Colorado River continues to cut down through sedimentary rock layers, exposing deep basements. This model of "tectonic uplift-erosion" makes both places a natural textbook for revealing geodynamic processes.

In addition, both are in a dynamic evolution. The Tiansheng Bridge Group in Wulong is still affected by the attack of the Wujiang River, and the canyon continues to deepen. The Grand Canyon is due to river erosion and climate change, and the cliff wall continues to collapse and retreat. This unfinished geomorphological evolution provides a vivid case for studying the processes on the earth's surface.

2. Comparison of core characteristics: dissolution and mechanical differences of river erosion

Causing mechanism and dominant external force
Wulong Tiansheng Three Bridges is a typical product of karst action. The core driving force is the chemical dissolution and mechanical erosion of soluble rocks (limestone, dolomite) by groundwater. The dissolved rock layer of the underground dark river causes the roof to collapse to form a sinkhole, and the residual rock mass constitutes a natural bridge, forming a unique structure of "collapse and residue". This process is centered on the dissolution capacity of water, which is manifested in the gradual hollowing out of rock layers along the fissures, and finally forming linear canyons and solitary peaks.

The Grand Canyon is dominated by mechanical erosion by rivers. The Colorado River carries large amounts of sediment, which are washed by eddies, abrasion, and lateral erosion, cutting alternating hard and soft sedimentary rock layers into stepped cliffs. The erosion energy is derived from the flow energy of the river and is manifested by the rapid stripping of soft rock layers (such as shale) and the slow grinding of hard sandstone. For example, the red sandstone layer resists weathering due to the presence of iron oxide, forming steep cliffs; Shale layers are susceptible to erosion, creating gentle slopes or plateaus.

Geomorphological and structural characteristics
The Wulong karst landform is dominated by negative terrain, including sinkholes, ground cracks, karst caves and Tiansheng Bridge. Its structure is complex, and it often presents a combination of "bridge-pit-hole" trinity. For example, the Tiansheng Three Bridges are connected by the Tianlong Bridge, the Qinglong Bridge, and the Black Dragon Bridge, each spanning a deep-cut canyon, and a funnel-type sinkhole and underground river system are developed under the bridges6. The core feature of this landform is vertical zoning: canyons and sinkholes on the surface, and a vast network of caves and dark river channels underground.

The Grand Canyon is a linear deep-cut canyon with V-shaped valleys, terraces and cliffs. Its cross-section is a multi-layered superimposed structure, from the bottom Precambrian metamorphic rocks to the top Cenozoic volcanic rocks, which completely record the sedimentary history of 2 billion years. The two walls of the canyon are clearly layered by the difference in the erosion resistance of the rock layers: the hard cocono sandstone forms the platform, and the soft shale is eroded into grooves. Alluvial fans and floodplains are occasionally found at the bottom of the valley, but the overall erosion landform is absolutely dominant.

Hydrology and climate response
The hydrological system of Wulong Karst is dominated by underground rivers. The Wujiang River and its tributary, the Yangshui River, change the direction of groundwater flow through raiding, forming a pattern of coexistence of surface dry valley and underground subterranean flow. Its ecology depends on the seasonal regulation of karst aquifers, the dry season depends on spring water recharge, and the rainy season is prone to flooding into karst caves. Heavy precipitation brought by the subtropical monsoon climate accelerates the dissolution, but also leads to system fragility, and the surface is susceptible to rocky desertification caused by vegetation destruction.

The hydrological processes of the Grand Canyon are entirely dependent on surface runoff. The erosion energy of the Colorado River fluctuates significantly with the seasons: spring snowmelt leads to a surge in flow, which exacerbates lateral erosion; Torrential rains in the summer cause flooding, carrying gravel to abrasive cliff walls. In semi-arid climates, water is scarce, but concentrated heavy rains and meltwater from ice and snow are the main causes of erosion. Under this climatic background, the vegetation in the canyon is sparse, mainly drought-resistant shrubs and herbaceous plants, and the ecological resilience is weak.

Geological record and scientific value
The scientific significance of Wulong's natural three bridges lies in the completeness of karst evolution. It records the whole process from the development of the underground river to the collapse of the sinkhole and then to the remnants of the Tiansheng Bridge, which provides chain evidence for the study of the "life cycle" of karst landforms. In addition, as a representative of subtropical karst, it contrasts with the Mediterranean region of Europe and Southeast Asia, highlighting the influence of climate on the rate of dissolution.

The scientific value of the Grand Canyon is reflected in the continuity of Earth's history. Its rock formations are like "geological history books", recording the complete history of the North American continent from the ancient igneous rock basement to the Paleozoic sedimentary environment, the Mesozoic dinosaur era and the Cenozoic climate change. The fossil layers (e.g., trilobite and fern imprints) and tectonic remains (e.g., folded rock formations) in the canyon provide a key basis for the study of plate tectonics, biological evolution and paleoclimate.

Conclusion: The dual narrative of the power of the earth and the universal value of natural heritage

Every fold and crack on the earth's surface is an epic written by the forces of nature. Chongqing Wulong Tiansheng Three Bridges and the Grand Canyon of the United States, as outstanding representatives of karst landforms and river erosion landforms, not only share the universal laws of geodynamics, but also interpret the diversity of natural laws in a completely different way. The comparison between them is not only the difference in geological genesis, but also the multiple symphonies of energy distribution, climate action and time scale of the earth system, which ultimately point to the deep understanding and responsibility of human beings for natural heritage.

Both were born in the grand context of the movement of the earth's crust. Due to the intermittent uplift of the Three Gorges area of the Yangtze River since the Neogene, the Wulong Karst has led to the dissolution and collapse of the Wujiang River, forming a negative topographic combination with Tiansheng Bridge and Tiankeng as the core； The Grand Canyon provides a potential energy base for river erosion due to the continuous uplift of the Colorado Plateau, which with astonishing patience cuts through sedimentary rock formations and exposes geological archives buried deep underground. This coupling model of "tectonic uplift and external erosion" reveals the core dynamics of the evolution of the earth's surface: internal forces create the initial conditions for terrain undulation, while external forces such as rain and rivers reshape the surface shape through chemical or physical processes. The dynamics of the two show that even the seemingly stable natural wonders are still in a slow but irreversible evolution, such as the continuous deepening of the Wulong Canyon and the collapse and retreat of the cliff wall of the Grand Canyon, both of which are concrete expressions of the vitality of the earth.

Although the evolutionary frameworks are similar, the process of formation of the two is fundamentally different. The Wulong Born Three Bridges is a masterpiece of water-rock chemistry. The dissolution of carbonate rocks (limestone, dolomite) by groundwater gradually hollows out the rock layer along the fractures, and eventually leads to the collapse of the roof to form a sinkhole, and the residual rock mass forms a natural bridge. Its landform is characterized by the trinity of "bridge-pit-hole", with underground rivers and cave networks forming a vertical zoning structure, and the surface of the land is represented by linear canyons and solitary peak clusters. Heavy precipitation from a humid climate accelerates the rate of dissolution, but it also leads to the fragility of ecosystems – the destruction of surface vegetation is prone to rocky desertification.

In contrast, the Grand Canyon is a model of mechanical erosion of rivers. The Colorado River carries large amounts of sediment, which are carved into stepped cliffs through eddy currents, abrasion, and lateral erosion. The topography is dominated by V-shaped valleys, terraces and cliffs, and the differences in the erosion resistance of the rock strata form clear layers: hard red sandstone (such as the Cocono sandstone) forms steep cliffs, and shale is eroded into gentle slopes or platforms. Seasonal fluctuations in river energy (snowmelt in spring and heavy rainfall in summer) and concentrated rainfall in semi-arid climates make erosion pulse-like. The mechanism of this landscape formation is based on the direct action of physical forces, and the game between rock hardness and river flow energy shapes the macroscopic shape of the canyon.

The scientific value of Wulong and the Grand Canyon lies in the fact that they respectively preserve the "memory" of the earth at different spatial and temporal scales. The Wulong Karst records the complete cycle of karst landform from development to decline under subtropical monsoon climate, and its "collapse-remnant" structure provides chain evidence for the study of karst life cycle. As a representative of the karst in southern China, it contrasts with the Mediterranean and tropical karsts, highlighting the influence of climate on the dissolution rate, and becoming a "laboratory" to reveal the details of regional geomorphological evolution.

The Grand Canyon carries the overall narrative of Earth's history. Its rock strata range from the Precambrian metamorphic rocks at the bottom to the Cenozoic volcanic rocks at the top, which has preserved a sedimentary history of 2 billion years, and the fossil layers (such as trilobite and fern imprints) and tectonic relics (such as folded rock formations) together write an epic of plate movement, biological evolution and paleoenvironmental changes. This continuity across geological time makes it a "textbook" for interpreting the evolution of the Earth system. Together, they expand the boundaries of human perception of nature: Wulong focuses on the detailed study of regional landforms, while the Grand Canyon provides a framework for geological events on a global scale.

As World Natural Heritage sites, the conservation process of the two places reflects the deepening of human awareness of the value of nature. Wulong, which was once caused by soil erosion due to logging and farming, was later declared a World Heritage Site (2007) to achieve a balance between conservation and development, and the cultural value of its film and television locations (such as "Transformers 4") further promoted ecotourism. The Grand Canyon fell into an ecological crisis due to overgrazing and tourism development, and was later restored through the national park system (1919) and scientific management, becoming a benchmark for global ecological protection. The practice of both shows that the excavation of cultural values can provide economic impetus for conservation, and scientific understanding is the premise for formulating strategies. More importantly, they serve as benchmarks for natural heritage, contributing to the global consensus on the conservation of geomorphological diversity – whether it is the fragile network of subterranean karsts or the magnificent systems of surface river canyons, which are witnesses to the non-renewable evolution of the planet.

The contrast between Wulong and the Grand Canyon ultimately points to the universal value of natural heritage. The former illustrates the art of geochemistry through hidden underground dissolution, and the latter illustrates the epic of physical erosion with majestic river cutting, both of which together illustrate the creativity and destructiveness of natural forces. They are not only the product of regional geological evolution, but also the "laboratory" of earth system research, with karst revealing the microscopic mechanisms of water-rock interactions, and river canyons interpreting macroscopic narratives of plate movement and climate change. At a time of climate change and intensifying human activities, these landscapes are not only early warning devices for fragile ecosystems, but also textbooks for awakening ecological ethics. Protecting them is not only to protect the historical archives of the planet, but also to preserve the key to deciphering the codes of nature for future generations. From the Tiansheng Bridge in Wulong to the cliffs of the Grand Canyon, the earth tells the same truth in different languages: human beings and nature are not opposites, but partners in writing the epic of life.