Development of Palimpsest Landscapes
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In Physical Geography, a palimpsest landscape is one where, in any given region, the different landforms that make up the landscape are not of the same age, with some surface landforms being very young because they are being shaped at the present time (such as gravel bars within today's rivers), and other surface landforms being old (sometimes millions of years old) because they were shaped under climatic conditions or by processes that are no longer present in that region (Bloom, 2002). Those landforms that are no longer developing under today's conditions are termed 'relict', because they were formed at some point in the past when conditions, processes or environments were different or operated at a different magnitude to present. Spatially, therefore, landscapes are composed of a mosaic of active and relict (inactive) landforms of different ages.
A useful way to conceptualize the development of a palimpsest landscape is to consider each successive episode of landscape change to be a different 'layer' laid as a blanket across the landscape's surface (Bailey, 2007). One way to consider how these different 'layers' might impact and their signatures to be retained on a landscape is to use the analogy of snowfall. When snowfall occurs, snow may settle as a layer across a region, but its ability to change the nature of a landscape depends on how thick and long-lived the snow layer is. In some places, the snow may be thick enough to completely transform the landscape and conceal what is underneath; in others, the snow is thin and patchy and the underlying land surface can be partially seen. If the snow is thick, it generally takes a long time to melt and its impact may be significant. If the snow is thin, it is soon gone and its impact may be minimal. The purpose of this analogy is to show that successive episodes of landscape-changing events (such as climate change, glaciation, mass movements, deforestation) can have different spatial and temporal manifestations, and can also yield very unequal signatures: some landscapes are very sensitive to external forcing by climate or human activity and can change very dynamically over short time scales, whereas other landscapes appear to have changed very little over time scales of millions of years.
Glaciation is a significant landscape-shaping event because glaciers are powerful erosive agents and so can dramatically change regional-scale landscape geomorphology. Several studies have been concerned with identifying palimpsest landscapes in areas that were glaciated during the Quaternary period (1.8 million–10,000 years ago). This is because glaciers can selectively preserve some land surfaces whilst completely destroying others (Goodfellow et al., 2008). This can result in a mosaic of older (relict) landscape elements being preserved amid a younger glaciated landscape. For example, upstanding weathered bedrock outcrops (tors) are particularly common in areas that have been affected by glacial and periglacial processes (Figure 1). Across Scandinavia, cosmogenic radionuclide dating of tor surfaces shows that some tors are 37,000–79,000 years old, and have survived two major episodes of Quaternary glaciation (Stroeven et al., 2002). Even on the scale of a single glacial valley, palimpsest landscape elements can be identified (Figure 2). A key issue hindering the identification of relict elements of palimpsest landscapes, however, is correct interpretation of their formational processes, climatic or environmental setting and their age. Often this information is not available, and so the true age and evolution of palimpsest landscapes remain unknown.
Physical landscapes worldwide have also been strongly affected by human activity since the development of settled agriculture around 7000 years ago, and humans are now the most powerful geomorphic agents on Earth (Hooke, 2000). Direct physical evidence for human impacts on landscapes can be seen through patterns of ecosystems, transport routes, settlements and archeology, amongst others (Bailey, 2007). This physical evidence also arises in part from more complex economic, social, political and technological processes, values, styles and cultures that are imprinted on the landscape in more subtle ways. The relationship between physical and human landscape elements can be considered in a hierarchical context (Figure 3), in which human activity responds to and is influenced by landscape physical patterns and the distribution of geological and geomorphological resources.
An important principle that this hierarchical relationship represents is the role of time in landscape evolution and thus in the development of landscape palimpsests. An example that illustrates the role of time in the evolution of both physical and human landscapes is shown in Figure 4. In west Cornwall, southwest England, the granite bedrock has been strongly affected by cold-climate weathering throughout the Quaternary, giving rise to a distinctive landscape relief. Metasomatism of the granite also resulted in the formation of Europe's largest tin and china clay resource, which has been exploited commercially for around 2000 years. This longstanding interplay between human activity and the physical landscape is seen though patterns of archeological features, settlements, roads, field boundaries and architectural styles (Hamilton et al., 2008), as well as more subtly through art, literature and mythology. The resulting palimpsest landscapes (Figure 4) are not only the material expressions of physical and human processes at work over different spatial and temporal scales, they also capture aspects of the nonmaterial expressions of cultural identity and sense-of-place that enable different landscapes to be distinguished from one another. From this it can be inferred that palimpsest landscapes are multidimensional expressions of physical and human processes, which is one reason why they are so interesting to study.
A useful way to conceptualize the development of a palimpsest landscape is to consider each successive episode of landscape change to be a different 'layer' laid as a blanket across the landscape's surface (Bailey, 2007). One way to consider how these different 'layers' might impact and their signatures to be retained on a landscape is to use the analogy of snowfall. When snowfall occurs, snow may settle as a layer across a region, but its ability to change the nature of a landscape depends on how thick and long-lived the snow layer is. In some places, the snow may be thick enough to completely transform the landscape and conceal what is underneath; in others, the snow is thin and patchy and the underlying land surface can be partially seen. If the snow is thick, it generally takes a long time to melt and its impact may be significant. If the snow is thin, it is soon gone and its impact may be minimal. The purpose of this analogy is to show that successive episodes of landscape-changing events (such as climate change, glaciation, mass movements, deforestation) can have different spatial and temporal manifestations, and can also yield very unequal signatures: some landscapes are very sensitive to external forcing by climate or human activity and can change very dynamically over short time scales, whereas other landscapes appear to have changed very little over time scales of millions of years.
Glaciation is a significant landscape-shaping event because glaciers are powerful erosive agents and so can dramatically change regional-scale landscape geomorphology. Several studies have been concerned with identifying palimpsest landscapes in areas that were glaciated during the Quaternary period (1.8 million–10,000 years ago). This is because glaciers can selectively preserve some land surfaces whilst completely destroying others (Goodfellow et al., 2008). This can result in a mosaic of older (relict) landscape elements being preserved amid a younger glaciated landscape. For example, upstanding weathered bedrock outcrops (tors) are particularly common in areas that have been affected by glacial and periglacial processes (Figure 1). Across Scandinavia, cosmogenic radionuclide dating of tor surfaces shows that some tors are 37,000–79,000 years old, and have survived two major episodes of Quaternary glaciation (Stroeven et al., 2002). Even on the scale of a single glacial valley, palimpsest landscape elements can be identified (Figure 2). A key issue hindering the identification of relict elements of palimpsest landscapes, however, is correct interpretation of their formational processes, climatic or environmental setting and their age. Often this information is not available, and so the true age and evolution of palimpsest landscapes remain unknown.
Physical landscapes worldwide have also been strongly affected by human activity since the development of settled agriculture around 7000 years ago, and humans are now the most powerful geomorphic agents on Earth (Hooke, 2000). Direct physical evidence for human impacts on landscapes can be seen through patterns of ecosystems, transport routes, settlements and archeology, amongst others (Bailey, 2007). This physical evidence also arises in part from more complex economic, social, political and technological processes, values, styles and cultures that are imprinted on the landscape in more subtle ways. The relationship between physical and human landscape elements can be considered in a hierarchical context (Figure 3), in which human activity responds to and is influenced by landscape physical patterns and the distribution of geological and geomorphological resources.
An important principle that this hierarchical relationship represents is the role of time in landscape evolution and thus in the development of landscape palimpsests. An example that illustrates the role of time in the evolution of both physical and human landscapes is shown in Figure 4. In west Cornwall, southwest England, the granite bedrock has been strongly affected by cold-climate weathering throughout the Quaternary, giving rise to a distinctive landscape relief. Metasomatism of the granite also resulted in the formation of Europe's largest tin and china clay resource, which has been exploited commercially for around 2000 years. This longstanding interplay between human activity and the physical landscape is seen though patterns of archeological features, settlements, roads, field boundaries and architectural styles (Hamilton et al., 2008), as well as more subtly through art, literature and mythology. The resulting palimpsest landscapes (Figure 4) are not only the material expressions of physical and human processes at work over different spatial and temporal scales, they also capture aspects of the nonmaterial expressions of cultural identity and sense-of-place that enable different landscapes to be distinguished from one another. From this it can be inferred that palimpsest landscapes are multidimensional expressions of physical and human processes, which is one reason why they are so interesting to study.
Associated References
- Bailey, G. 2007. Time perspectives, palimpsests and the archaeology of time. Journal of Anthropological Archaeology, 26, 198-223.
- Bloom, A.L. 2002. Teaching about relict, no-analog landscapes. Geomorphology, 47, 303-311.
- Goodfellow, B.W., Stroeven, A.P., Hättestrand, C., Kleman, J. and Jansson, K.N. 2008. Deciphering a non-glacial/glacial landscape mosaic in the northern Swedish mountains. Geomorphology, 93, 213-232.
- Hamilton, S., Harrison, S., Bender, B. 2008. Conflicting imaginations: Archaeology, anthropology and geomorphology on Leskernick Hill, Bodmin Moor, southwest Britain. Geoforum, 39, 602-615.
- Hooke, R.LeB. 2000. On the history of humans as geomorphic agents. Geology, 28, 843-846.
- Stroeven, A.P., Fabel, D. Hättestrand, C. and Harbor, J. 2002. A relict landscape in the centre of Fennoscandian glaciation: cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles. Geomorphology, 44, 145-154.