Human–landscape interactions in the Bologna area (northern Italy) during the mid–late Holocene,with focus on the Roman period

Fluvial-channel facies association

Description

This facies association is made up of coarse-grained (gravel and sand) sedimentary bodies, with erosional lower boundaries and either sharp or gradual transition to the overlying muds. Gravel bodies are commonly matrix-supported, with an upward increasing proportion of sand and silt. The gravel/mud ratio decreases with the distance from the Apenninic chain. In the most proximal outcrops, gravel bodies are poorly sorted and disorganized, while moving downstream, they consist of vertically stacked sets with horizontal and high-angle cross-stratification. Gravel bodies are commonly amalgamated, with a overall thickness that may exceed 10 m. Their lateral extension generally exceeds the maximum visible width in quarries (several hundred metres). Tree trunks are commonly found in the lower part of these bodies. In contrast, sand bodies are markedly lenticular, thinner (2–6 m) and significantly narrower (15–40 m). Inclined bedding is a common feature (Figure 2), although high-angle cross-stratification is frequently found. Basal pebble layers are frequently encountered. Organic-rich layers are common atop this facies association. Fossils are absent.

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Figure 2.

Example of facies interpretation from outcrop (see Figure 1, for location), showing stratigraphic location of the two Holocene palaeosols.

Crevasse and levee facies association

Floodplain facies association

Anthropogenic facies association

Holocene stratigraphy

The stratigraphic unconformity close to the Pleistocene–Holocene boundary represents the most valuable stratigraphic marker within the study succession (Figure 3). This surface is a characteristic indurated horizon associated with a remarkable depositional hiatus (Figure 4).

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Figure 4.

Stratigraphic cross-sections (for section traces, see Figure 1) showing contrasting facies architecture between the periurban (section CC′) and urban (section DD′) Bologna areas. Anthropogenic units are amalgamated beneath the town centre, whereas they are separated by overbank deposits north of the urban area.

Two closely spaced palaeosols represent additional stratigraphic markers within the Holocene succession of the Bologna area (Figures 3 and 4). Given their stratigraphic position, just few metres below the ground surface, these palaeosols can be identified using not only core data but also deep excavations (Figure 2). The lower palaeosol, commonly encountered 5–7 m below the ground surface, is highly discontinuous (Figures 3 and 4). A variable thickness (commonly about 2 m) of overbank and crevasse deposits separates this palaeosol from the upper one (Figure 2), which exhibits instead significantly greater lateral continuity (Figures 3 and 4). Based upon existing and new radiocarbon dates (see Supplementary material), we were able to establish a precise chronological attribution for these two palaeosols. Particularly, the older palaeosol was radiocarbon dated to about 8000–5000 cal. yr BP, while an age of about 3200–1500 cal. yr BP was assigned to the upper palaeosol, based upon combined ¹⁴C dates and archaeological evidence (see section ‘Development and burial of the Iron Age–Roman palaeosol’).

Archaeological digs and excavations in the subsurface of Bologna unearthed an abundance of archaeological remains associated with the Holocene palaeosols. The older palaeosol bears evidence of human settlements and artefacts of Neolithic and Early Eneolithic age. Late Eneolithic rests, dated to 4100–3800 cal. yr BP, were documented uniquely close to the Reno outlet and in the small town of Castenaso, 10 km east of Bologna (Cadeddu et al., 2011). The younger palaeosol contains material from the Late Bronze Age to the Roman period (Figure 5). However, remains of Late Bronze Age are relatively uncommon, and the palaeosol can be regarded as developed almost entirely during the Iron Age–Roman period. There is no evidence in the Bologna area of earlier (Early–Middle Bronze Age) settlements, such as the terramare culture, which are instead well documented in other sectors of the Po Plain (Barfield, 1994; Cardarelli, 1997; Cremaschi et al., 2006; Greig, 1984).

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Figure 5.

Examples of composite palaeosol recording distinct stages (Late Bronze, Iron Age and Roman) of human occupation (see Figure 1, for location).

The next sections explore the archaeological characteristics of the Iron Age–Roman palaeosol, for which a wealth of archaeological material are available (Figure 6).

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Figure 6.

Types of archaeological findings in the subsurface of Bologna: (a) rest of a Roman farm (villa rustica) in plan view, discovered during the excavation for the construction of the new Municipality of Bologna, north of the historical centre (Archive of SAER); (b) Roman road (glareata), N-S oriented, along the N prolongation of the cardine maximum (see R5 in Figure 7) (SAER); (c) Villanovan double (bi-conical) cinerary urn (Civic Archaeological Museum of Bologna); (d) typical Late Antiquity inhumation (SAER); (e) coin from the grave goods of a Roman inhumation assigned to the 3rd century bc (SAER); (f) plan view and (g) cross-section of an anthropogenic channel dated to the Iron Age (SAER).

Types of archaeological material

The abundance of archaeological remains from the Iron Age–Roman palaeosol and their stacking at distinct stratigraphic levels provide evidence for a sequence of settlements indicative of different cultures and traditions (Figure 5). Each culture was identified by particular architectural styles and death rituals. Archaeological dating of coins and ceramics enabled the attribution of distinct anthropogenic units to specific phases of human settlement. The archaeological material can be classified in four classes. These are summarized below.

Street, bridges and communication routes

The most notable archaeological rests are roads of Roman period, with only subordinate roads of Etruscan age. Roman roads allowed rapid diffusion of the materials and methods of construction (Berechman, 2003), which for these reasons represent temporal markers throughout the Roman Empire. The current knowledge of road construction techniques derives mainly from archaeological surveys, while rare notes are present in the Roman historiography (Statius, Silvae, IV, 3, 40 segg.; Vitruvium, De architectura, VII, 1 segg.; Plinius, Naturalis Historia, XXXVI, 186 segg.). Findings of Roman roads are frequent in the Roman urban area, where a network (partly inherited by the Etruscans) of W-E and N-S oriented streets has been reconstructed (Bergonzoni and Bonora, 1976). The two major roads, the Cardine Maximum (N-S oriented) and the Decumanus Maximum (W-E oriented), subdivide the urban area into four sectors, and represent the link between urban and rural communication route networks (Figure 7). Only sparse evidence of Roman roads is available from rural areas (Figure 6b).

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Figure 7.

Reconstruction of the Roman landscape by integrated archaeological, historical and stratigraphical documentation.

Reconstruction of the Roman landscape

The Roman colony of Bononia was located close to the Apenninic foothills, in a topographically elevated area particularly favourable for human settlement, bordered on the west and on the east by two river incisions (c3 and c4, Figure 7). Archaeological evidence indicates the existence of stable human settlements in this area since already the Iron Age. Archaeological data, integrated with historical documentation and stratigraphic (palaeosol) correlations (Figures 3 and 4), enable the accurate reconstruction of the Roman landscape in the Bologna area. Particularly, three major features of the Roman topography are outlined (Figure 7): (1) surface morphology, depicted by means of 2-m-spaced topographic contours; (2) the river network and (3) communication routes.

In the Bologna area, the modern alluvial plain still preserves traces of the ancient Roman topography. Centuriation, for example, is identifiable from the modern road layout, which is partially organized according to the typical 700-m square grids (Figure 7). An Inceptisol of Roman period, with partly decalcified ‘A’ horizon (up to 80 cm thick), crops out patchily at the Apenninic foothills and in N-S elongated rural areas, W and E of Bologna. In these areas, archaeological rests of Roman period can simply be unearthed by deep ploughing. In contrast, in the town centre, the Roman topography is buried beneath about 5 m of alluvial deposits (Figure 4, section CC′) or is part of a complex succession of stacked anthropogenic units (Figure 4, section DD′).

The contour map reconstructed for the Roman period shows an overall N-dipping topographic surface (Figure 7). Higher gradients are observed close to the Apenninic chain, around the town centre (Bologna interfluve), whereas low-angle dip slopes are reconstructed from relatively more distal positions. The Reno and Savena rivers flowed in significantly different position compared with the modern paths (Figure 1), and merged about 5 km north of Bologna. Five short creeks draining the adjacent Apenninic chain and flowing in S-N direction (c1–c5 in Figure 7) acted as tributaries of Savena River. Contour lines at the basin margin suggest the existence of small, coalescing alluvial fans, the deposition of which likely preceded the formation of the Iron Age–Roman palaesol. These landforms are cut by river incisions which most likely induced soil development.

Significant modification of the river network by human activities is documented close to the Roman urban area. For example, Aposa creek (c4 in Figure 7) is rectilinear and narrower near the ancient urban centre, where it flowed confined by walls. Vallescura creek (c3 in Figure 7), which was active during the Iron Age, was deactivated during the Roman Empire and filled with landfill material. During the Roman Republican age, up to the fall of the Roman Empire, an anthropogenic c3, running through the urban area became active along the Cardine maximum (Curina et al., 2010).

The Roman road layout, partly inherited by the Etruscans, was made up of long, rectilinear stretches connecting the colony of Bologna with the main towns of the Roman Empire. The Via Aemilia (Figure 7) is probably the best example, with a straight stretch 450 km long. The layout of the main Roman roadways displays strong similarities with the modern road network and was significantly influenced by the river network. Roman roads were designed in order to cross the rivers perpendicularly in as few points as possible, and for this reason, their layout provides additional information about the ancient fluvial paths. A striking example is the Decumanus Maximum (Figure 7), which just off the Roman urban area (after crossing channels c3 and c4) splits in three roads (R2, R3 and R4) on the west, and four roads (R3, R6, R7 and R8) on the east (Figure 7). The anomalous cross-cutting relationships between R1 and c3 confirm that during the Roman period, channel c3 was at least partially filled.

Development and burial of the Iron Age–Roman palaeosol

The combination of sedimentological, archaeological and historical data into a high-resolution stratigraphic framework enables us to trace the late Holocene palaeoenvironmental evolution of the Bologna area. With respect to the development and burial of the Iron Age–Roman palaeosol, for which the most detailed stratigraphic information is available, two distinct phases can be reconstructed: (1) 9th century bc to 3rd century ad phase, characterized by high floodplain stability, expansion of human settlements and onset of the Iron Age paedogenesis and (2) post-3rd century ad phase, characterized instead by high fluvial activity, crisis of the human society, contraction of the urban area, mass abandonment of rural areas and burial of the Iron Age–Roman palaeosol.

The first permanent settlements during the Iron Age took place in a topographically slightly elevated tract of land between two rivers. The Iron Age populations started to control the river network and to protect their settlements from flooding, probably also through widespread erection of artificial levees. Stable settlements were also associated with the development of agriculture and the setting of regular patterns of irrigation channels in which flood waters were conveyed (Ortalli, 1993, 1995). Soil development likely started in this context.

During the ensuing Roman period, the control of the river network was pervasive (Figure 7) and human influence on fluvial dynamics prevailed over other factors (Marchetti, 2002). Wide areas of the Po Plain were reclaimed and subdivided into regular square grids of centuriae. River courses in the proximity of the urban area were strictly controlled, maintained and kept cleaned from sediments, preventing channels from silting up. The continuous expansion of human settlements in the town centre area, documented by archaeological material spanning the Iron Age–Late Antiquity period, was favoured by the slightly raised, interchannel location (Figure 7), less susceptible to flooding than the adjacent regions. Distinct topographic surfaces, attributable to the Roman period, the Iron Age and even the Late Bronze Age are locally identifiable in the Bologna periurban area (Figure 5), whereas in the historical centre, they merge into a complex occupational sequence (Figure 4, section DD′).

Starting from the 3rd century ad, the Po Plain experienced a generalized crisis of human society that was controlled by two major events: (1) a widespread plague epidemic led to strong depopulation of the area (Fears, 2004; Gilliam, 1961), as testified by the contraction of the major cities along the Via Aemilia. Some of these once flourishing cities, such as Claterna, a few kilometres east of Bologna, disappeared in this period, to be rediscovered in archaeological surveys only during the last centuries (Brizio, 1892); (2) from northern Europe, which was instead experiencing generalized overpopulation (Pauli Diaconi, Historia Longobardorum, book 1), many groups moved to the Po plain. This stage, known as ‘barbaric invasions’, was probably one of the most important causes in the crisis of the Roman Empire (Anderson and Lewit, 1988). Several towns at the southern margin of the Po Plain were besieged, and the population abandoned the countryside to take refuge in fortified towns, with subsequent loss of control of the river network and degradation of the water flow system (Castaldini et al., 2007).

In the geoarchaeological record of the Bologna area, this period is well documented. During the Late Antiquity, periurban and rural houses were abandoned and partially disassembled to obtain construction materials. The latter stages of use of these dwellings are dated, based on associated coins, to the 3rd to 4th century ad and testify to a decline in the construction techniques, with abundant use of recycling material.

The burial of the Roman settlement by overbank sedimentation realistically took place during the Early Mediaeval Age, consistent with the coeval phase of fluvial aggradation recorded a few kilometres south of Bologna in the Reno River valley (Eppes et al., 2008). In this period, Bologna was a fortified town (Foschi, 1992), reduced to one-fourth of the Roman town. Outside the Early Mediaeval city walls, extremely rare rests of Early Mediaeval Age are documented. In several excavations, the youngest inhumations in the uppermost part of the Roman palaeosol, just below the overbank facies, are confidently dated to the 4th century ad based on the associated grave goods. The oldest archaeological material above the overbank facies is attributed to the Late Mediaeval Age.

Although historical events clearly played a major role in controlling the burial of the Roman settlements in the Bologna area, a contributing effect of climate cannot be ruled out. The possible impact of climate change on the expansion and fall of Rome has long intrigued historians, and it is widely established that exceptional climate stability (i.e. climate warmer than in later centuries) characterized the centuries of the Roman Empire’s rise, especially between 100 bc and ad 200 (Desprat et al., 2003; McCormick et al., 2012; Wang et al., 2012). Recent studies from the adjacent Apenninic and Alpine regions show a good match between the burial of the Roman settlement in the study area and regional climate proxies. For example, advance/retreat reconstructions for the Alpine glaciers during the last three millennia (Holzhauser et al., 2005) indicate that glaciers were larger than today around the 6th century bc (Iron Age) and after the Roman Empire’s decline, with major advances during the course of the 6th century ad (Early Mediaeval Age) and around the 16th century ad (Little Ice Age). A synchronous sharp increase in climatic humidity between 1800 and 1500 yr BP has been documented across the central Mediterranean (Alps, Italy and Turkey) and Central Europe, using a vast range of proxy archives and model simulations (Büntgen et al., 2011; Dermody et al., 2012). This is also confirmed by several historical notes (Pauli Diaconi, Historia Longobardorum, book 3).

In the light of these remarks, it is apparent that overbank deposition atop the Iron Age–Roman palaeosol took place under increasingly cooler and wetter conditions. The burial of the Roman settlements soon after 500 ad can thus be viewed as the result of combined historical events and disadvantageous climate conditions. Climatic deterioration increased the incidence of large floods (Macklin and Lewin, 2003), whose devastating effect was enhanced by the sudden blackout in the control of the river network, resulting in the widespread flooding of previously protected interfluvial areas.