Multi-Channel Ground-Penetrating Radar Array Surveys of the Iron Age and Medieval Ringfort Bårby on the Island of Öland, Sweden
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remote sensing Article Multi-Channel Ground-Penetrating Radar Array Surveys of the Iron Age and Medieval Ringfort Bårby on the Island of Öland, Sweden Andreas Viberg 1,* , Christer Gustafsson 2 and Anders Andrén 3 1 Archaeological Research Laboratory, Department of Archaeology and Classical Studies, Stockholm University, SE-106 91 Stockholm, Sweden 2 ImpulseRadar AB, Storgatan 78, SE–939 32 Malå, Sweden; [email protected] 3 Department of Archaeology and Classical Studies, Stockholm University, SE-106 91 Stockholm, Sweden; [email protected] * Correspondence: [email protected] Received: 20 December 2019; Accepted: 4 January 2020; Published: 9 January 2020 Abstract: As a part of the project “The Big Five”, large-scale multi-channel ground-penetrating radar surveys were carried out at Bårby ringfort (Swedish: borg), Öland, Sweden. The surveys were carried out using a MALÅ Imaging Radar Array (MIRA) system and aimed at mapping possible buried Iron Age and Medieval remains through the interior in order to better understand the purpose of the fort during its periods of use. An additional goal was to evaluate the impact of earlier farming on the preservation of the archaeological remains. The data provided clear evidence of well-preserved Iron Age and Medieval buildings inside the fort. The size and the pattern of the Iron Age houses suggest close similarities with, for example, the previously excavated fort at Eketorp on Öland. Given the presence of a substantial cultural layer together with a large number of artefacts recovered during a metal detection survey, it is suggested that Bårby borg’s primary function during the Iron Age was as a fortified village. The Medieval houses partly cover some of the Iron Age buildings. They are placed in a U-shape with an open square in the middle facing the edge of a limestone cliff. As in the case of Eketorp, it is suggested that the activities during Medieval times changed, but the precise purpose of the Medieval Bårby settlement is still a question open for debate. Future targeted archaeological investigations are needed in order to better understand its purpose. Rescue excavations may also be necessary, as the western steep cliff ledge is eroding and the well-preserved archaeological remains are at risk of being destroyed. Keywords: ground-penetrating radar; ringfort; Sweden; Iron Age; Medieval; archaeology; geophysical survey; multi-channel GPR 1. Introduction The island of Öland, situated east of the Swedish mainland, is home to several large ringforts dated to approximately AD 300–700 (Figure1). Eighteen ringforts are currently known from historical maps and sources, but only 15 are still visible in the landscape today. The best-preserved fort on the island is Ismantorp which contains the remains of approximately 95 stone foundations of houses still visible inside the fort’s perimeter wall [1,2] (Figure2). Additional smaller archaeological inventories and excavations have confirmed the presence of similar stone house foundations in at least 10 other forts on the island [3,4]. Remote Sens. 2020, 12, 227; doi:10.3390/rs12020227 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 11, x FOR PEER REVIEW 2 of 16 Remote Sens. 2020, 12, 227 2 of 16 Remote Sens. 2020, 11, x FOR PEER REVIEW 2 of 16 Figure 1. Map of the island of Öland and the location of its ringforts. The best-preserved fort on the island is Ismantorp which contains the remains of approximately 95 stone foundations of houses still visible inside the fort’s perimeter wall [1,2] (Figure 2). Additional smaller archaeological inventories and excavations have confirmed the presence of similar stone Figure 1. Map of the island of Öland and the location of its ringforts. house foundations inFigure at least 1. Map 10 otherof the fortsisland on of theÖland island and the[3,4]. location of its ringforts. The best-preserved fort on the island is Ismantorp which contains the remains of approximately 95 stone foundations of houses still visible inside the fort’s perimeter wall [1,2] (Figure 2). Additional smaller archaeological inventories and excavations have confirmed the presence of similar stone house foundations in at least 10 other forts on the island [3,4]. Figure 2. The Ölandic fort Ismantorp with well-preserved Iron Age building foundations. Photo: J. Figure 2. The Ölandic fort Ismantorp with well-preserved Iron Age building foundations. Photo: J. Norrman (1997), ©RAÄ. Norrman (1997), ©RAÄ. Given that the Ölandic forts are very large, any archaeological excavations within them are bound to be both expensive and time consuming. This has led to a situation where only limited archaeological excavations have been carried out over the last 40 years. One example is the fort at Löt inFigure north-eastern 2. The Ölandic Öland, fort where Ismantorp three with square well-preserved metres have Iron been Age excavated building foun in adations. fort measuring Photo: J. over Norrman (1997), ©RAÄ. Remote Sens. 2020, 11, x FOR PEER REVIEW 3 of 16 Given that the Ölandic forts are very large, any archaeological excavations within them are bound to be both expensive and time consuming. This has led to a situation where only limited archaeological excavations have been carried out over the last 40 years. One example is the fort at Löt Remote Sens. 2020, 12, 227 3 of 16 in north-eastern Öland, where three square metres have been excavated in a fort measuring over 1.6 ha in size [5]. Needless to say, it is difficult to provide any accurate interpretations based on such a limited1.6 ha in source size [5 material.]. Needless As to a say,consequence, it is difficult and to providein order anyto properly accurate understand interpretations how based and onwhy such the a fortslimited were source constructed, material. other As aways consequence, of investigating and in these order structures to properly are understand needed. how and why the fortsTo were provide constructed, new insights other ways into ofpast investigating activities within these structures the large are Ölandic needed. ringforts, geophysical investigationsTo provide were new suggested insights into as pasta suitable activities starting within point. the large The Ölandicnon-destructive ringforts, character geophysical of geophysicalinvestigations surveys were suggested and their as ability a suitable to startingcover large point. areas The non-destructivewithin a reasonable character timeframe of geophysical were importantsurveys and factors their when ability deciding to cover on large a way areas forward. within a reasonable timeframe were important factors whenTherefore, deciding onthe a project way forward. “The Big Five” was initiated in 2014 (see Reference [6]). The project, fundedTherefore, by the theSwedish project Research “The Big Five”Council was and initiated the inSwedish 2014 (see Academy Reference of [6 ]).Letters, The project, History funded and Antiquities,by the Swedish aimed Research at using Councilmulti-channel and the ground-penetrating Swedish Academy radar of Letters,(GPR) instruments History and to Antiquities,investigate severalaimed atlarge using Ölandic multi-channel ringforts. ground-penetrating The purpose of radar the (GPR)surveys instruments was to assess to investigate the preservation several large of archaeologicalÖlandic ringforts. remains The purpose within ofthe the forts surveys as well was toas assessthe effect the preservation of past and of extensive archaeological agricultural remains activities.within the The forts surveys as well also as theaimed effect at oflocating past and house extensive foundations agricultural and internal activities. house The features surveys (e.g., also postholes,aimed at locating hearths) house within foundations the forts. If and spatial internal plans house could features be created, (e.g., postholes,it would provide hearths) a within valuable the empiricalforts. If spatial foundation plans for could future be created,archaeolog it wouldical research provide and a valuable targeted empiricalexcavations. foundation for future archaeologicalPrevious geophysical research and surveys, targeted conducted excavations. in one of the smaller forts on Öland, Sandby borg, in 2010, Previousindicated geophysical that GPR surveys, would conductedbe a suitable in one geophysical of the smaller method forts on Öland,for the Sandby detection borg, of in such 2010, archaeologicalindicated that GPRfeatures would (see be Figure a suitable 3) [7], geophysical and yearly method excavations for the have detection highlighted of such the archaeological benefits of geophysicalfeatures (see surveys Figure3)[ as7 ],the and starting yearly excavationspoint and foundation have highlighted for archaeological the benefits of research geophysical within surveys the Ölandicas the starting forts [8]. point and foundation for archaeological research within the Ölandic forts [8]. Figure 3. (a) GPR results from Sandby borg. (b) Interpretation of the GPR results from Sandby borg Figure 3. (a) GPR results from Sandby borg. (b) Interpretation of the GPR results from Sandby borg showing multiple Iron Age house foundations within the fort (results and interpretations presented in showing multiple Iron Age house foundations within the fort (results and interpretations presented Reference [7]). (c) Spatial layout of the already excavated Ölandic ringfort Eketorp II [9]. in Reference [7]). (c) Spatial layout of the already excavated Ölandic ringfort Eketorp II [9]. Multi-channel array surveys have subsequently been carried out in four