Granite Coasts

Although no estimate of the aggregate length of granite rock coasts around the world is available, they surely make up quite a significant proportion of the total, especially around the Fennoscandian and Canadian Shield (Bird and Schwartz, 1985). However, in contrast to the vast amount of literature about inland granite landforms, granite coastal scenery has attracted significantly less attention, in spite of the fact that some of the most spectacular coastal landscapes are supported by granite (Plate 6.1). Detailed studies of granite coastal geomorphology are surprisingly few, although the structural adjustment of the coastline in plan at the regional scale is a recurrent observation (Bird and Schwartz, 1985). One probable reason for this discrepancy between the length of granite coasts, their scenic values, and scientific knowledge are the low rates of geomorphic change expected along them. Therefore they are poor candidates for any process-oriented studies, which dominate contemporary coastal geomorphology. It is probably because of this scarcity of information that contrasting opinions have been expressed about the specifics of granite coasts. Whereas Twidale (1982: 2) asserts that: ‘In coastal contexts, too, the gross assemblage of forms is due to the processes operating there and not to properties peculiar to granites. . . . Orthogonal fracture sets also find marked expression but, with few exceptions, granite coasts are much the same as most others’; Trenhaile (1987: 173) goes on to say: ‘Igneous coasts are usually quite different from other rock coasts’. On the one hand, many granite coasts consist of an all-too-familiar assemblage of cliffs, coves, joint-aligned inlets, stacks, and sea arches. From this point of view, no components of coastal morphology are likely to be demonstrated to be unique to granite. But this is also true for granite landforms in general, as was indicated in the introduction to this book. On the other hand, there seems to be enough observational material to claim that certain granite coastal landforms have developed specific characteristics, different from those supported by other rocks, as well as that there exist certain very specific sections of granite coasts which hardly have parallels in other lithologies.

Granite Weathering

Weathering is a necessary precursor for landform development. However, in the context of granite it acquires a particular importance for various reasons. First, many granite terrains show an extensive development of deep weathering profiles, which can be extremely varied in terms of their depth, vertical zonation, degree of rock decomposition, and mineralogical and chemical change. Moreover, the transitional zone between the weathering mantle and the solid rock, for which the term ‘weathering front’ is used (Mabbutt, 1961b), may be very thin. There is now sufficient evidence that many geomorphic features of granite landscapes, including boulders, domes, and plains, have been sculpted at the solid rock/weathering mantle interface and they are essentially elements of an exposed weathering front. Therefore, the origin of granite landscapes cannot be satisfactorily explained and understood without a proper understanding of the phenomenon of deep weathering. Second, granites break down via a range of weathering mechanisms, both physical and chemical, which interact to produce an extreme diversity of small-scale surface features and minor landforms. In this respect, it is only limestones and some sandstones which show a similar wealth of weathering-related surface phenomena. Third, both superficial and deep weathering of granite act very selectively, exploiting a variety of structural and textural features, including fractures, microfractures, veins, enclaves, and textural inhomogeneities. In effect, the patterns of rock breakdown may differ very much between adjacent localities, and so the resultant landforms differ. In the context of deep weathering, selectivity is evident in significant changes of profile thickness and its properties over short distances, and in the presence of unweathered compartments (corestones) within an altered rock mass. Fourth, it is emphasized that granites are particularly sensitive to the amount of moisture in the environment (Bremer, 1971; Twidale, 1982). They alter very fast in moist environments, whereas moisture deficit enhances rock resistance and makes it very durable. Hence, a bare rock slope shedding rainwater and drying up quickly after rain will be very much immune to weathering, whereas at its foot a surplus of moisture will accelerate decomposition.

Geological Controls in the Evolution of Granite Areas

The preceding chapters have already indicated that granite properties and structures play a key role in the progress of rock weathering, the development of many medium and small-scale landforms, and in the patterns of mass movement phenomena on slopes. But the influence of geological factors sensu lato is by no means limited to these, rather restricted spatial scales. Geotectonic settings, modes of emplacement, and long-term geological histories are all relevant to the understanding of the diversity of granite landforms and landscapes. The aim of this section is show the variety of geological controls and how they are reflected in granite landscapes, moving progressively from large to small spatial scales. If plate tectonics is used as a framework, then granite intrusions form in two major settings: orogenic, including transitional, and anorogenic (see Chapter 2). Geographically, the former take place at convergent plate margins, whereas the latter take place at divergent plate margins (rift zones) and within continental interiors, at hot spots. However, for the purpose of a geomorphological approach to granite landscapes of the world, the time-independent plate tectonics framework is less useful. This is because many granite intrusions occur in settings different than those in which they formed millions of years ago, and it is their post-emplacement long-term geological history and current location that are crucial to understanding the landscapes that have developed upon them. For example, late Paleozoic granite intrusions in central and western Europe took place within the Hercynian orogenic belt, hence in a convergent plate margin setting, but their present-day morphology is mainly the legacy of long-term evolution in an anorogenic regime and late Cainozoic rejuvenation, including plateau uplift and faulting. In a somewhat similar manner, ancient orogenic granite intrusions have been incorporated into shield interiors and passive margins. Figure 8.1 is an attempt to relate the tectonic settings of granite intrusions to the distribution of granite areas, as we see them today, against the background of global tectonics. From this point of view, granite landscapes occur within the following five main geodynamic settings: (1) orogenic zones along convergent plate margins, (2) eroded and rejuvenated ancient orogenic belts, subject to geologically recent plateau uplift, (3) passive margins at divergent plate boundaries, (4) stable shield interiors, and (5) oceanic islands.

Typology of Natural Granite Landscapes

There are two major recurrent themes in geomorphological research into granite landscapes. On the one hand, there is a recognition of the extraordinary diversity of landforms supported by granitic rocks, on a variety of scales, from microrelief on exposed rock surfaces to regional landscape types. On the other hand, there are striking geomorphic similarities between basement regions across the world, noted again at the scale of individual, almost omnipresent landforms, as well as in larger landform assemblages. To account for both diversity and similarity, various attempts have been made to produce a typology of granite landscapes. One of the early systematic approaches was that presented by Wilhelmy (1958) in his Klimamorphologie der Massengesteine. In line with the dominant paradigm in German geomorphology, he was an advocate of strong climatic control on the development of landforms and, accordingly, used climatic zonation of the globe as a basis for his classification system. Seven major morphoclimatic zones with allegedly distinctive phenomena of granite weathering and denudation have been distinguished. In addition, Wilhelmy emphasized climate-controlled change in landform inventories with altitude, citing examples from Corsica, the Sinai Peninsula, and Korea. Following Lautensach (1950), he specifically mentions mountain ranges in the Korean Peninsula, where a dissected landscape typified by deep ferruginous weathering gradually gives way to assemblages typical of harsh periglacial environments as altitude increases. The problem with the climatic approach is the likely co-existence of landforms and weathering patterns of different ages, hence formed in different environmental conditions, especially in middle and high latitudes. In fairness, it has to be said that inherited components have been recognized by Wilhelmy, but the evidence for inheritance is not always conclusive. Selected controversial examples have been presented in the previous chapters. These difficulties also raise a wider methodological issue, if climatic zonation of the globe is to be an appropriate framework to analyse granite landforms. Stoddart (1969) and more recently Twidale and Lageat (1994) offered insightful reviews in this respect, concluding that the uncertainties and limitations of climatic geomorphology are far too many to make it a preferred paradigm. Thomas (1974) adopted a different approach, a morphological one, attempting to identify characteristic granite landform systems.

Slope Development in Granite Terrains

Rock slopes developed in granite may take different forms, as reflected in their longitudinal profiles. Field observations and a literature survey (e.g. Dumanowski, 1964; Young, 1972) allow us to distinguish at least five major categories of slopes: straight, convex-upward, concave, stepped, and vertical rock walls. In addition, overhang slopes may occur, but their height is seldom more than 10 m high and their occurrence is very localized. These basic categories may combine to form compound slopes, for example convex-upward in the upper part and vertical towards the footslope. Somewhat different is Young’s (1972) attempt to identify most common morphologies of granite slopes. He lists six major categories: (1) bare rock domes, smoothly rounded or faceted; (2) steep and irregular bare rock slopes of castellated residual hills, tending towards rectangular forms; (3) concave slopes crowned by a free face; (4) downslope succession of free face, boulder-covered section and pediment; (5) roughly straight or concave slopes, but having irregular, stepped microrelief; (6) smooth convex-concave profile with a continuous regolith cover. The latter, lacking any outcrops of sound bedrock, are not considered as rock slopes for the purposes of this section. Young (1972) appears to seek explanation of this variety in climatic differences between regions, claiming that ‘Variations of slope form associated with climatic differences are as great as or greater, on both granite and limestone, than the similarity of form arising from lithology’ (Young, 1972: 219). This is a debatable statement and apparently contradicted by numerous field examples of co-existence of different forms in relatively small areas. Slope forms do not appear specifically subordinate to larger landforms but occur in different local and regional geomorphic settings. For example, the slopes of the Tenaya Creek valley in the Yosemite National Park include, in different sections of the valley, straight, vertical, convex-upward, and concave variants (Plate 5.1). Apparently, multiple glaciation was unable to give the valley a uniform cross-sectional shape.

Boulders, Tors, and Inselbergs

Boulders, tors and inselbergs (Plates III, IV, V) are regarded as the most characteristic individual geomorphological features of granite landscapes and it is their assemblages extending over large areas that give granite terrains their unmistakable appearance. Although none of these landforms is unique to granite, nor even specific to basement rocks, it is perhaps true that the most astounding ones occur within granite areas. Twidale (1982) in his Granite Landforms considered boulders and inselbergs as two key individual components of granite landscapes and devoted to them almost 100 pages, whereas the other major landforms received only 35. Likewise, rock-built residual hills figure prominently in Klimamorphologie des Massengesteine by Wilhelmy (1958). In the voluminous literature about inselbergs, papers focused on those developed in granite evidently prevail (see the reviews by Kesel, 1973 and Thomas, 1978). Likewise, granite tors, especially in classic areas such as Dartmoor (Gerrard, 1994a) do not cease to attract the attention of geomorphologists. The unifying characteristic of all three landforms considered in this section is that they are essentially outcrops of solid rock rising above a surface cut across a weathering mantle, even if the thickness and lithology of the weathering mantle may be very variable. Outside arid areas there are very few examples of tors and inselbergs, surrounded by a rock-cut platform. Therefore, the discussion about their origin and significance has inevitably been tied to the increasing recognition of the significance of deep weathering. Twidale (1981a, 1982, 2002) reviewed many early accounts and concluded that selective subsurface weathering and subsequent exposure of unweathered cores to form boulders and inselbergs had been appreciated as early as the end of the eighteenth century. Nowadays, there is little doubt that the majority of individual medium-scale granite landforms are due to selective subsurface weathering. Before the presentation of residual granite landforms commences, a few terminological issues need to be raised. Although it may appear that the distinction between boulders, tors, and inselbergs is a simple task, it is in fact not at all straightforward.

Geology of Granite

The unifying theme for granite landscapes of the world is the granite itself, hence it is logical to start with a brief account of granite geology. For obvious reasons of space and relevance, this chapter cannot provide a comprehensive and extensive treatment of granite as a rock. Rather, its aim is to provide background information on those aspects of granite geology which are relevant to geomorphology and may help to explain the variety of landforms and landscapes supported by granite. The survey of literature about the geomorphology of granite areas reveals that in too many studies the lithology of granite and the structure of their intrusive bodies have not received adequate attention, especially if a ruling paradigm was one of climatic, or climato-genetic geomorphology. Granites were usually described in terms of their average grain size, but much less often of their geochemistry, fabric, or physical properties. Even the usage of the very term ‘granite’ may have lacked accuracy, and many landforms described as supported by granite may in fact have developed in granodiorite. On the other hand, it is true that granite may give way to granodiorites without an accompanying change in scenery. In the Yosemite National Park, Sierra Nevada, California, these two variants occur side by side and both support deeply incised valleys, precipitous slopes and the famous Sierran domes. Likewise, wider structural relationships within plutons and batholiths, and with respect to the country rock, have been considered in detail rather seldom. In analyses of discontinuities, long demonstrated to be highly significant for geomorphology, terms such as ‘joints’, ‘faults’, and ‘fractures’ have not been used with sufficient rigour. But it has to be noted in defence of many such geologically poorly based studies that adequate geological data were either hardly available or restricted to a few specific localities within extensive areas, therefore of limited use for any spatial analysis of granite landforms. Notwithstanding the above, there exist a number of studies in which landforms have been carefully analysed in their relationships to various aspects of the lithology, structure, and tectonics of granite intrusions.

Granite Landscapes Transformed

An analysis of granite landscapes would not be complete if the modifying human factor were ignored (Godard, 1977). Over the millennia humans have used the resources provided by granite, whether in a solid or weathered state, taken advantage of the spatial configuration of granite landforms, or mimicked natural granite features for various purposes. The combination of rock outcrops, regolith-mantled surfaces, and soil characteristics has acted as a significant constraint on many human activities, especially in the past. Hence many granite areas have very specific histories of human impact. The monumentality of many granite landforms has inspired fear, awe, and spiritual experience, whereas in the modern era the distinctiveness of many granite terrains has become a magnet for tourism. Each of these activities has left its imprint on granite landscapes, to the extent that some of them easily fall into the category of ‘cultural landscapes’, while in others, man-made features have evidently overwhelmed the natural configuration of the land. In this closing chapter of the book a few aspects of human transformation on natural granite landscapes will be briefly addressed. The coverage, and particularly the selection, of examples are by no means exhaustive, and the historical context has not been explored. The intention is rather to review some of the most characteristic relationships between humans and granite landscapes and to show that the characteristics of natural granite landforms dictate very specific adjustments, uses, and strategies of landscape change. Therefore, extensive referencing has also been avoided. The middle and late Neolithic in western Europe (3500–1700 BC) was a period of extraordinary construction activity using local and imported stone. It was not limited to granite lands, but the availability of durable monumental stone was certainly important. Therefore, uplands and rolling plains underlain by granitoid rocks abound in a variety of megalithic structures, including standing stones, stone circles and rows, passage tombs, simple dolmens, burial mounds (cairns), and stone enclosures. Extensive assemblages of Neolithic monuments occur on the Alentejo plain in southern Portugal, in western Spain, in Brittany, France, and on the uplands of south-west England, from Dartmoor through Bodmin Moor, Carnmenellis to Land’s End.

Cold-Climate Granite Landscapes

Inselbergs, tors, boulder fields, and pediments are repetitive landforms of many low- to mid-latitude granite landscapes, whether in humid or in arid environments. Although there have been attempts to link these landforms to certain specific climatic environments, their actual distribution, as shown in the preceding chapters, speaks clearly for minor climatic control in their development. Therefore, identification of a ‘typical’ granite rainforest, or savanna, or desert landscape does not seem possible. Each of these environments is known to host a variety of distinctive landscapes supported by granite, which will be explored in the next chapter. Likewise, cold environments in high latitudes have long been considered as having a very distinctive geomorphology, in which the factor of rock control matters little, but repeated freezing and thawing is critical. This view is difficult to maintain any longer, especially in the light of recent progress in periglacial geomorphology. The effects of glaciation are more evident, but even there the role of bedrock must not be neglected and formerly glaciated granite terrains do show certain specific features. Many granite terrains are located in cold environments, or have experienced cold-climate conditions in the relatively recent past of the Pleistocene. Therefore, it is reasonable to expect that their geomorphic evolution has been influenced by a suite of surface processes characteristic of such settings, collectively termed as ‘periglacial’. Present-day periglacial conditions typify such granite areas as the uplands of Alaska, Yukon, and the northern Rocky Mountains, much of the Canadian Shield, coastal strips of Greenland, northern Scandinavia, extensive tracts of Siberia, and the Tibetan Plateau. Granite areas located further south, in the British Isles, the Iberian Peninsula, the Massif Central, the Harz Mountains, and the Bohemian Massif, were affected by periglacial conditions for most of the Pleistocene. In fact, the most elevated parts of these mountains and uplands experience a mild periglacial environment even today and winter temperatures may remain below 0°C for weeks. The efficacy of present-day frost action is however limited by the insulating snow cover. Some of the granite areas of the southern hemisphere are, or were, within the periglacial realm too.

Minor Landforms

Perhaps the most characteristic of all minor landforms on exposed granite surfaces approaching horizontality are flat-bottomed or, less commonly, hemispherical hollows ranging in diameter from 15–20 cm to a few metres. They are known under a variety of local names, such as Opferkessel in German, pias in Spanish, vasques in French, or gnamma, which is an Aboriginal word occasionally used in Australia (e.g. Twidale and Corbin, 1963). In English, these superficial features are collectively described as weathering pits. They are not unique to granite, but are also abundant in sandstone and occur in other lithologies too. A remarkable flatness of floors of many shallow pits is reflected in another name present throughout the literature, namely that of a ‘pan’ (e.g. Twidale and Corbin, 1963; Fairbridge, 1968; Dzulynski and Kotarba, 1979). However, and despite a more accurate reflection of the form, the term ‘pan’ for weathering pits has fallen into disfavour, apparently because an identical name is used to describe much larger, closed topographic depressions within low-angle surfaces in arid lands. The majority of weathering pits are either closed features or there is a narrow outlet in the form of a channel trending away from the pit (Plate 4.1). Another type is an ‘armchair pit’, which grows into the rock surface from the side of an outcrop. These are hemispherical and wide open. At many localities pits may coalesce to form extensive networks, or else they are joined by channel-like features. Weathering pits in granite show a wide range of dimensions. Hollows in excess of 10 m long and 3 m deep have been reported, and the largest ever described is probably one in Australia, measuring 18.3 x 4.6 x 1.8 m (Twidale and Corbin, 1963). Unfortunately, there are very few systematic measurements of large populations of pits, and this severely restricts any attempts to generalize about the size of pits. Goudie and Migoń (1997) provided such a data set for two outcrops in the central Namib Desert. An interesting observation is that weathering pits in this arid area are much larger than their counterparts in humid temperate latitudes.