The aim of pedological mapping is to inventoriate the environmental compartment "soil", intended as the earth's surface layer. Soil is the container of most of the archaeological remains and also an indicator of (paleo-) environmental conditions.
In mapping, soils are not considered as isolated land elements, but are placed in the "physiographical" context of their geomorphological, lithological and sedimentary environment.
The working method of soil mapping consists in a survey of the spatial variability of soils. This is basically field activity, with the use, during the preparatory phase, of geological maps and aerial photographs.
The legend of the pedological map lists the various soil types of the area, first in descriptive and then in applicative terms. For instance, a single legend unit of a soil map may be thus composed (from the Soil Map of Rome, 1999):
Specific indications for the archaeological purpose may be added to the legend, such as:
The advantages of soil maps emerge mainly in archaeological survey projects. In the first place, before starting the activity, the survey can be planned more accurately. During the survey, soil characteristics may supply indications on primary or secundary position of the finds. In the interpretative phase of the project, the soil map allows a confrontation between the landscape units of various "ages" and the spatial distribution of the finds of various periods, and offers therefore potentially a more real vision of the original distribution of the relicts for each period.
The minimum soil map scale for a survey project is 1:25.000, but 1:50.000 may still be sufficiently detailed. n some case, a more map scale is necessary.
Costs of soil mapping vary with scale, terrain morphology, observation density and the aims of the mapping. For instance, a normal soil survey (escluding laboratory analyses) at scale 1:50.000 may cost between tra 3 and 5 Euro per hectare, and at scale 1:25.000 between 6 e 18 Euro per hectare.
Other thematic maps may be useful for archaeological studies. Some examples are: distances of sites to valleys, land units probably existing in a specific period, distribution of resources (water, clay, flint, etc.).
A provenience study was executed for the lithic instruments fgound at "La Polledrara di Cecanibbio", a Lower Palaeolithic site located in the northwestern part of the region of Rome. The site is associated to the Aurelia Formation, correlated with isotope Stage 9, its age being about 0.300 m.y., but is probably slighter older.
Six excavation campaigns from 1985 to 1995 have been executed by the "Soprintendenza Archeologica di Roma". Over 7,000 faunal remains have already been revealed. Elephant bones are very abundant. Up to now, the remains of at least ten individuals, mostly adult, have been recovered. Two well preserved elephant skulls were identified, both belonging to adult males.
Human activity is confirmed by the presence of a lithic industry on siliceous pebbles, by some worked bones, and by lava stones. All flint pebbles and lava stones were evidently brought by man, since they do not belong to the immediately surrounding geological context. The siliceous pebbles discovered in the deposit show mainly a dark grey colors, with a black or grey, thick cortex.
A preliminary geological study and counting of the flint types in samples of the surrounding pebbly formations, has led to the mapping of four different "flint provinces" for Rome.
Flint pebbles of similar size, colour and texture as those encountered at the Polledrara, occur in a part of the "Ponte Galeria Formation", presently exposed about 3 km to the south of the site. Also other formations contain the same dark-grey flint, among which the pebbly part of the Aurelia formation, representing a river bed contemporaneous to the site.
The whole geological complex containing this dark grey pebble type was defined as the "Tiber flint province". Typical are total flint percentage of 10 - 30 %, of which about one fourth of the Polledrara type.
It turned out that during the Middle Paleolithic period of the Roman area, these pebbles were used very scarsely, being prefered by then evidently the flint from the formations identified as the "Pontina province". The latter is characterized by much higher total flint percentages (50-70 %) and an almost total absence of the Polledrara type.
In order that the archaeological remains of a specific period can be detected, a series of specific conditions must be fulfilled for the time span between the living culture and the excavation. In the first place, the material must be abandoned by the culture and not have been reused afterwards, then the material must not have undergone strong natural erosive or accumulative processes, and finally, it must not be degraded too strongly.
Soil characteristics may express the past and present conditions of stability, erosion or accumulation, allowing thus to determine the probability that the finds of a specific period may be preserved in a protion of the landscape. That's to say: normally, a soil profile is always older than the relicts contained in it.
The probability map of archaeological finds is therefore an excellent document for use in surveys, both of the surface as well as through trench digging.
Existing already the soil map, the creation of the probability map is a rather quick operation. For the calibration of the criteria, the ideal situation is the presence in the area of an already an excavated zone.
A single legend unit of the archaeological probability map may state as follows (from the map of Poggibonsi, Siena):
In preparation of the archaeological survey of the Ostian coastal belt, a probability map for archaeological finds was redacted, based upon the present knowledge of soil distribution and local geological stratigraphy, with the aims to optimize the survey economics.
The Maccarese plain is a reclaimed marshy area between Rome and Ostia, until some time ago considered of recent formation ("recent" considered in geological terms: the sedimentation supposedly terminated ca. 4000 years ago). Detailed research on the surface (soil map, systematic archaeological survey) and in depth (drillings) have revailed the existence of coastal terrace, almost, but not completely, buried beneath recent lagoonal clays.
The terrace was populated, as testified by tombs and settlement traces, dating from 4000 to 3000 years ago and probably even older. Those remains are preserved along the border of the terrace, just there where the cover of lagoonal clay was sufficiently thick to protect the relicts against modern ploughing, but not too thick to hamper their discovery.
In this case, the soil map and the derived probability map indicated the existence of the terrace before it's confermation through geological drilling.
Antonia Arnoldus-Huyzendveld, Mario Mineo & Paola Pascucci (in print): "Piana di Maccarese: dati e materiali da un sito costiero delletà del Bronzo"