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Contents:
1) The principles
2) The applications of Geoarchaeology
3) The difficulties
4) A case study: the Tiber valley

1)
The principles

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A major part of the historical-cultural heritage is still buried in the subsoil, treated by an advancing degradation. This patrimony does not consist in single archaeological sites, but in areas and landscapes of a certain extension.

The inclusion of the earth sciences among those traditionally applied at the archaeological aims, may bring about a more correct and efficient data interpretation and allows moreover the tracing of better space-time relations between the various sites.

Only part of the earth sciences can be aplied usefully, i.e. that part that refers to the last geological periods (Quaternary) and that concers specifically the dynamics of the processes at the earth surface.

A definition of  geoarchaeology is: archaeological research, applying the concepts and methods of earth sciences. Thus, geoarchaeology shares with  archaeology the field of research and part of the aims.

Geoarchaeological initiated, together with modern geology, in the first half of the last century, around the question of the age of mankind. It was rediscovered only from the seventies on, together with the development of modern archaeology. It is now fully recognized that, before correlating the excavation data with culture, the possible modifications brought about by depositional and post-depositional processes should be evaluated.

The application of the techniques and methods of  geological sciences to archaeological research is justified by the consistency of earth shaping processes and their resulting stratigraphies. And it is necessary since it has been realized that the association of items within the deposits at the excavation is not merely the result of contemporaneous or episodic human activity, but often a consequence of the influences of numerous natural formation processes acting through time.

Useful techniques and methods are available within the wide range of possibilities of geoarchaeology, among which particularly: physical geography (the study of earth surface dynamics), sedimentology (the study of sedimentation processes and sediments), geophysics (offering the techniques for non destructive prospection) and  soil science (the study of soils and the transformation processes of the earth surface).

2)
The applications of Geoarchaeology

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Hereafter follows a short outline about how and when geoarchaeology can be involved in the study and management of the historical-cultural heritage (escluding willingly the important aspects related to the study of contruction material and restauration, of dating techniques, of paleontology, etc.; see Websites on Geoarchaeology).

Archaeological prospections
Geophysics offers a wide range of non destructive prospection techniques, among which georadar, geoelectrics, geomagnetics and thermic infrared measures. Also in the case of trench prospection, the methods of geoarchaeology allow a more efficient layout planning and therefore a more favourable cost/benefit rate for the operation.

Archaeological survey
A large part is of the archaeological heritage still remains buried just below the earth surface, essentially within the soil (study object of pedology or soil science). The application of the methods of geoarchaeology, particularly those of soil science, allows a correcter prevision, identification and interpretation of the archaeological survey data, especially in the case of pre-protohistorical material.

The archaeological excavation
The excavation site is generally composed of naturally or culturally formed layers containing archaelogical items. The sedimentary processes are in may cases similar to those in natural sedimentary basins, since the laws of sedimentation, determined by the gravitational and hydraulical forces, are equally valid both in natural and anthropical environments. The involvement of geoarchaeology implies the optimization of the excavation project. 

Interpretation of the data
The reconstruction of the original environmental conditions requires a profound knowledge of the dynamics of earth surface processes. A correct interpretation of the paleo-environment is necessary not only from the scientific viewpoint, but also for education and divulgation scopes. 

Management of the cultural heritage
The idea is spreading of an archaeological research all-in, from prospections and survey, to excavation and management. At the Italian national level the possibilities of such an approach are still largely unexplored. Also in this case, the application of geoarchaeological techniques may be cost and time saving.

3)
The difficulties

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The application of earth sciences to archaeology merits a few short reflections, based upon practical problems encountered. 

The time scale
The events interesting to archaeology occurred within the range of Upper Pleistocene and Holocene, rarely within the Middle Pleistocene. In the practice of geoarchaeology, it is therefore necessary to adapt to the short and detailed time scale of archaeology. Case by case, the possible impact of terrestrial changes on the paleo-environmental balance should be evaluated: neotectonics, volcanism, seismics, sea level variations, climatic variations, shifting of river meanders, slope processes (erosion, accumulation), loss of topsoil due to (pre-) historical woodcut, variations in spring flows related to changing land use, effect of erosion on the watershed's hydrological balance, etc.

Limited space
Archaeological relicts are scarse, the space at the excavation is limited. Therefore it necessary for the earth scientist to adapt to archaeological practice also in this respect. Often, a paleo-environmental reconstruction must be carried out on the base of a few data, often collected on sites not properly ideal from the geological viewpoint. Moreover, both the extreme detailed level applied in archaeological stratigraphy and the criteria used for distinction of the layers, the latter often quite different from those used in sedimentology, require a revision of the traditional geological approach. 

The partially "urbanized" environment
Earth sciences are traditionally applied in strongly natural environments, whereas the archaeological paleo-environmental conditions are often more or less urbanized. Therefore, the geoarchaeologist must keep in mind the principal aim of archaeology, which is the reconstruction of the past cultures through the "archaeological correlation", a process in which geoarchaeology assumes necessary a subordinate role. 

The unique character of the archaeological data
In contrast to the geological layers, the archaeological strata, ones excavated, cannot be reexamined: always (if not in extremely important cases) the excavation brings about the destruction of the stratigraphical context. 

The dynamics of earth processes
To the question most frequently asked to the geoarchaeologist - how much time did it take for this layer to deposit? - there is almost never a useful answer, particularly not if the layer has a strong natural component (e.g. layers deposited by running water). The reason for this difficulty must be sought in the dynamics of earth environments, characterized by cycles of formation and destruction of layers, with a frequence variable between daily and thousands of years or still wider. The archaeologist would like to know: for how long was this spot dominated by the specific environmental conditions testified by this layer. But what the geologist sees and can answer about is only the ephymeral expression, moreover casually preserved, of a situation that may have lasted a few days but also hundreds of years. 

4)
A case study: the Tiber valley

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Since Roman times, both the coastline near Rome and the course of the Tiber river, have been subject to remarkable variations. The coastline near the mouth of the Tiber river has moved seawards for various kms, with the consequential silting up of the harbours constructed by the Roman emperors Claudius and Trajan.

The phases of coastline advancement have been reconstructed through the position of the Roman buildings and the medieval coast towers. According to the geological map of Cerveteri, about half of the advancement has occurred between 100 and 1570 A.D., whereas the other half is from even later date. There are reasons to presume that the strong advancement phase started only after 500 A.D.

The reasons for this displacement must sought in a combination of factors. In the first place, in land use changes within the Tiber watershed, like deforestation and agricultural expansion. In the second place in climate change. And finally, in sea level variations. In fact, historical sea level changes are indeed known: in Roman times (between 600 and 100 b.C.), the sea level was about 1 mt. lower than today, whereas in the IV-V century A.D. it seems to have been about 1 mt. higher.

The degradation, from the XVI century on, of the agricultural landscape in the hills and mountains behind Rome is well known: steeper slopes then formerly where brought into culture, thus provoking stronger erosion. A climate change, occurring from 1200 A.D. on, consisted in a lower average temperature, heavier winter rains and less summer rains. Such a change leads to less favourable conditions for a protective vegetation cover, and thus potentially to stronger erosion.

Increment of sediment load in this period implied also more frequent and heavier floodings. In fact, the XVI century is characterized by the heaviest floodings known in historical times. Another contribution to these phenomena must have been the obstruction of the Tiber course in the city of Rome, in full urbanistical revival by this time, through buildings, bridges and floating water mills.

Thus, together with the advancement of the coastline, the level of the Tiber valley floor was raised and the equilibrium of its lenght profile restored. As a consequence, in some parts of the lower Tiber valley, tracts of the "Roman landscape" are preserved under fluvial deposits.

Buried below various meters of fluvial sediments, near the Magliana area in the Tiber valley, the relicts of a Roman bridge and dam were found close to each other. Presently, both dam and bridge are quite distant from any river course. The bridge, with a lenght of ca. 40 meters, was anyway too small to have crossed the Tiber river. The dam, with a lenght of about 20 meters, was constructed about halfway the I century A.D.

During the archaeological excavation by the Soprintendenza Archeologica di Roma in 1993, special attention was paid to the natural layers exposed below and above the dam, which allowed the reconstruction of the natural historical events for this area, and which lead moreover to the formulation of an hypothesis on the original function of both the bridge and the dam.

Near the dam and the bridge, a long and narrow depression was encountered, filled in with grey loam. The line of this depression was visible on the aereal photographs for over 700 meters, and turned out to cross the bridge at a right angle. This depression was interpreted as the old course of the Magliana tributary, crossed by the bridge. Moreover, aerial photographs showed traces of the displacement of the course of the Tiber river in historical times.

The dam was oriented parallel to the former course of the Magliana tributary. At a close distance behind the dam, a marshy area was known from medieval times on ("Campo Merlo"), that could well have existed already in Roman times. The hypothesis was forwarded that the dam was constructed to protect the marshy area from the increasing floods in the I century. In CATALLI et alii (1995), detailed maps show both the present river courses as well as their reconstruction for Roman times.

The natural layers exposed near the dam showed below the structure the traces of at least two strong floods, that could be dated (considering the age of the dam) as probably having occurred in the years 15 and 36 A.D. The layers deposited after the construction of the dam showed, through a clear differentation in characteristics before and behind the structure, it's effective functioning, maybe until the IV century A.D., but finally it's total overriding by the later and higher floods.

(Dr. Antonia Arnoldus-Huyzendveld)

Reference

Catalli F., Coletti C.M., Arnoldus-Huyzendveld A. (1995) - Acquisizioni archeologiche e paleoambientali nell'area della Magliana ; in: Archeologia Laziale 1995 XII: 333-340

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