Mapping landslides at different scales

James S. Griffiths, Anne E. Mather and Martin Stokes
Quarterly Journal of Engineering Geology and Hydrogeology, 48, 29-40, 28 January 2015, https://doi.org/10.1144/qjegh2014-038

Abstract

Landslides can be mapped for purely academic reasons to understand their nature and properties, or for engineering purposes because they constitute a hazard to an existing human activity or a proposed development. The scale of the mapping to be carried out will be influenced by the requirements of the investigation. For engineering investigations large-scale mapping (>1:10000) is generally the most appropriate whereas for academic studies smaller-scale mapping can be just as valuable as it can reveal much about the role landsliding has in overall landscape development. In this paper three examples of mapping landslides at different scales are presented and the implications for the mapping procedures employed are explored. These examples are: (1) large-scale mapping of an active landslide affecting a road and mobile home site on the SW coast of the Isle of Wight, UK; (2) medium-scale mapping of landslides in the proximity of a river capture site in SE Spain; (3) small-scale mapping of large coastal landslides in northern Chile and the implications for mass sediment transfer along an active subduction margin. All three mapping programmes had a clear focus and intent but the method employed varied with the purpose of the investigation and the scale of the mapping that was carried out. It was concluded that it is important for any landslide mapping programme to have clearly identified objectives from the outset if time, and money, is not be wasted. However, even in site-specific investigations it is imperative for landslides to be seen in the wider context of their geomorphological situation with their role in landscape development assessed so as to understand the likelihood and potential magnitude of any hazards.

Dramis et al. (2011, p. 43) stated that ‘scale is one of the main issues in geomorphological mapping’, and this comment is particularly relevant to the geomorphological and engineering geological investigation of landslides. The investigation of landslides is a longstanding pursuit given these are one of the most deadly natural hazards and each year are responsible for hundreds of millions of dollars of damage and the loss of more than 1000 lives (Clague & Roberts 2012). Indeed, CRED (2012) suggested that landslides were responsible for 10000 deaths and left 2.5 million people homeless over the period 2001–2010.

One of the most important techniques for determining the nature and extent of landsliding is through a systematic process of geomorphological and engineering geological mapping. However, before any mapping is undertaken it is necessary to establish the purpose of the investigation; fundamentally the question is whether the investigation is going to concentrate on a discrete landslide or whether the work is a much broader evaluation of landslide hazard and risk (Lee & Jones 2014). Also, it must be accepted that mapping to solve an engineering issue, such as the potential for a landslide to damage an existing or proposed development, is distinct from more academic geomorphological investigations related to long-term landscape development. In this paper these issues are explored through three case studies at different scales and for distinctly different purposes: (1) large-scale mapping of an active landslide on the Isle of Wight, UK, that was affecting a main road and mobile home site; (2) medium-scale mapping initially undertaken as part of a hazard and risk assessment within a single river catchment in SE Spain; (3) small-scale mapping of the coastal cordillera in northern Chile to evaluate the role of landsliding in the mass transfer of sediment in an active subduction margin.

The question of scale

In mapping landslides it is possible to envisage three broad scale categories (large, medium, and small) being utilized. These categories are presented in Table 1, and although the boundaries to the categories should be regarded as flexible, there are differences in the way the maps are compiled, the techniques employed in their construction, and their intended end-use.

View this table:
Table 1.

Scale of landslide mapping necessary to meet different requirements (developed from Griffiths & Whitworth 2012; Shilston et al. 2012)

There are many instances of large-scale mapping of landslides; excellent examples used for landslide hazard assessment in Hong Kong have been given by Parry (2011). Medium-scale engineering geology maps are probably the most widely used and examples can be found throughout the world; for example, the 1:25000 scale ZERMOS maps in France (Humbert 1977), and the 1:20000 scale GASP series in Hong Kong and notably its use in the compilation of a landslide inventory (Burnett et al. 1985). Small-scale maps generally are best exemplified by the PUCE (Pattern–Unit–Component–Evaluation) system developed by CSIRO in Australia (Finlayson 1984). This programme produced maps at a scale of 1:250000 that defined broad terrain patterns, within which particular assemblages of landforms, pedological soils and vegetation sequences occur. However, a small-scale (1:100000) version of the French ZERMOS system was used in some areas for the specific purpose of investigating landslide hazard (Landry 1980).

It is not just the maps themselves where ‘scale’ is an issue, and Figure 1 provides a standard geomorphological nomenclature for landform size, illustrating this with a landslide example. There may even be difficulty when investigating landslides in actually defining what constitutes a ‘landslide’ (Griffiths 1999). The scale and purpose of the mapping will determine what size of geomorphological feature is recorded. For example, even large-scale mapping for engineering purposes may not record micro-features, such as spalling or small rockfalls, as they are below the threshold of what can actually be recorded on a map sheet, whereas single large landslide blocks and the nature of the landslide movement can be mapped in detail. In contrast, typical medium-scale maps of national geological surveys (1:10000 to 1:50000) will record a landslide only as a feature with its spatial extent delimited rather than identifying details of its internal structure. Small-scale mapping is one further step removed from this and is likely to record only the location of a landslide, normally as the first stage in compiling an inventory.

Fig. 1.
Fig. 1.

The size of geomorphological features (after Summerfield 1991).

Therefore, it emerges that some of the critical decisions that need to be made before embarking on any programme of landslide mapping are as follows.

  • What is the purpose of the investigation?

  • What scale of mapping is most appropriate for the required purpose, as this will affect the techniques that have to be employed (Table 1)?

  • What is the minimum size of landslide feature that can be recorded?

  • What level of detail is needed about any single landslide to meet the required purpose?

Mapping approaches

The techniques of mapping landslides have been widely documented and do not need to be repeated here. A detailed review of the engineering geomorphological investigation of landslides has been given by Griffiths & Whitworth (2012). An account of general geomorphological mapping for applied purposes, along with a collection of case studies at a range of mapping scales, has been given by Smith et al. (2011). For engineering geological mapping the book by Dearman (1991) remains the definitive volume. The use of medium- and large-scale terrain classification techniques in landslide investigations is best exemplified by the work in Hong Kong originally outlined by Brand et al. (1982) and Burnett & Styles (1982). For an overall approach to the identification of landslide hazard and risk reference should be made to the book by Lee & Jones (2014). A summary of the techniques to be employed in general terrain assessment has been given by Lawrance et al. (1993) and Griffiths (2001), and the continuing relevance of the terrain evaluation approach for engineering is demonstrated by the work of Hearn (2011a,b) and Shilston et al. (2012).

In the following sections three case studies of the investigation of landslides are presented. In each case the original reason for and purpose of the investigation is defined, followed by a description of the geology and geomorphology of the area investigated and then some conclusions from the work.

Large-scale mapping of an active landslide (Binnel Bay, Isle of Wight, UK)

In January 2001 the Binnel Bay landslide on the Isle of Wight, located off the south coast of the UK, caused damage to the main road in the south of the island and virtually destroyed a mobile home site. Large-scale geomorphological mapping was used to identify the nature of the landslide problem, and to provide a framework for subsequent ground investigations and design of remedial measures.

The Binnel Bay landslide lies within the feature known as the Undercliff landslide complex, which is one of the largest areas of landsliding in Europe (Chandler 1984). The Undercliff is a dip slope landslide complex that has developed off the south coast of the island (Fig. 2) with a terrestrial component up to 1 km wide and some 10 km long. There is extensive offshore evidence to indicate that the landslide has been part of a retrogressive landslide system that has migrated shoreward in response to rising sea levels since melting of terrestrial glacial ice at the end of the Younger Dryas Period, c. 11500 years ago (Lee et al. 1991).

Fig. 2.
Fig. 2.

Basic geology map of the South Downs of the Isle of Wight (after BGS 2013).

The geology of the South Downs of the Isle of Wight comprises a conformable series of Cretaceous strata dipping c. 2° to the south, forming an escarpment (Fig. 2). The highest part of the sequence exposed on the southern coast is the fossiliferous calcarenites and calcareous mudstones of the Chalk, comprising the Holywell Nodular Chalk Formation, the Zig Zag Chalk Formation and the West Melbury Marly Chalk Formation. Of most relevance to the landsliding at Binnel Bay is the geological sequence below the Chalk, starting with the ≥40 m of the Upper Greensand Formation. The weak sandstones of the Upper Greensand Formation form a significant aquifer developed on the 45 m thick Gault Formation aquiclude (Fig. 3). The Upper Greensand Formation conformably overlies the Gault Formation, which can be divided into two distinct lithological units, an upper plastic clay and a lower, much more silty, less plastic clay. The base of the Gault Formation is identified by the 3.7 m thick Monk’s Bay Sandstone Formation. The underlying Sandrock Formation sequence is mainly a weak sandstone containing clay layers up to 1 m thick. The most substantial movements on the basal shear surface of the main Undercliff landslide complex have occurred along these Sandrock Formation clay layers, but movements both within and in the upper parts of the main landslide occur at the boundary between the plastic clay and silty clay lithologies in the Gault Formation.

Fig. 3.
Fig. 3.

Geomorphological map of the Binnel Bay landslide.

The Binnel Bay landslide occurred towards the western end of the Undercliff landslide complex, close to the St. Catherine’s Point landslide investigated by Hutchinson et al. (1991). Investigations at the site comprised geomorphological field mapping at 1:2500 scale by a two person team over 1½ days using global positioning system (GPS), compass–clinometer, Suunto clinometer, and 50 m tape. The main landslide movements were essentially north to south with the backscar some 2 m downslope of the road, although tension cracks were appearing in the road at the time of the site visit. By reference to base maps from 1995 it was established that the backscar had retrogressed 5 m over a 5 year period. Within the landslide complex itself there was extensive evidence of active movement, notably exposed shear surfaces, stretched roots and vegetation, debris runout over beach gravels, disrupted drainage, fallen trees, Reidel shears on the slide margins, out of place geological units, and damage to property. The upper part of the landslide contained detached boulders from the Upper Greensand Formation, but the main part of the landslide constituted debris of disrupted and remoulded Gault Formation.

Geomorphological mapping identified four distinct landslide components, labelled A, B, C and D in Figure 3. There was no indication that these landslides were part of a large-scale reactivation of the overall Undercliff landslide complex but they represented continuing degradational movements within the debris apron. Landslides A and C were part of an upper landslide movement that appeared to be failing along the lithological boundary within the Gault Formation. The main movement was landslide A, with landslide C occurring as a result of the loss of support along the eastern boundary caused by the landslide reactivation. Landslide A appeared to be spilling over an in situ bench in the Sandrock Formation. Landslide D was a discrete part of this in situ bench that had also started to retrogress.

Based on the mapping, the cross-section shown in Figure 4 was drawn up as a basis for designing and interpreting the subsequent ground investigations. Given there was no indication of basal erosion by the sea, it was hypothesized that the cause of the reactivation of the landslide was high porewater pressures in the landslide system resulting from the previous very wet autumn. It was suggested that the landslide movements would continue if rainfall remained high. The mapping established that to determine the exact nature of the landslide would require ground investigation and monitoring, and it was recommended that any remedial measures would have to be based on effective drainage rather than techniques such as toe-loading, anchoring or retaining walls.

Fig. 4.
Fig. 4.

Cross-section through the Binnel Bay landslide.

What emerged from this study was that to obtain the detail needed for the design of an engineering investigation and solution to the consequences of the landslide movement necessitated mapping by somebody experienced in engineering requirements. This project utilized large-scale field mapping techniques as it was necessay to identify all the various components of the landslide complex to gain an understanding of the landslide mechanisms and their likely cause (Figs 3 and 4). It was recognized, however, that the investigation of the site was aided by the extensive previous work that had been undertaken on the whole Undercliff landslide complex (Chandler 1984; Hutchinson et al. 1991; Lee et al. 1991) and the data from these investigations were compiled during desk studies carried out prior to going on site.

Medium-scale mapping to establish the geomorphological controls on a landslide cluster (Río Aguas Basin, SE Spain)

The requirement for an investigation of landslides in the Río Aguas Basin, which is located in the semi-arid SE part of Spain, emerged from research into the Cenozoic evolution of the drainage system (Mather 1991, 2000; Mather & Harvey 1995; Mather et al. 2002; Stokes et al. 2002). It was recognized that landsliding was a significant process in the development of the contemporary landscape (e.g. Mather et al. 2003) and an important contributor to the sediment flux (Mather et al. 2002; Stokes et al. 2002). In March 1998 it was decided to develop a research programme to investigate the scale and extent of landsliding with the specific objectives of evaluating landslide causes and mechanisms, ascertaining what had initiated landslide activity, identifying the controls on susceptibility and hazard, and establishing the role of landslide processes in landscape development. This work was based on field mapping, remote sensing interpretation, data collation and analyses (Hart 2004; Griffiths et al. 2005).

The general geology of the study area is presented in Figure 5. In broad geomorphological terms the area can be described as a ‘basin and range’ with predominantly Neogene rocks infilling fault-bounded sedimentary basins surrounded by mountains (sierras) formed in Mesozoic and older basement rocks that make up a complex nappe sequence (Sanz de Galdeano 1990). The Sorbas and Vera basins (Fig. 5) are part of the Trans-Alboran shear zone of Larouziére et al. (1988), a zone dominated by sinstral movement within the internal zone of the Betics. A detailed description of the development of these Neogene basins and their subsequent Quaternary erosional history has been given by Mather et al. (2001).

Fig. 5.
Fig. 5.

Simplified geology map of the study area in SE Spain (from Mather & Harvey 1995).

The region has been affected by epeirogenic uplift and the area remains seismically active; it is these processes that have had a significant effect on the nature and distribution of landsliding in the area. The uplift since the Late Pliocene is calculated as 80 m Ma−1 in the central areas of the Sorbas Basin and 15 m Ma−1 in the Vera Basin (Mather 1991; Mather et al. 2001). The differential uplift rates between the sedimentary basins increased regional gradients, which, when combined with the variable lithologies of the basin fill (generally comprising weak calcareous mudstones–marls, weak gypsum, medium strong and strong sandstones and limestones) led to fluvial incision, the creation of steep hillside slopes, and river capture. The sediment and water routing within the Río Aguas catchment was severely altered by a river capture event estimated to have occurred at about 100 ka, based on the U/Th isochron dating of carbonate soils from river terrace landforms at c. 88 ka (68–104 ka) (Kelly et al. 2000). The river capture lowered base levels at the point of capture (Fig. 5) by c. 90 m (Harvey et al. 1995), and resulted in c. 73% of the Sorbas Basin drainage being re-routed from its original southerly route through the Níjar–Carboneras Basin via the Rio Feos, into an easterly one through the Vera Basin (Mather & Harvey 1995; Mather 2000). It has been estimated that one consequence of the base-level change was a five-fold increase in incision rates over most of the river network, and close to the point of capture incision rates may have increased 10-fold (Mather 2000; Mather et al. 2002; Stokes et al. 2002).

The first phase of the landslide research was to compile an inventory of landslide occurrence within a single river catchment, the 425 km2 Río Aguas. This was completed through a combination of medium-scale terrain evaluation and selected medium- to large-scale geomorphological mapping from aerial photograph interpretation and in the field. This work revealed an unexpected landslide distribution, illustrated by the isopleth map (Fig. 6) with a concentration immediately upstream of the identified main river capture site rather than in the higher and steeper ground in the mountains that formed the boundaries of the watershed in the north and south. Clearly, the river capture had had a profound effect on the geomorphology (Mather et al. 2002). In the Sorbas Basin, deep canyons had developed in the lithological units that were more resistant (the strong sandstones and limestones), and large V-shaped valleys had formed in the weaker lithologies (notably the weak calcareous mudstones–marls). In the formation of this terrain sediment had been liberated from the slopes to supply both suspended and bedload material to the fluvial system through a variety of hillslope processes. The topography generated by the enhanced incision provided the ideal situation for landslides to become a significant mechanism for this sediment transfer. Also, given the rapid rates of incision, the landscape had little time to adjust and hence many hillside slopes were over-steepened, leaving them in a state of marginal stability. This had led to a concentration of landslides around the river capture site with a ‘wave of aggression’ (Bjerrum & Jorstad 1968) making its way through the catchment, initially created by the differential uplift between the Vera and Sorbas basins, and then continuing as a result of the increase in stream power following the river capture (Griffiths et al. 2005). Because the river capture event had occurred relatively recently in the geological record the wave of headward erosion has yet to extend more than 25 km upstream of the capture point, hence the concentration of landslide activity (Mather et al. 2002; Mather et al. 2003). The landsliding at the river capture site is active to the present day, with massive tension cracks visible in the area close to the original elbow of capture. Elsewhere within the zone of the migrating wave of aggression there are a number of large landslides, including Maleguica (Fig. 7), located 8 km upstream from the capture site with a volume of >1 million m3.

Fig. 6.
Fig. 6.

Isopleth map of the landslide distribution in the Río Aguas catchment (from Hart 2004).

Fig. 7.
Fig. 7.

The Maleguica landslide located 8 km upstream of the river capture site.

Large-scale mapping of the landslides in the capture col area was subsequently undertaken to establish the local controls on the failures. This demonstrated that the local lithology was the major control on the nature of the landslides, which ranged in type from large rock falls to translational slides and rock avalanches. However, it was the original medium-scale mapping that identified the geomorphologically anomalous landslide concentration and provoked a more detailed examination of the causes of landsliding within the catchment. Essentially, this study led to the identification of a unique geomorphological control on the landslide hazard for this area; namely, a river capture event.

Small-scale mapping: coastal landslide occurrence with an active subduction plate boundary (northern Chile)

A very different landscape development question was raised during geological studies of the Coastal Cordillera in a hyper-arid region of northern Chile. During vehicular ground reconnaissance surveys covering some 450 km of the seaward scarp of the coastal cordillera (Fig. 8a and b) a series of arcuate embayments 1–10 km across were observed (Fig. 9). The geometry of the embayments was typical of those reported from the backscarps of giant landslides associated with volcanic island terrains (e.g. Masson et al. 2002). This raised a number of questions. First, were these features landslides or just examples of ‘landform convergence’ or ‘equifinality’, where the processes of hillslope, fluvial and coastal erosion combine to create features that resemble landslides? Second, if they were landslides, what had caused them? Third, what was the role of landsliding in the evolution of the contemporary landscape of the coastal cordillera?

Fig. 8.
Fig. 8.

Main terrain units of northern Chile: (a) location map showing identified coastal landslides plus the inland Latagulla landslide (LL), the Lluta collapse (LC), and landslide prone areas within a drainage basin (LPD); (b) representative cross-section.

Fig. 9.
Fig. 9.

A landslide generated on the 1 km high western scarp of the Coastal Cordillera (see small settlement on left of image for scale).

The study area is developed within Jurassic volcanic rocks, lies between the Pacific coast and western edge of the Coastal Cordillera of northern Chile, and forms part of the Andean fore-arc (Von Huene et al. 1999; Taylor et al. 2007). The presence of Oligocene and Miocene marine sediments along the coastline (Ibaraki 2001) indicated that uplift had dominated and that subsidence had been minimal during the Neogene. Application of a geomorphological index (Stream Gradient Index; Hack 1957) to the region revealed that the study site was one of the most actively uplifting areas of the coastline, with postulated uplift rates of the order of 480–1152 mm ka−1 (Mather & Hartley 2006). Although the area lies within seismically active terrain (Spence et al. 1999), historically it is part of a well-known seismic gap with recurrence intervals for major seismic events in excess of 100 years. The last major seismic event was in 1877 and generated a 25 m run-up tsunami in Arica (Kulikov et al. 2005).

Initial studies of the site had concentrated on understanding the drainage systems, regional tectonics and seismicity, and the relationship between this fore-arc system and the offshore subduction trench (Hartley et al. 2000; Mather & Hartley 2006). However, once the gross morphology of the region came under scrutiny the mechanisms for onshore to offshore sediment transfer emerged as a specific research question and raised the issue of the role of landsliding in this process.

The geomorphology of the coastal region is dominated by two main terrain units within the Andean land system (Fig. 8), as follows.

(1) A coastal plain. This is a narrow and discontinuous plain present at the foot of the coastal scarp and is defined as the area beneath the 200 m contour line. It comprises alluvial fan material, landslide debris and some marine terraces.

(2) The coastal Cordillera. This is up to 2.2 km high and 20–50 km wide. Delineation of the cordillera as a separate morphotectonic unit commenced in the Oligocene and continues to the present day (Hartley et al. 2000). It largely comprises basaltic andesites and granodiorites cut by north–south- to NE–SW-trending, landward-dipping, normal faults that bound a series of half-grabens. The eastern flank of the Cordillera is bounded by the 900 km long Atacama Fault Zone. The western margin of the Coastal Cordillera is represented by a coastal scarp, which forms a prominent break in slope 600–1500 m high.

Where the coastal cordillera scarp is separated from active coastal erosion by the coastal plain it is considered to represent a weathered palaeo-cliffline (Mortimer & Saric 1972). It is on these areas that large terrestrial landslides have been observed (Fig. 9). The age of the scarp is unknown but stratigraphical relationships suggest that it is composite and extends back to at least the mid-Miocene. Where the scarp is located on the coast it is suggested that the large embayments identified represent the degradational zones of large landslides that have run out offshore (Mather et al. 2014). The interpretation of the coastal embayments as landslides was based on the combination of field reconnaissance and remote sensing interpretation (Soeters & van Westen 1996) set within a process of terrain evaluation (Lawrance et al. 1993). If this interpretation is correct then over the length of the 450 km long study area there are 60 landslides each with volumes up to 9 km3, giving a landslide density of 1 per 7.5 km of coastline. These are very large landslides but not as large as the collapsing flanks of hotspot volcanic islands such as Hawaii (Moore 1964) and the Canary Islands (Carracedo et al. 1999). For example, McMurtry et al. (2004) estimated that the Alia landslide that occurred on the southwestern flanks of Mauna Loa, Hawaii, c. 120000 years ago had a minimum volume of 400 km3. There is no evidence that the embayments were created by single massive events and it is possible that the observed morphology represents the combination of a number of smaller mass movement features that have coalesced. Large coastal and submarine landslides have previously been associated with mid-ocean volcanic islands formed over mantle hotspots (Keating & McGuire 2000; Masson et al. 2002; Mitchell et al. 2002). Collapse of these volcanic edifices is believed to have been responsible for mega-tsunamis (Ward 2001; Whelan & Kelletat 2003). It has, therefore, been suggested that these Chilean landslides may be one cause for tsunamis emanating from this coastal region (Mather et al. 2014). Furthermore, in this hyper-arid region it is hypothesized that intermittent and large-scale mass movement failures of the coastal escarpment, in response to tectonic uplift and seismic activity, are the main processes of erosion of the coastal scarp and transfer sediment from the land surface into the subduction trench (Fig. 10).

Fig. 10.
Fig. 10.

Conceptual model of the relationship between landsliding and regional tectonic activity, northern Chile (from Mather et al. 2014).

For this study the investigation of the landslides was carried out as part of the research into mass sediment transfer. As it was necessary to identify the nature and extent of landslide activity over 450 km of coastline it was not feasible, or essential, to map each landslide in detail, so rapid reconnaissance methods were used. These included the interpretation of remote sensing and ground truthing within an overall terrain evaluation framework. The chronology of the landslide model that has been developed (Fig. 10) remains speculative and further research is under way to check both its validity within the study area and its applicability to other convergent plate boundaries (Mather et al. 2014).

Conclusion

Three very different case studies have been provided, all of which fall under the umbrella of landslide mapping. It is apparent that for engineering measures to be designed on a discrete landslide, large-scale mapping by experienced practitioners is required (Case Study 1). These maps should form the basis for the design and interpretation of the ground investigations and must remain live documents that are updated as new information emerges. For landslide hazard investigations, commonly required for engineering feasibility studies and planning, medium-scale mapping is the most appropriate. At this scale landslide inventories can be compiled that can form the basis for hazard and risk maps, although they also have a role in understanding aspects of the geomorphological development of the landscape (Case Study 2). Small-scale mapping (Case Study 3) lends itself only to long-term landscape development investigations to answer the ‘mega-landscape’ issues. In all the case studies the scale of mapping undertaken and the techniques that were employed were determined by the requirements of the investigation. It is important that any landslide mapping programme has clearly identified objectives from the outset if time, and money, is not to be wasted. However, even in site-specific investigations (Case Study 1) it is imperative for landslides to be seen in the wider context of their geomorphological situation and their role in landscape development to be assessed so as to understand the likelihood and potential magnitude of any hazard.

Acknowledgments

The authors wish to thank The GeoMapping Unit, Plymouth University for Figures 2, 3 and 4; our former PhD student A. Hart, now at Atkins, for his field mapping in SE Spain and for Figure 6; Google Earth for Figure 9; and L. Walsh for many years of support at the Urra field centre, SE Spain. J.S.G. would like to acknowledge the support of his wife, Sarah, as his field assistant during mapping on the Isle of Wight. A.E.M. would like to acknowledge her co-worker in Chile, A. Hartley from Aberdeen University, and support from Royal Society Research Grant 24407 for herself and A. Hartley to undertake fieldwork in Chile. J.S.G. acknowledges the support of the Hong Kong Regional Group of the Geological Society for funding support when initially presenting this paper in Hong Kong in February 2014.

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