Why conduct new non-destructive surveys of known, excavated sites?

The Celtic oppida of Bohemia are a representative of a particular type of extensive hillfort that originated in the late La Tène period, and which has been investigated in the form of usually prolonged, intensive archaeological excavations. The subsequent processing of such investigations of what are normally complex archaeological situations with a large number of artefacts has required, and will yet require, many years of intensive archaeological work. In terms of the areas of the oppida, only small fractions of the total have been subjected to detailed excavation. As it is unlikely, given costs, time constraints and required labour intensity, that these sites will ever be excavated in toto, it would seem to be appropriate to attempt complementary forms of the monitoring, survey, identification and assessment of whole localities. The application of modern, less financially demanding,non-destructive methods can now offer further opportunities for the systematic survey and research into hillforts, conducted in co-operation with archaeologists.

The first systematically implemented wide-area surveys of selected hillforts in Bohemia were possible only after certain investments had been made in 1998 (Reference KrivánekKrivánek 2001c, 2003a) as part of the archaeological "Settlement Patterns in Prehistoric Bohemia" project, supported by the Grant Agency of the Czech Republic (1997-2002 - Gojda et al.). The advantages of wide-area geophysical measurements within fortifications with no structures surviving on the surface were proven by the outcome of the pioneer study in Bohemia on "The identification of destroyed fortifications and internal settlement structures at hillforts", conducted with the support of the Dept. of Monument Preservation of the Czech Ministry of Culture (Grant no. PK99P04OPP007 - Krivánek 1999-2000). The project results, applicable within both archaeology and heritage management (Reference KrivánekKrivánek 2000, 2001ab), demonstrated the validity of a new, fast and inexpensive method of detailed area survey for significant fortified archaeological sites.

The optimal methodological combination for the extensive and intensive survey of non-excavated parts of oppida would seem to be the use of wide-area geophysical survey along with other non-destructive techniques (such as surface survey, surface artefact collection). Their rapid and notably inexpensive results can play a critical role in the processes of monitoring site structure, evaluating large territorial units fromprevious archaeological excavations and more effectively protecting entire archaeological sites. The primary field method in the project is sufficiently detailed (yet nevertheless wide-area) geophysical survey of oppida selected on the basis of earlier surveys and excavations, sometimes undertaken on ploughed fields along with systematic surface artefact collection, the processing of historical maps and selective verification in the form of test pitting or employing the results of aerial archaeology (or the verification of significant new findings by microprobe under the supervision of archaeologists). The composition of the research team, and the anticipated contribution of the individual scientists and specialists (see below) reflects the nature of almost exclusively non-destructive wide-area survey of oppida as unique and specific sites.

Site Selection

Celtic oppida commonly extend over up to several dozen hectares of highly variable landscape (slopes, hills and/or plains; wooded, grassy and/or ploughed; etc.). The selection of appropriate terrain within an oppidum for wide-area survey should be based on the archaeological data available and on a knowledge of the site, its components and current state, all as determined by the researcher and archaeologists involved in the project. It is therefore necessary from the outset to respect the practical aspects of measurement efficiency, and select propitious, suitable and accessible areas for investigation, eliminating unacceptable areas, areas with a significant intensity of recent activity (including earlier excavations) and areas otherwise subject to disturbing influences. For the purposes of this project, the following oppida in Central and South Bohemia were selected (Figure 1).

Figure 1.

Locations of Celtic oppida in Central and South Bohemia selected for non-destructive survey within the project.

Hrazany (Radíc cadastre, Príbram district)

The oppidum is located in the area known locally as 'Na Hrádnici', on the right bank of the Vltava river, next to the confluence with the Mastník stream. The greater part of the site is on meadowland (former fields), with some smaller parts in the surrounding woods. The overall fortified area, including the baileys, runs to some 39 ha. Systematic archaeological investigations were conducted by the Institute of Archaeology in Prague in 1951-1963 (Reference JansováJansová 1986, 1988, 1992; Drda & Rybová 1997). Human activity on the site has been identified from as far back as the Palaeolithic and Mesolithic periods, an Eneolithic settlement and another settlement dating to the Bronze Age and Hallstatt period. The oppidum is divided into an inner, central part (containing farmsteads, iron-processing and smithing areas, and with four gates) and two fortified baileys (with two further gates). The oppidum(probably used from 120-50 BC) was destroyed by fire, restored, and destroyed by fire again. The site has been proposed as a national heritage monument.

Nevezice (Písek district)

The oppidum is located in the area known locally as 'Hrad' (lit. 'castle, stronghold'), on the left bank of the Vltava river, above an indented stream mouth. The inner parts of the site are on meadowland with some woods, while the outer parts lie on arable fields. The fortified area covers around 13 ha. Survey and rescue excavations were conducted here in 1949-1951 (Figure 2) and 1980 (Reference DrdaDrda 1987; Drda & Rybová 1997); the earliest settlement identified dates to the late Bronze Age. The inner area is undivided, and entry to it is gained through double ramparts to the north-east; there is another gate in the south-eastern part of the fortification and the slopes are terraced. This oppidum was destroyed by fire.

Figure 2.

Photo-documentation of the old archaeological excavation of the Nevezice oppidum by Svoboda 1949-1951, outside the main rampart with outer ditch now located on arable fields.

Stradonice (Beroun district)

The oppidum is located in the area known locally as 'Hradište' (lit. 'hillfort'), west of the village and above the confluence of the Berounka river and Habrový stream. The larger, central part of the site lies on arable fields (Figure 3), with a smaller area lying on meadows. The fortified area covers 90.3 ha. Numerous finds including hoards of gold coins were obtained from the site at the end of the nineteenth century. The first archaeological investigations were conducted by Stocký in 1929, while rescue excavations linked to the construction of a gas pipeline were undertaken by the Prague Institute of Archaeology in 1981 and 1982-1984 (Rybová & Drda 1994; Drda & Rybová 1997). The earliest settlement identified dates to the Hallstatt period. Judging from the surviving stretches of the fortifications, the oppidum can be divided into an inner area and wide outer fortified zone (with craft workshops and a mint), originally with 4 major gates. The oppidum was destroyed in the second half of the first century BC. The site has been proposed as a national heritage monument.

Figure 3.

Aerial photography (by Gojda 2003) of the intensively ploughed-out central part of the Stradonice oppidum.

Trísov (Ceský Krumlov district)

The oppidum is located in the area known locally as 'Hradište - V hradišti' (lit. 'Hillfort - in the hillfort'), north of the village, in a meander of the Vltava by the mouth of the Kremžský stream. The inner and outer parts comprise recently ploughed fields or meadows (formerly fields), with the peaks mainly in the surrounding woods. The fortified area covers some 26 ha. Systematic archaeological investigations were conducted by theNational Museum in Prague from 1954-1982 (Reference BrenBren 1966, 1975; Drda & Rybová 1997). The oppidum is divided in two plataeus (hills) and an extensive inner area (wooden structures - workshops) with a doublefortification system along the western side, and west (Figure 4) and east gate. The oppidum, dating to the second half of the first century BC, was abandoned after half a century, around the turn of the millennium. The site is a national heritage monument.

Figure 4.

Archaeological reconstruction (by Reference BrenBren 1966) of the western gate of the Ttrísov oppidum.

Závist (Lhota cadastre, Prague-West & Prague-Tocná districts)

The oppidum is located on two hills known locally as 'Hradište' (lit. 'hillfort') and 'L ance' (lit. 'rampart') on the left bank of the Vltava, above the former confluence with the Berounka and the indented mouth of the Brežany valley. Most of the site is covered by wood, but a large part of the fortified outer area is on arable fields with a new residential development nearby. The fortified area covers approximately 157 ha. Systematic archaeological investigation was carried out by the Institute of Archaeology in Prague from 1963-1989 (Reference CižmárCižmár 1989; Motyková, Drda & Rybová 1984; Drda & Rybová 1993, 1997). A settlement on the site has been identified from as early as the Eneolithic period, along with an extensive Bronze Age hill-fort and intensive settlement of the Bylany culture, the late Hallstatt period and the Early Middle Ages. The fortification shows numerous gates (13 known to date) and is divided into an acropolis, internal settlements, external settlements (baileys) and other components. Five main developmental stages have been identified for the fortification. The oppidum dates to the second to late first centuries BC. The site is a national heritagemonument.

Figure 5.

The magnetometric survey (approx. 3.2 ha; Reference KrivánekKrivánek 2003) at Nevezice distinguished concentrated anthropogenic activities, empty zones and geological changes in the inner part of the oppidum.

Figure 6.

Schematic plan of the Závist oppidum (Drda & Rybová 1997), showing the areas where geophysical surveys were conducted in 2003 until summer 2004.

Problems and Aims

The subjects addressed by the submitted project for large scale non-destructive oppidum survey can be divided into several related categories, with a varying view of the applicability of the results. The results of the geophysical methods, originally associated with the natural sciences but now interdisciplinary, will be applicable to the resolution of partial (individual oppida) and general archaeological expert issues, the recording and protection of significant archaeological monuments and with respect to the methodological presentation of new and unusual applications of geophysics to archaeology. It is necessary that a suitable methodology is employed in the non-destructive mapping within extensive non-excavated parts of oppida areas over the course of the whole project, reflecting the needs of the archaeologists particularly as regards wide-area geophysical measurement (continuous result assessment for the most effective survey), followed by other complementary non-destructive methods. The following may be regarded as feasible project objectives:

1. 1. The identification of the fortification system remnants not surviving on the surface, of the internal structure and the other significant and resolvable structures or activities on arable areas within oppida.

2. 2. The identification of other indistinct or non-preserved parts of fortification systems, of gates, of interior structures and/or of roads within selected grassy or wooded inner areas of oppida.

3. 3. The identification of significant and resolvable structures not surviving on the surface, of their concentrations or the concentration of activities within the surveyed inner areas of oppida, and of presumed structures and activities outside the fortification.

4. 4. Determination of the probable fortification (and/or structure) types and of their current states of sub-surface preservation (or of the scope of later and modern surface modifications and site adjustments to the archaeological contexts).

5. 5. The preliminary or general dating of artefacts or diverse anthropogenic activities identified by geophysical investigation of arable or otherwise degraded areas by archaeologists (the processing of systematic surface artefact collection and surface survey assemblages, etc.).

6. 6. Composition of general maps for the exact identification and registration of the sites, with the verification of other areas or internal structures of oppida for the adequate protection of the entire archaeological monuments.

7. 7. Comparison of the results of non-destructive methods with current knowledge regarding the oppida based on archaeological investigation, the filling out of current information, new maps and possibly new interpretations of the use of individual areas within the sites.

8. 8. An assessment of the benefits of, and further potential for, non-destructive surveys of the oppida, and of the common traits and specific features within individual sites.

Survey Methods

The great advantage held by all modern and often inter-disciplinary non-destructive archaeological survey methods is that they can be applied without disturbing the archaeological features, contexts, artefacts and/or natural environment of the sites concerned. The primary project methodology is based on the wide-area geophysical survey of selected locations. The major geophysical method applicable on such a large scale is wide-area magnetometric survey, the magnetometry being the most suitable geophysical method for the detection of diverse sunken or burned archaeological features surviving beneath the surface. On arable fields and meadows, as well as in wooded areas without low vegetation, it is possible to identify sections of linear structures, various fortification systems, and enclosures/fencing or other zoning (dikes, trenches, palisades, channels, terraces, arable land) - including areas of discontinuity (entrances, gates) - which do not survive on the surface; it is also possible to distinguish larger and smaller sunken isometric structures (houses, clay pits, postholes, graves etc.), structures built of stone (houses, walls etc.), and various manufacturing and burned features (furnaces, mine tailings, spoil heaps, smithies, burned sections of fortifications etc.). On the basis of the geophysical anomalies identified it is possible to make comparisons with otherwide-area measurements and traces settlement structure, the character of anthropogenic activities within the oppida, and the current surface preservation or disturbance to various situations and areas.

Since 1998 both detailed and wide-area magnetometric surveys ofarchaeological sites have been conducted using high sensitive, precise and powerful caesium magnetometers (Smartmag SM-4g model, Scintrex, Canada), with the possibility of making instant measurements of the local magnetic field. From previous experience and the results of geophysical surveys conducted at hill-forts and other fortified locations it may be expected that the project as designed (depending on the measurement density applied - e.g. 1 x 0.25m or 0.5 x 0.2 m with caesium magnetometers) is capable of investigating a total of some 30-60 ha (6-12 ha per year). Where more complex, detailed survey of partial situations and stony features would be appropriate geo-electrical apparent resistivity can be measured (using an RM-15 instrument by Geoscan Research, United Kingdom), as can electromagnetic apparent conductivity or magnetic susceptibility (using an EM-38b instrument by Geonics, Canada). The large anticipated volumes of measured geophysical data (usually running to several tens of thousands of points in a single day!) will be processed using the geophysical software Oasis-montaj (Geosoft, Canada), Envimap (Scintrex, Canada), Surfer (Golden software, USA) and Geoplot (Geoscan Research, United Kingdom).

Figure 7.

The magnetometric survey in the outer, wooded part (aprox. 1.1 ha; Reference KrivánekKrivánek 2003) of the Závist oppidum distinguished the sub-surface remains of 2 separate and previously unknown entrances through the outer multiple ditch and rampart fortification.

Figure 8.

The results of experimental magnetometric prospection in the 'šance' area (approx. 0.45 ha; Reference KrivánekKrivánek 2003) of the Závist oppidum demonstrated the earlier, different use of this recently wooded area as arable fields.

Another non-destructive method used in the project is systematic surface artefact collection. On the arable parts of oppida (and e.g. beyond the perimeter established by the fortifications) geophysical survey results will be complemented by systematic surface survey conducted using a regular, uniform grid; it will be then possible to compare the distribution of artefacts with the interpretation of the geophysical research results. The artefacts collected during surface survey will be evaluated by archaeologists. The grids used for geophysical investigation, artefact collection etc. will be localised using GPS (e.g. a Trimble Pathfinder GPS unit). Where new archaeological situations are identified that no longer survive on the surface, it is also possible to employ a complementary detailed vertical geodetic survey to obtain adetailed vertical model of the terrain. Another complementary and almost non-destructive method that can be used at selected points chosen on the basis of the geophysical survey results is test pitting. The likely presence of metal artefacts on intensively ploughed fields will also be verified using systematic metal detector prospection (equipment by White's Electronics). The application of other destructive methods(microtrenching) for the verification of the most significant results will be considered by archaeologists according to the results of non-destructive investigative methods.

The First New Results

The first year of the non-destructive survey project (2003) brought some new and somewhat unexpected results. Geophysical surveys of selected areas were conducted at four oppida over a total surveyed area of approx. 8.5 ha (Reference KrivánekKrivánek 2004). The wide-area magnetometric survey of the inner, meadow part (approx. 3.2 ha) of the Nevezice oppidum identified various sub-surface situations: concentrated anthropogenic activities (sunken settlement features and the remains of shorter ditch enclosures), empty zones (possible lines of communication) and geological changes in the bedrock (Figure 5). Another magnetometric survey in the outer, wooded part (approx. 1.1 ha) of the Závist oppidum (Figure 6) identified the sub-surface remains of 2 separate and previously unknown entrances (a possible gate and another simple entrance; Reference KrivánekKrivánek 2003b) in the outer system of multiple ditch and rampart fortification (Figure 7). Further, experimental magnetometric prospection of another area at Závist known as 'šance' (approx. 0.45 ha) proved from the distinguished parallel lines of ploughing that this recently wooded area was previously occupied by arable fields (since at least 1780 forest is shown here even on the old maps of the first Austrian military (Josephine) mapping -Figure 8).

Research Team

• • Project lead investigator: Institute of Archaeology of the Academy of Sciences of the Czech Republic RNDr. Roman Krivánek - geophysicist (project leader)

• • PhDr. Petr Drda - archaeologisting.

• • Cenek Cišecký- computer specialist

• • Project co-investigator: National Museum, Prague - Mgr. Zuzana Karasová - archaeologist; PhDr. Pavel Sankot - archaeologist

• • Project co-investigator: Mining Museum, Príbram - Mgr. Rostislav Korený - archaeologist

No. of the project in the Grant Agency of the Academy of the Sciences of the Czech republic: A8002301 (2003-2007).