The upper layers of the earth form a continuum along the earth's surface, but their characteristics vary from place to place, due to variation in formation processes in time and space and in the sources from which they originated.
Various disciplines are involved in the study of earth layers, but with different aims and often operative in different places.
So, earth layers will be considered worthwile for archaeological excavation only when they (and their contents) possess an obvious relationship with a determinate socio-cultural system.
In most cases, only a small portion of a site is man-made and most of it is formed naturally, implying that an excavation site should be studied as a natural phenomenon in which traces of human activities are manifest.
The various natural processes that contribute to the formation of an archaeological stratigraphy, may result in layers (due to accumulation), to interfaces (due to erosion) or to soils (due to exposition on the surface, without accumulation or erosion).
The geoarchaeological consultancy at the excavation consists in the analysis of the exposed stratigraphy from an Earth Sciences viewpoint and in the integration of the site in a wider landscape context.
Near Ostia, in Roman times the Tiber river followed a narrow meander. The area enclosed by the meander belonged structurally to the city of Ostia. During the disastrous flood of 1557, the meander was naturally cut off, leaving behind a characteristic oxbow lake and moreover an isolated area, known from that moment on as the "Fiume Morto" (Dead River).
Undated engraving entitled "Ostia e il suo
During the excavation by the "Soprintendenza Archeologica di Ostia" in 1992, along the inner bank of the abandoned river course, three phases of meander shift could be distinguished: one of Roman times (end of the first century A.D.) and two much later ones, probably from 1530 and 1557. The flooding events were dated by combining the archaeological data with the list of known historical floods.
Each cycle of events could clearly be deduced from the layer structure: first erosion, than sand deposition, followed by a horizontal fine layering. Lateral river bed displacement was, in this tract of the river, for each event about 4 meters.
The evident stability of this tract of the river course between the I and XVI centuries coincides with the functioning of an artificial river outlet near Fiumicino, the "Fossa Traiana", which was dug near the end of the I century A.D. This rather precise time overlap seems to indicate the effective functioning of this channel, not only as a water way but also as a buffer against the energy surplus of the river during high water stands.
Antonia Arnoldus-Huyzendveld & Lidia Paroli, 1995: "Alcune considerazioni sullo sviluppo storico dell'ansa del Tevere presso Ostia"; in Archeologia Laziale 1995 XII, pp 383-392.
The small statue of the Venus of Savignano, older than 30.000 years, was discovered in 1925 in "Savignano sul Panaro" (province of Modena), on a fluvial terrace of Upper Pleistocene age. The exact finding place of the statue is not known, but there are arguments to presume that it is was present at a depth of about a meter, i.e. within the soil of the terrace.
From the excavation report of 1926, one deduces that the soil profile on the site was composed of a thin organic layer overlying a compact clayey layer, strongly colored, first yellowish and then reddish, with strong iron and mangenese mottling. At a depth of about 1.50 meter the clay overlaid the strongly weathered pebble of the fluvial terrace.
On the basis of this description and of the observation of the indisturbed soil samples, the profile could be collocated among the soil mapping units of the maps of the Region Emilia Romagna.
The strong soil development suggests an initial pedogenesis in the relatively warm period preluding the Wurmian Pleniglacial. Moreover, the dominantly loamy texture (60% silt) observable in the profile samples, points to an aeolian origin, i.e. to a loess deposit on pebble layers.
So, in this paleo-environmental study, a precise consistency turned out to exist between the archaeological, geological, pedological and paleo-climatic data, all converging towards the hypothesis of a primary position of the small statue.
Antonia Arnoldus-Huyzendveld, 1996: "Contexte pédologique", inserted in: Margherita Mussi: "Problèmes réecents et découvertes anciennes: la statuette de Savignano" in: Bulletin de la Société Préhistorique del lAriége, Tome LI, Année 1996.
Two of the many analytical techniques available for archaeological purposes will be illustrated here: phosphate analysis and diffractometry.
The purpose of the technic is the detection and delineation of archaeological sites through the measurement of the quantity of phosphates present in the soil. Phosphates are chemical substances indicating a specific human activities, such as the preparation and conservation of food, the deposit of liquid and solid wastes and burial.
The detection can be executed directly in the field ("spot method") or else in the laboratory on soil samples. In the latter case, the total phosphate content is normally measured.
The main advantage of the technic is the immediate response to traces of specific organic materials. Especially field measurements are quick and rather cheap. Laboratory techniques are more precise, but more expensive.
A limitation of the technic consist in disturbances due to other phosphate sources, e.g. from fertilizers used in modern agriculture.
The best results with this methods have been reached in England, Ireland, Skandinavia and the United States, e.i. in the humid climatic zones. In mediterranean conditions, the result of phosphate analysis have not been always satisfactory up to now.
Diffractometric analysis (X ray analysis)
Through X ray analysis, the mineralogical composition of a sediment or object can be determined.
The procedure consists in examining the X rays diffracted by the atoms contained in a sample, when excited by a bundle of energy. The radiation emitted by the sample propagates in all directions and phenomena of interference occur between the radiation from different atoms.
If the material hit by the X ray bundle has a crystalline structure, even at a micro scale, the diffraction pattern presents a characteristic regularity, from which important data can be derived regarding the composition and structure of the crystall.
A case study: the fossilization of the Polledrara faunal remains
The importance of the Polledrara site near Rome is linked to the discovery not only of Lower Paleolithic industry but also of fossil vertebrate remains. The species are dominantly large mammals, like elephant, ox, deer, horse and wolf.
The bones are compact and well fossilized. Through diffractometric analysis their compositon has been established as microcrystalline fluoro-apatite. The faunal remains were associated with layers containing whitish and friabile nodules, identified by X ray analysis as pure fluorite (CaF2).
The process of fluoritazation of the bones is supposedly related to the closing period of the volcanic activity near Rome, and in particular to the circulation of deep water and mineralizing magmatic fluids, reaching the surface through main fractures, in the form of liquid or gas.
Anzidei A.P., Angelelli A., Arnoldus-Huyzendveld A., Caloi L., Palombo M.R., Segre A.G., 1989: "Le gisement pleistocène de La Polledrara di Cecanibbio (Rome, Italie)", in -L'Antropologie-.