Landform Recognition in Granite Mountains in East Asia (Seoraksan, Republic of Korea, and Huangshan and Sanqingshan, China) – A Contribution of Geomorphology to the UNESCO World Heritage

Huangshan (Yellow Mountains) is located in the south-eastern part of Anhui Province, along the water divide between yangtze river in the north and Qiantang river in the south. Granites of Cretaceous age, intruded in several phases from 132 to 124 Ma ago, occupy an area of 107 km², including the central, most elevated part of Huangshan where four peaks exceed 1800 m a.s.l. (Cui et al. 2009). The gross morphology of Huangshan is one of a dissected structural dome, subject to strong uplift and related erosion during the younger Cenozoic. Radial drainage is a characteristic feature of the local river network.

As a consequence of uplift pattern and progress of headward erosion, superimposed on lithological variation within the granite intrusion, a concentric arrangement of first-order landforms has developed (Cui et al. 2009). The core area is a high-altitude surface of low relief up to 200 m, with broad troughs and several major domes at its peripheries (Fig. 2A). This conspicuous geomorphic surface is interpreted as a relict surface from pre-uplift phase of relief evolution and provisionally dated for 30 Ma (Huang et al. 1999). It has not yet been reached by headward erosion and several major knickpoints typify the edge part. Moving beyond the edge of the central area local relief increases to hundreds of metres and isolated peaks with precipitous rock slopes dominate the morphology (Fig. 2B). Cui et al. (2009) claim that steep-sided domes gradually give way to castellated peaks and then pillars and pointed peaks, but quantitative evidence of such a transition is yet to be provided. The outer area represents ridge-and-valley topography, with altitudes up to 1300–1400 m and granite peaks less conspicuous. It coincides with the outcrop area of older granites, i.e. from early stages of intrusive history. At the transition to the metamorphic rock area lithologically-controlled knickpoints with waterfalls occur. In the past the presence of local glaciers in Huangshan was inferred but the purportedly glacial deposits were shown to be of mixed alluvial/colluvial origin (Helland et al. 1997).

Fig. 2

At the local scale, granite morphology of Huangshan is clearly controlled by jointing. Joint directions vary, with NNW, NNE, NE and W–E trends being most evident (Huang et al. 2002). Joint spacing in the central part and in the inner periphery varies but is generally large. In many places one can observe continuous sub-vertical joints being tens of metres apart and horizontal joints absent. In these structural circumstances, massive domes separated by deep clefts and ravines due to preferential weathering and erosion of densely jointed zones have formed. More dense joint spacing gives rise to angular towers and pillars, but they are still quite massive. Indeed, the bulky appearance of granite peaks has been shown as the distinctive feature of Huangshan in a review of Chinese granite landscapes (Chen et al. 2009).

Sanqingshan is a distinctive granite mountain massif located in the eastern part of Jiangxi Province that rises above the rather monotonous ridge-and-valley topography that has developed within the metamorphic rocks. Similar to Huangshan, granites of Sanqingshan are of Cretaceous age but nevertheless slightly younger. According to K-Ar dating the intrusion of the main body of granite occurred c. 115 Ma ago, but the stock building the highest peak of the massif, Yujing (1816 m a.s.l.), is even younger, dated for 87–97 Ma (Yin et al. 2006). Differential crustal movements in the late Cenozoic resulted in considerable uplift, at rates apparently exceeding the mean uplift rate of the host Huaiyu Mountains, hence the Chinese geologists describe this situation as uplift-on-uplift (Yin et al. 2006). The boundaries of the elevated block are made of three fault zones, trending SSW–NNE, NW–SE, and SW–NE, respectively. Together, they enclose an area of some 30 km², built not only of the granite of Sanqingshan, but also, in marginal parts, of Proterozoic limestone, Ordovician slate and limestone. Thus, Sanqingshan occupies much smaller area than Huangshan but with the similar range of altitudes between the valley floors and the highest peaks it appears steeper.

In contrast to Huangshan, Sanqingshan lacks an extensive planar summit surface and its most elevated part is essentially a chain of sharply pointed peaks (Migoń 2007) (Fig. 2C). The overall jointing pattern is different too. Massive compartments are very rare and there is paucity of horizontal fractures, whereas vertical jointing is ubiquitous. In consequence, the key landforms are closely spaced high conical peaks and pinnacles, tall vertical rock faces with infrequent intervening ledges and solitary columns separated by deep clefts and ravines (Fig. 2D). Among them is the 128 m high free-standing column of Giant Boa, claimed to be highest residual landform of this kind in the world. Some towers and pinnacles have acquired curious shapes due to long-term surface weathering and have been named after motives from Chinese mythology and legends.

Vertical zones of dense jointing, locally mylonitization, are weathered out to form deep and narrow clefts with no surface drainage which meet at passes separating the individual peaks. Regular valleys begin at an elevation of c. 1400 m a.s.l. and show V-shaped cross profiles and steep longitudinal gradients, with some residual boulders derived from rock falls and slides from the valley sides. Waterfalls occur locally at lower elevations. Chen et al. (2009) suggested that Sanqingshan represents a more evolved topography than Huangshan, with remnants of ancient planation surface completely erased, but given the prominence of vertical jointing and the limited area it is unlikely that a broad planation surface ever existed here.

Seoraksan, peaking at Daecheongbong (1708 m a.s.l.), is located in the middle of the Korean Peninsula, in the northern part of the Republic of Korea, overlooking the East Sea (Sea of Japan) (Fig. 3). It forms part of the coastal mountain range of Taebaeg which runs along the eastern coast of the Korean Peninsula in NNW–SSE direction (Jo 2000). No evident topographic boundaries of Seoraksan exist on either northern or southern side. Both towards the south and the north the mountainous terrain continues, although reaching slightly lower altitudes of 1000–1500 m a.s.l. Regional tilting of the Taebaeg range to the west accounts for the topographic asymmetry of Seoraksan too. To the west the general elevation decreases gradually, while to the east it drops sharply and the narrow strip of coastal plain (2–3 km in the south; 5–10 km in the north) separates Seoraksan and the neighbouring mountains from the sea.

Fig. 3

Seoraksan represents classic mountainous topography with narrow, often sharp-crested ridges separated by numerous deeply incised valleys as the main components (Fig. 4). In contrast to both Chinese granite mountains presented earlier, the first-order topographic feature of Seoraksan is a c. 30 km long ridge of west-east extension which forms the morphological axis of the mountain area and includes the highest peaks which exceed 1600 m a.s.l. Secondary ridges radiate towards the north whereas an east-west fault-aligned valley separates the northern and the southern part of Seoraksan, the latter being slightly lower, peaking at 1424 m a.s.l.

Fig. 4

Seoraksan is built of various igneous and metamorphic rocks which considerably differ in age and testify to different stages of geotectonic evolution of the Korean Peninsula. Three main generations of rock complexes can be distinguished, of Proterozoic/early Palaeozoic, Jurassic and Cretaceous age, respectively and granites are an important component of each (Kee et al. 2010). Proterozoic rocks are represented by gneisses, locally intercalated with quartzites and amphibolites, into which several lithological variants of granites intruded. The latter have been subject to subsequent deformation and acquired certain features of metamorphic rocks such as foliation and banding. The next generation of granites is of Jurassic age, collectively known as the Daebo Granites but it consists of a few distinct lithological variants. Zircon Pb-U ages for these granites range from 170 to 190 Ma. The youngest granites are of Cretaceous age and dated for about 88 Ma. Again, several lithological variants are present, including coarser Seoraksan granites, with porphyritic texture and locally with large, a few cm long potassium feldspar crystals, and finer Gwittaegicheong granites which form localized occurrences (stocks) within the more widespread Seoraksan granites.

Lithological diversity of Seoraksan is reflected in diverse landform inventories in different bedrock types. Among them, landform assemblages in the youngest, Cretaceous granites are the most distinctive and include characteristic medium-size and minor features. The former include: –

domes and half-domes – steep-sided elevations, often with vertical slopes in lower sections and inclined surfaces in the upper sections, giving the overall convex shape. They occur in isolation or in juxtaposition. Rock slopes of domes are the highest in the area and may exceed 300 m.

The most ubiquitous granite landforms in Seoraksan are boulders, i.e. monolithic compartments 1–10 m long, scattered on slopes and in valley floors. They have more than one origin. Some are derived from rock fall from rock slopes, while others are excavated from deeply weathered rock mass. Minor features due to surface weathering present on outcrops of Seoraksan granites are weathering pits, runnels and flutes. However, they are neither as common nor as large as similar features of this kind present in other granite terrains (see Twidale 1982, Migoń 2006).

Morphology of terrains underlain by Proterozoic granites and gneisses stands in stark contrast to that developed upon the Creteceous granites (Fig. 5). High degree of jointing is probably responsible for the lack of impressive rock slopes, domes and fins which typify the younger granites. Slopes are more gentle, although still considerably steep (30–40°), and rock outcrops are rather few and subordinate to the general hillslope morphology (Fig. 3). In terms of shape and setting, some outcrops in water divide positions resemble classic tors known from many other granite terrains, e.g. from central and northern European countries.

Fig. 5

Finally, landform inventories on Jurassic granites in the southern part of Seoraksan occupy an intermediate position, although morphology seems closer to that developed on Proterozoic deformed granites rather than on younger, Cretaceous granites. Dense jointing favours the development of narrow columns and slender shapes of residual rock landforms and may account for the absence of domes typical for the Cretaceous granites.

The evidence of mass movements is ubiquitous in Seoraksan and the resultant landforms indicate that these gravity-driven processes are often catastrophic in nature. Two most common processes are: rock falls and debris flows. Both are favoured by geological conditions and rock properties, but the actual triggers are different and the effects on geomorphology of Seoraksan are different too.

Rock falls occur on very steep rock slopes, abundant especially in the part built of the Cretaceous granites. These rocks, subject to high tensile stresses, have developed a fracture system which consists of both primary sub-vertical joints, arranged in more or less regular pattern, and secondary joints related to erosional unloading of the rock mass, i.e. sheeting joints. The latter are broadly parallel with the topographic surface and typically steeply inclined, at an angle of 40–70°. Along these intersecting joint planes large rock compartments are separated from the rest of the rock mass and due to combined effects of size (weight), initial position high on the slope and steepness, the movement is very fast and run-out distance is high. In this way huge granite blocks fell, rolled or slid down the slopes, eventually reaching the footslopes or the valley floors. The size of detached blocks not uncommonly reaches 10 m and they are essentially monolithic, with no second-order fractures. The complementary evidence are ubiquitous scars in the rock slopes above, now seen as large overhangs, alcoves, steps and wedge-shaped hollows.

Debris flows are distinctly weather-controlled phenomena, typically initiated by heavy rains, when infiltration capacity of bedrock becomes insufficient. In contrast to rock falls, they tend to occur on regolith-covered slopes. A feature favouring debris flows is the presence of steeply inclined sheeting joints at depth, beneath the regolith. Thus, the process of movement starts with slow sliding of excessively water-bearing regolith over the sheeting plane and later, after the sliding mass reaches a ravine or headwater valley, its movement becomes constrained by topography and turns into flow due to surplus water. In Seoraksan, debris flows may travel for many kilometres, completely transforming the pre-existing morphology of valley floors. Debris flow deposits are subsequently washed out, with finer material transported further away, and larger rock compartments left as residuals. The geomorphic evidence for debris flows includes scars in the upper slopes, with bedrock exposed within otherwise forested slopes, big boulders scattered in the valley floors, lateral ridges (levees) and debris fans at the junction with a main valley. The presence of all these features under variable coverage of vegetation indicates that debris flows are persistent components of the morphological system of Seoraksan.

Seoraksan boasts a variety of fluvial landforms, many of them indicative of ongoing, fairly rapid incision into bedrock (Fig. 6). Thus, bedrock channels are abundant, especially in the headwater sections of valleys, although at many places bedrock is concealed under recent debris flow deposits. Longitudinal profiles of streams are highly irregular, with series of steps and more evident knickpoint zones, separating channel sections of different gradients. The most characteristic fluvial landforms testifying to ongoing incision are slot canyons and waterfalls. The former occur if bedrock discontinuities provide zones of preferential weakness and favour fluvial incision. They are tens or even hundreds of metres long, although in the latter case slot sections tend to alternate with short sections dominated by gravel deposition. Among impressive features of this kind is the slot canyon in the Cheonbuldong Valley. If such discontinuities are absent and bedrock is very massive, or the stream has low discharge and hence low erosional capacity, water flows over inclined bedrock slabs, often in staircase-like arrangement.

Fig. 6

Waterfalls are abundant and occur in different settings. Some formed at steps across the valley floor, accounting for highly irregular stream profiles (e.g. Daeseung Fall, Seorak Fall, Biryong Falls), whereas others occur in places where a tributary stream that carries less water joins the main river with much higher discharge and is unable to keep pace with the progress of incision in the main valley. They show variable morphology too, some being simple vertical drops of water (e.g. Daeseung Fall), whereas others are more complex and consist of a series of steps and runnels oriented at different angles. The height of waterfalls varies from a mere few metres to 88 m at Daeseung Fall. Waterfalls and bedrock channels are typically associated with potholes. Some potholes are small-scale features c. 1 m across and less than 1 m deep, but others, especially those below large waterfalls, may have c. 10 m in diameter and the depth of several metres. The massiveness of granite favours the origin and enlargement of potholes.

The most obvious evidence of cold-climate conditions are block fields and block slopes (Fig. 7). Depending on setting, the term block field is used for ridge-top and crest position, whereas block slope applies if the slope is steep (>20°) and some gravity-driven movement of blocks may have occurred. Block fields/slopes are generally products of in situ breakdown but some were supplied by rock fall-derived debris from steep rock walls occurring further upslope. There is correspondence between block fields/slopes occurrence and bedrock lithology. Very massive and poorly jointed Seorak granites do not readily give rise to block fields, whereas finer-grained and more jointed variants such as the Gwittaegicheongbong granite are more prone to block field formation. Around the peak of Gwittaegicheongbong the entire mountain crest is covered by blocks, which further downslope give way to block streams. Likewise, some metamorphic rocks support blocky accumulation. The size of blocks within block fields varies from less than 1 m to 3–4 m long, they are loosely packed and there may be large voids in between them. In many places block fields are unstable. It is difficult to ascertain the thickness of block fields from visual field observations only, but in numerous places it is at least 3–4 m.

Fig. 7

Although selected block fields in Seoraksan have been studied in the past (Park 2000, 2003), many issues and problems are yet to be resolved, particularly regarding the complex surface morphology of the block slopes and its meaning. Field observations suggest that the relief of block fields is complex and various secondary features indicative of slow gravitational movement may be identified such as convex steps, lobes, linear furrows and closed depressions. It is possible that some extensive blocky accumulations in the upper part of Chohangnyong valley may be remnants of degraded rock glaciers but this hypothesis should be verified by further research.