Abstract Various physical attributes of the Earth’s surface are factors that influence local topography and indirectly influence human behaviour in terms of habitation locations. The determination of geomorphological setting plays an important role in archaeological landscape research. Several landform types can be distinguished by characteristic geomorphic attributes that portray the landscape surrounding a settlement and influence its ability to sustain a population. Geomorphometric landform information, derived from digital elevation models (DEMs), such as the ASTER Global DEM, can provide useful insights into the processes shaping landscapes. This work examines the influence of landform classification on the settlement locations of Bronze Age (Minoan) Crete, focusing on the districts of Phaistos, Kavousi and Vrokastro. The landform classification was based on the topographic position index (TPI) and deviation from mean elevation (DEV) analysis to highlight slope steepness of various landform classes, characterizing the surrounding landscape environment of the settlements locations. The outcomes indicate no interrelationship between the settlement locations and topography during the Early Minoan period, but a significant interrelationship exists during the later Minoan periods with the presence of more organised societies. The landform classification can provide insights into factors favouring human habitation and can contribute to archaeological predictive modelling.

Citation: Argyriou AV, Teeuw RM, Sarris A (2017) GIS-based landform classification of Bronze Age archaeological sites on Crete Island. PLoS ONE 12(2): e0170727. https://doi.org/10.1371/journal.pone.0170727 Editor: John P. Hart, New York State Museum, UNITED STATES Received: March 29, 2016; Accepted: January 10, 2017; Published: February 21, 2017 Copyright: © 2017 Argyriou et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: Primary-level data are available from the Laboratory of Geophysical-Satellite Remote Sensing and Archaeo-environment (GeoSat ReSeArch Lab) of the Foundation for Research and Technology, Hellas (FORTH). The data have been compiled under various research projects and are also available from the Web portal of Digital Crete (http://digitalcrete.ims.forth.gr/index.php?l=1) which is continuously maintained by GeoSat ReSeArch Lab of FORTH. Maps with distribution of the sites and their chronological and typological attributes are openly available from Watrous et al. 1993, Gesell et al. 2004 and Hayden et al. 1992, 2004. Raw data that concern published material (archaeological sites) can be available upon request from GeoSat ReSeArch Lab. Any other unpublished data need to be requested by the local Archaeological Ephorates. The availability of the archaeological data falls under the restrictions of the Archaeological Law of Greece. The ASTER Global DEM dataset used for this manuscript can be downloaded from: http://gdem.ersdac.jspacesystems.or.jp/. The Fragstats freeware used in the data analysis can be downloaded from: http://www.umass.edu/landeco/research/fragstats/fragstats.html. Funding: The authors received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist.

Introduction During the Middle Minoan period of the Bronze Age period in Crete, there appears to have been a widespread increase of sites in low elevation areas suitable for arable farming. Systematic archaeological surveys across eastern and central Crete have examined the settlement dynamics and in all cases highlight a generalised population movement from high elevation areas with limited arable land to lower elevation areas, particularly plains favourable to cultivation and efficiency in irrigation [1]. Most of the studies hypothesized that this population movement tendency was caused by economic and political reasons but only a few of them considered the possibility that this tendency could be a result of environmental conditions and resource exploitation [2,3,4]. Mountainous sites at higher elevations (~600–800 m) seem not to attract further exploitation during the Middle Minoan (and were rarely re-occupied permanently during the later Minoan periods). That was perhaps due to the development of new practices for intensive agriculture which could not be applied in steeply sloping terrain [1]. As a result, people relocated to lower elevations (~300 m) and plains, which have high agricultural and irrigation potential to support population growth, as in Kavousi and Vrokastro districts [5,6]. This study aims to examine the hypothesis outlined above by studying settlement dynamics and landform characteristics during the Minoan periods, to check whether the population movement to lower elevation areas was a random tendency, or was interlinked with agricultural practices. It also examines the more general trend that the clusters of settlements followed, despite their local “micro-regional identity” [5]. Investigation of heterogeneity in geological and geomorphological properties can lead to the quantification of landscapes and to a better understanding of their complexity [7,8,9]. The Earth’s surface consist of large geomorphic features (e.g. plains, mountain ranges), through to smaller features (e.g. valleys) and their component landforms, such as valley slopes, floodplains and terraces [10]. Such landscape classification information can be of use for archaeological studies because the two sets of data can be interlinked to gain insights into the factors driving settlement evolution [11,12]. Topographic prominence was one of the first approaches to find application on archaeological studies while examination of other geomorphometric parameters followed (e.g. terrain ruggedness, amplitude of relief). Such was the case with archaeological studies of Cyprus, where an evaluation of the exposure of cultural heritage sites to natural hazards took place [13,14,15,16,17]. Most of these studies were mainly investigating the local or relative topographic position of archaeological sites in the landscape [18,19]. More recently, the development of computer technology, software packages and free access to datasets attracted the interest of analysts to develop computer algorithms to examine geomorphometric attributes and the topography of the Earth’s surface [20,21,22]. In particular, in recent years there has been an increase of interest in use of GIS-based analyses to classify landforms, over various scientific fields such as geomorphology, geology, agriculture [23,21]. Despite that, only a few studies exist with applications of archaeological interest [24,19]. The majority the studies consist of automated or semi-automated approaches, mostly evaluating homogeneous landscapes [25,26]. In contrast, this study examines both homogenous and heterogeneous landscapes. The information derived by terrain analysis, via DEMs and geomorphometrics, offers to analysts a powerful approach to describe the topographic position of features [27,21]. Understanding the relationship between the types of archaeological sites (e.g. settlements, defending sites or burial sites) and their surrounding landscape type is an important aspect of archaeological investigations. Landscape geomorphometrics can reveal insights into variations in the distribution of settlements over time [28,12]. Archaeological landscapes can be quantified by using the topographic position index (TPI) (or difference from mean elevation, DIFF), which can classify the landscape, both in terms of slope position and landform categories, into morphological classes based on the geomorphology [29,30,31]. In addition, the deviation from mean elevation (DEV) can be useful tool for geo-archaeological studies, because it highlights the subtle topographic features [32]. These two approaches are investigated in this study because they can be significantly enlightening from an archaeological perspective. The periods of the Early Minoan (5,600–4,000 BP), Middle Minoan (4,000–3,600 BP) and Late Minoan (3,600–3,000 BP) are the focus of this study [6,33,34].

Study area Crete is located in the southern part of Greece and has a significant archaeological heritage. Evidence from stone tools reveals human presence on the island of Crete as early as 130,000 years ago. However, evidence for modern human presence dates to 10,000–12,000 years ago and it was not until the Neolithic period (8500–4900 BP) when the first signs of advanced agriculture appeared in the Aegean, to open the way for the subsequent emergence of the Minoan civilization (5,600 to 3,000 BP), which is considered as the birthplace of the earliest “high culture” in Europe [35,36,37] (Fig 1A). The Phaistos, Kavousi and Vrokastro districts were selected as case study sites because of their rich archaeological heritage (Fig 1B). The information derived from past archaeological surveys is sufficiently detailed for the analysis of variations in settlement locations during the Minoan periods [6,33,38]. Earlier prehistoric geomorphological conditions were not considered in this methodological framework, as a freely-available Digital Elevation Model (DEM) of recent years is being used in the analysis: further research is needed to determine palaeo-geomorphological features of the study region. In general, the landscape of the study region has remained stable since the earliest phases of human settlement, with the main terrain features being formed in the Pleistocene (i.e., mostly within the past 2 million years) [39]. Regarding palaeo-climatic conditions, at present no conclusive observations exist for Crete from Bronze Age to the present. The role of climatic fluctuations during that period is in general an unexplored field, although a few studies have attempted to evaluate climatic changes with societal developments in areas around eastern Mediterranean [40,41]. [34] reviewed existing indirect measurements of palaeo-climate, so-called climate proxies (e.g. stable isotopes, fossil microshells), with few climatic fluctuations (drought or wetter conditions) being observed in Crete during the Minoan period. As [34] describes, the existing data is unevenly distributed, with some periods having one or two proxies and others by seven or eight. That highlights the importance of integrating climate and environmental history because a relationship between minor Alpine glacial advances in Europe and periods of extreme weather in the Aegean seems to have existed in the study region [39]. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 1. (a): The island of Crete, with red tones highlighting mountainous relief (LO: Lefka Ori; P: Psiloritis); (b) Crete shaded relief and contour intervals. The black boxes indicate the case study sites. https://doi.org/10.1371/journal.pone.0170727.g001

Minoan land use patterns During Early Minoan, in the case of the Kavousi-Vrokastro district, the settlements are preferably established on heterogeneous landscapes and many are found at higher elevations with complex rough terrain, mainly hills and ridges [6]. The Mirabello Bay area consists of small coastal valleys: most of the settlements seem to be placed on slopes rather than on the best (flat, lowland) agricultural land [42]. The site pattern was defensive, providing lines of sight between the settlements and out across the Mirabello gulf. Thus the Vrokastro site pattern may be in some accord with [43] suggestion that “people were forced to look for their safety in barely accessible places”. During this period it seems there was a dichotomy between occupying low-lying coastal sites and upland locations, indicating a tendency of the population to occupy diverse types of terrain, seeking to gain control of water sources and sites with the greatest visibility [6]. The evidence for settlement hierarchy during this period has been doubted [44], with clusters of settlements found around the arable coastal basins and not around large settlements. The Middle Minoan is characterized by settlements in homogeneous landscapes, such as the Kavousi plains, which were well-suited for cultivation [38]. There appears to have been a movement of population into the lowlands and arable upland areas close to water resources, with an associated increase in number of settlements near the best agricultural land during Middle and Late Minoan [6,33,38]. [5] notes that the pattern of settlements exploitation in the Kavousi area is in accordance with the Argolid model during this period, with concentrated population around arable areas, similar to the expansion pattern of small farms inland in Chania district. Between the Early and Middle Minoan, population growth occurred in the Gournia valley, with continuous expansion of settlements and coastal trading interests [6]. In addition, during the Middle Minoan there is exploitation of the landscape at relatively high elevations, between 450 m and 700 m above sea level. River terraces and bedrock of conglomerates and marl provided fertile soil for cultivation of the Gournia valley, along with fault fractures that provided water access via springs at higher elevation [6]. In the Late Minoan an overall reduction of the number of settlements is observed, especially in upland sites. Despite that, rural settlement is oriented to arable land and water resources, preferentially between 50–200 m elevation range, while a retreat from coastal zones (0–50m) seems to have occurred [6]. An event that might be able to explain this reduction of settlements is the impact of the Theran eruption, with evidence of a thick layer of tephra/ash found in the Mochlos area, near Gournia. Population centralization is also observed during the Late Minoan, for example at Pyrgos or Gournia. Some new large building structures during this period can be found (e.g. around the Istron river valley) which appear to be linked to trade routes, access to arable land and water resources [45].

Discussion This study has quantified the spatio-temporal variations of Early, Middle and Late Minoan settlement locations in the landscapes of Crete. A general trend is observed during the Early to Middle Minoan, with the population moving inland in search of arable land, most settlements initially being located within 1.5 km of the coast. During the Middle Minoan, there was an increase of settlements over low-gradient slopes, upper slopes and mesas in hilly terrain, where they remained during the Late Minoan, probably because of their strong defensive advantage (Fig 6). The analysis of settlement areas indicates that during the Middle Minoan there was: i) an increase of settlements in deeply incised valleys, with people looking for access to perennial water supplies and; ii) a reduction of the number of settlements in shallow U-shaped valleys, mild slopes & mid-slope ridges, perhaps due to movement of population to more arable land on the plains and U-shaped valley floors (Fig 6) [6,33,38]. In Phaistos district, the Early Minoan settlements are located in heterogeneous landscapes, distributed sparsely over variable terrain. In the Middle Minoan the settlements are found over homogeneous landscapes, such as the plains. In the Late Minoan the largest settlements are found on the plains and lowlands. During the Middle to Late Minoan, the number of settlements decreased, especially at higher elevations; the settlements on the plains remained, while farming increased and population concentrated in the larger settlements [6,33,38].Based on the analysis of the area percentage of the Minoan settlements in Phaistos district, population increased within the deeply incised valleys, perhaps due to the need for perennial water, and on the plains and foot slopes, perhaps due to increased arable land access. However, population decreased on the upper slopes, mesas and high ridges, perhaps as a result of the observed movement to arable lowlands (Fig 6). The accuracy assessment, comparing the findings of this study with the archaeological surveys of [6] (Table 8), shows a low percentage of no agreement (14%) while there is a substantial percentage of agreement (64% of moderate agreement and 22% of high agreement). That implies that the rapid low-cost geoinformatic techniques used in this study are a cost-effective technique that can complement conventional archaeological surveys. PPT PowerPoint slide

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larger image TIFF original image Download: Table 8. Accuracy assessment, whether there is an agreement or not, comparing the coverage percentage of the individual landforms classes, based on TPI, and the settlement geological-geomorphological description provided from the archaeological surveys of Hayden et al. (2004). The qualitative agreement type was based on the overall areal percentage coverage of associated landform types: Low (<50%), Moderate (50–70%) and High (>70%). https://doi.org/10.1371/journal.pone.0170727.t008

Conclusion During the last decade the quantification of landform types has attracted the interest of the geoinformatic research community, with various studies using GIS-based approaches. The evaluation of geomorphometric datasets can be integrated with GIS techniques, highlighting the information within the interlinked geographic data. The extracted information can be particularly useful for landform classification, as this case study of Crete has illustrated. Such information is useful when linked to geospatial data about the distributions of archaeological sites over space and time. This investigation of Phaistos, Kavousi and Vrokastro districts has produced valuable information regarding the distribution of settlements during the Early, Middle and Late Minoan periods. Based on the analysis of this study, a general trend is observed during the Early to Middle Minoan, with population moving from heterogeneous terrain at higher elevation, to the lowlands (Fig 6). During the Middle to Late Minoan, the population remained in the arable lowlands, with better organization and concentration in larger settlements. This study was constrained by the limited amount of paleo-environmental data available for Crete, resulting in a knowledge gap that adds to the uncertainty associated with the interpretations made about the factors driving the variations in the Minoan settlement distributions. Nevertheless, the methodology presented here can provide useful spatio-temporal analyses, at district scales, for future studies to examine at local scales, with associated studies of palaeo-environmental conditions or archaeological predictive modelling. In addition, this study offers valuable information for further research, where socio-economic or political factors can be considered for settlement hierarchy assessments.

Supporting information S1 Fig. TPI or DIFF for EM, LM and MM period on Kavousi-Vrokastro region, with six morphologic classes for the neighbourhood sizes: a) 150 m; b) 300 m; c) 600 m; d) 1200 m. https://doi.org/10.1371/journal.pone.0170727.s001 (TIF) S2 Fig. DEV for EM, LM and MM period on Kavousi-Vrokastro region, with six morphologic classes for the neighbourhood sizes: a) 150 m; b) 300 m; c) 600 m; d) 1200 m. https://doi.org/10.1371/journal.pone.0170727.s002 (TIF) S3 Fig. Slope position classification based on TPI of the case study sites of Kavousi-Vrokastro, for EM, LM and MM periods, with six morphological classes for the neighbourhood sizes: a) 100 m; b) 300 m; c) 600 m; d) 1200 m; e) 2000 m. https://doi.org/10.1371/journal.pone.0170727.s003 (TIF) S4 Fig. Landform classification based on TPI of the case study sites of Phaistos, for EM, LM and MM periods, with ten landform types for the combined neighbourhood sizes: a) 100 m and 600 m; b) 300 m and 1000 m; c) 300 m and 2000 m; d) 600 m and 2000 m. https://doi.org/10.1371/journal.pone.0170727.s004 (TIF)

Acknowledgments The authors would like to thank the anonymous reviewers for their comments that improved the quality of this manuscript.

Author Contributions Conceptualization: AA AS RT. Data curation: AA. Formal analysis: AA. Funding acquisition: AS. Investigation: AA. Methodology: AA AS RT. Project administration: AA. Resources: AA AS. Software: AA. Supervision: AA AS RT. Validation: AA. Visualization: AA. Writing – original draft: AA. Writing – review & editing: AA AS RT.