The Danxia landform, put forward by Chinese scholars (Chen, 1939; Huang, 1982) and named after the Danxiashan Mountain (Mt. Danxiashan) in Guangdong Province of China, is developed in the strata of red continental sandstones of the Cretaceous period, including such morphologic shapes as horizontal grooves, red cliffs, mesas, stone walls, stone peaks, stone pillars, narrow gorges, rock lanes, and rock caves (Zhu et al., 2010; Peng et al., 2013; Zhao et al., 2014). Over 1000 Danxia landform sites have been discovered in China (Huang et al., 1992; Huang, 2014) and six of them, including Danxiashan, Chishui, Taining, Langshan, Longhushan, and Jianglangshan, were inscribed as the World Heritage Sites by UNESCO in 2010, which have attracted great attention and boosted the tourism of the regions. However, research on the landscape morphogenesis of the Danxia landform has been lagged far behind the progress of tourism development in the regions (Peng, 2001; Zhu et al., 2009; Huang, 2010; Ouyang et al., 2011) and has been largely published in Chinese (Chen, 1935; Zeng, 1978; Huang, 1982; Peng, 2001; Zhu et al., 2009). The methods employed by the previous studies are basically field investigations except for identifications of lithological slices in labs (Zhang and Zhang, 1986; Bureau of Geology of Foshan City and 706 Geological Team of Bureau of Geology of Guangdong Province, 2008; Chen et al., 2010). As a type of rocky landform, the development processes of the Danxia landscapes are very slow for us to observe in our life time and the paleoclimates in which the landscapes formed are also significantly different from the present. The Mt. Danxiashan is presently humid subtropical climates but was dominated by periglacial conditions in the Quaternary glacial periods and frequently influenced by volcanic eruptions in the region in Mesozoic (Zhu, 2000; Shi et al., 1989; Zhu et al., 2012). In this study, we adopted experimental approaches to study the landscape morphogenesis by testing rock strength of the samples taken from all major types of Danxia landscape morphologies under freezing-thaw, acidic and dry-wet conditions, and by examining the lithological differences of the various Danxia landscapes. An understanding of the Danxia landscape morphogenesis is not only a contribution to the field of geomorphology but also can meet the increasing curiosity of tourists about the spectacular scenes associated with the Danxia landform.

Mt. Danxiashan is located in Renhua County of Shaoguan City in Guangdong Province (24°51′48″-25°04′12″N, 113°36′25″-113°47′53″E), covering a total area of 292 km² and comprising more than 600 peaks of the Danxia landscape and a density of stone peaks, stone fortresses, stone walls and stone pillars up to 20 per square km (Figure 1). It has an average annual precipitation of 1700 mm or so. Under warm and wet climatic conditions, subtropical evergreen broad-leaved forests cover most of the region, and subtropical monsoonal rainforest can be found growing in the ravines as well. Mt. Danxiashan is famous for its beauty and has earned the titles of world natural heritage site, global geopark, state reserve, national AAAAA scenic spot, and national geopark. The NNE Shaoguan-Renhua fault is a large regional fault passing through the middle of the Danxia Basin, dividing the region into the eastern and western basins (Figure 1). Under all kinds of internal and external forces, various spectacular Danxia landscape topographies came into view as can be seen today. Table 1 shows the stratum and lithology in the Mt. Danxiashan region.

Mechanical (compressive) experiments were performed on 40 rock samples collected from the Fengcheyan through cave, the base of Yangyuanshi stone pillar, the grooves near Tongtai bridge, the grooves of Jinshiyan Crag, and the grooves of Hunyuan Cave. The results showed that the average dry uniaxial mechanical strength of sandstone in the Fengcheyan through cave is only 10.77 MPa which is much weaker than that of Yangyuanshi stone pillar (39.86 MPa), the grooves near Tongtai bridge (29.28 MPa), as well as the grooves of Jinshiyan Crag (59.65 MPa). The average mechanical (compressive) strength of Yangyuanshi conglomerate is 63.78 MPa, which is greater than that of all other landscape sites. These results suggest that the Fengcheyan through cave developed at the sandstone layer with the lowest anti-compressive strength, while the Yangyuanshi stone pillar developed at the sandy conglomerate layer with fairly high anti-compressive strength.

The average dry and wet uniaxial mechanical strengths are 43.51 MPa and 35.47 MPa for conglomerate and 41.70 MPa and 30.30 MPa for sandstone, respectively. We can see that dry uniaxial mechanical strength is higher than wet uniaxial mechanical strength for both conglomerate and sandstone, which suggests Danxia stratum is more susceptible to erosion and weathering processes in rainy seasons or under fluvial conditions.

The testing core samples declined in compressive strength after being immersed into diluted sulphuric acid solution. The compressive strength of the sandstone in Fengchenyan through cave decreased from 10.77 MPa to only 3.71 MPa; and the Yangyuanshi sandstone, from 39.86 MPa to 26.52 MPa; the Jinshiyan Crag groove, from 59.65 MPa to 53.57 MPa. This suggests that erosion on Danxia stratum could be accelerated if acid rains were formed by the volcanic eruptions in the Mesozoic Cretaceous period.

After the testing core samples of sandstone having experienced freezing and thawing cycles in the lab, their dry and wet uniaxial mechanical strength declined as well. The average dry uniaxial mechanical strength of sandstone declined from 41.70 MPa and to 36.15 MPa, and the average wet uniaxial mechanical strength, from 30.30 MPa to 23.32 MPa. This suggests that the climate fluctuation cycles during the Quaternary glacial periods and interglacial periods might have also had influence on the resistance of rocks against weathering in Mt. Danxiashan.

The formation of Danxia landform is too slow for us to observe in our life time and the climate conditions in which the Danxia landscapes developed are significantly different from present. Thus, this study adopted experimental approaches to examine the lithological and paleoclimatic control on the formation of various landscape morphologies in the Mt. Danxiashan, South China. Through the comprehensive experimental analyses aided with field investigations, we can derive the following conclusions:

(1) The uniaxial mechanical strengths of conglomerates from all the research sites of the Danxia landscape were ranked from the highest to the lowest as follows: Yangyuanshi independent stone pillar (59.61 MPa), →groove near Tongtai natural bridge (45.47 MPa)→ Jinshiyan groove (41.54 MPa)→through cave of Fengcheyan (41.14 MPa). All the uniaxial mechanical strengths of sandstones were arranged from the largest to the smallest as follows: groove of Hunyuan Cave (69.13 MPa)→Jinshiyan groove (62.46 MPa)→Yangyuanshi stone pillar (49.30 MPa)→through cave of Fengcheyan (11.80 MPa). All the uniaxial mechanical strengths of conglomerate with resistance against freezing and thawing were arranged from the highest to the lowest as follows: Yangyuanshi stone pillar (61.10 MPa)→groove near Tongtai bridge (49.76 MPa)→through cave of Fengcheyan (42.89 MPa)→Jinshiyan groove (42.33 MPa). All the uniaxial mechanical strengths of sandstones with resistance against freezing and thawing were arranged from the highest to the lowest as follows: Jinshiyan groove (72.12 MPa)→groove of Hunyuan Cave (61.06 MPa)→through cave of Fengcheyan (48.72 MPa)→Yangyuanshi stone pillar (41.82 MPa)→groove near Tongtai bridge (28.70 MPa). As proved by the experiments, in the same area of the Danxia landscape, the dry uniaxial mechanical strengths of both sandstones and conglomerates are universally higher than the wet uniaxial mechanical strengths, which indicates that Danxia rocks tend to crack and collapse more easily in rainy and water-saturated seasons. The conglomerate of Yangyuanshi stone pillar has the highest compressive strength and resistance against freezing and thawing actions among all the research sites whereas the conglomerate and sandstone of the Fengcheyan through cave have the lowest compressive strength and resistance against freezing and thawing actions among all the research sites. Certainly, roles of microclimate and lithologic differences in the development of different Danxia landscapes should be researched further.

(2) As also indicated by the experiments, the sandstones and conglomerates in the research sites of the Danxia landscape are highly sensitive to the action of acid erosion: after being immersed in the dilute sulfuric acid solution for 25 days, the compressive strengths of almost all the rock specimens collected in the sites of landscape morphologies decreased. For example, the compressive strength of the sandstones in the groove of Hunyuan Cave was 67.08 MPa before being immersed in acid solution, and decreased to 46.29 MPa afterward; the compressive strength of the sandstone of Jinshiyan groove decreased from 59.65 MPa to 43.62 MPa; the compressive strength of the sandstones of Yangyuanshi stone pillar decreased from 39.52 MPa to 25.57 MPa; the compressive strength of the sandstones of groove near Tongtai natural bridge decreased from 29.28 MPa to 22.46 MPa; the compressive strength of the sandstone of the Fengcheyan through cave decreased from 10.77 MPa to 3.71 MPa. The experimental results suggest that the water erosion in a humid monsoon climate environment of South China would promote the development of flat grooves and caves in the Danxia landscape areas.

(3) The experimental results show that great variations exist in the resistance against freezing and thawing among the rocks from the research sites: the compressive strength of Jinshiyan groove is much lower than that of Yangyuanshi stone pillar and Fengcheyan through cave; both the wet and dry compressive strength of conglomerate is higher than that of sandstone, and the sandstone in compressive experiments showed instantaneous bursts while the conglomerate showed significant spalling; the specimens collected from the groove near Tongtai natural bridge showed no significant changes in the compressive experiments; the dry and wet compressive strength of conglomerate decreased significantly after freezing-thawing. In the Danxiashan region, frequent intense actions of freezing and thawing happened in the glacial and interglacial periods in Quaternary. Even in the present winter time with air temperature fluctuating around 0^oC, the quasi periglacial actions occur (Zhu, 1996). Accordingly, freezing and thawing actions in the region could have greatly promoted the development of flat grooves resulting from physical weathering and collapses.

(4) The differences of uniaxial mechanical strength, the ability of resistance against erosion, freezing, and thawing of Danxia rocks are closely related to the physical composition and structure of rocks. The sandstone of Fengcheyan is fine-grained sandstone with well-developed intergranular porosity and poor consolidation and cementation, and the loose sands will peel from rocks even by touching. Although there are calcareous cements in the conglomerate of Fengcheyan, gravel particles account for about 50% and only little iron filing cements are distributed at the edge of clastic particles in the form of a very thin film. The above results show that the compositional and structural defects are the fundamental reasons for why the sandstone and conglomerate in Fengcheyan through cave have the weakest compressive strength in Mt. Danxiashan. In contrast, conglomerates at Yangyuanshi stone pillar not only have calcite and ferruginous cements, but also contain polycrystal quartz, and therefore have the highest compressive strength.

(5) According to identifications of rock thin sections under a polarizing microscope as well as the XRF and ICP-MS determination and analysis, we found that the white spots and their surrounding red sandstones are highly similar in grain size, roundness, sedimentary structures, etc. Thus, the white spots must have resulted from secondary reactions within the red sandstone. As the red sandstone of the Danxia Formation is a kind of permeable rock, rainwater infiltrating from the surface into the rock can form many cylindrical seepage zones. As a result, the ions such as K⁺, Zn²⁺, Ti²⁺, Cu²⁺ and PO₄^3- in the sandstone are prone to flowing towards and being enriched in the infiltration zones. In the reduction environment, the red Fe³⁺ is reduced to Fe²⁺ and leached away, resulting in bleaching and the formation of white spots at the seepage zones. Also, the role of microbes cannot be ignored, and that are currently being studied.

(6) According to the analysis of composition and structure of honeycomb caves in Yunvlnajiang Cave, the small through caves and honeycomb caves near the Mengjueguan of Jinshiyan and Bazhai areas, the development of small through caves and honeycomb caves on precipice walls might have been related to the infiltration zones. When water moving vertically through the cylindrical infiltration zones in sandstone rocks and coming across the aquiclude layer at the bottom, it would change directions and move laterally, which eventually leads to the formation of honeycomb caves and small through caves as a result of dissolution and collapses.

(7) By integrating the field investigations with the experimental analysis, we found that the development of the large horizontal grooves in Jinshiyan, Sengmao Peak and Chahu Peak might have experienced the following processes: a large groove, mainly located within the weak sandstone layer, was developed by the lateral erosion of spiral circulations at the river bend. The later tectonic uplift raised the groove above the river. Since then, the geologic structure was stable, and the lateral erosion of river flows formed the second groove right below the first one. Such a cycle repeated again and again and eventually multiple-level grooves developed.

(8) According to the analysis of squama stone and the bedrock underneath, we believe squama stone in Jinshiyan Temple should be formed by the percolation of rainwater through the bedrock, leading to the enrichment and recrystallization of Ca²⁺ and other minerals in the shape of grids. The greenish materials on some of the squama stone surfaces are algae and other microorganisms growing under the condition of high humidity in the caves.

(9) It should be noted that the rock samples of Danxia landscape topographies in this study were mainly taken from K₂d Danxia Formation stratum except white spots which were sampled from K₁c Changba Formation. The K₂d Danxia Formation was about 1000 m thick and formed during the late Cretaceous dating back to 96 Ma to 65 Ma. This 31 Ma long period witnessed not only the dinosaur extinction event but also the frequent volcanic eruptions, acid rain events, climate changes of wet/dry and warm/cold. As a result, the stratum developed in this period in the Mt. Danxiashan region showed enormous lithological difference and experienced the Quaternary neotectonic movement and freezing-thawing alternation between glacial and interglacial stages. In a word, many internal and external forces jointly created and shaped the present unusual Danxia landscape topographies.