Viene riproposto in versione sfoltita il lavoro della dottoressa Marta Cinque (premiato come miglior poster del convegno Metrology for Archaeology 2015) sulla valutazione del potenziale archeologico di Agerola in base alle variabili morfologich e stratigrafiche del territorio. Per il potenziale relativo a reperti di epoca romana riveste grande importanza la presenza e lo spessore dei depositi primari (Wpa) e rimaneggiati (WPb) dell’eruzione vesuviana del 79 d. C. Come mostrato in fig. 9, nella conca di Agerola si distinguono
- a) zone ripide (>20%) dove la coltre delle pomici del 79 d. C. (WPa) è oggi mancante o ridotta a pochissimi decimetri di spessore;
- b) porzioni di ripiani orografici poste alla base di versanti e impluvi ripidi dove il paleosuolo d’epoca romana si trova normalmente tra 3 e 5 m di profondità (sotto la coppia WPa+ WPb);
- c) porzioni subpianeggianti e lontane da versanti in aggetto dove WPa è poco o niente assottigliato dall’erosione e il paleosuolo romano è di norma a 1,5 – 2 m di profondità;
- d) zone a pendenza medio-bassa (7 ÷ 20 %) dove l’unità WP è mediamente assottigliata dall’erosione e il paleosuolo romano si pone di solito tra 0,7 e 1,5 m di profondità.
Le conseguenze di dette variazioni di profondità in termini di prospettive archeologiche sono schematizzate in figura 10.
I – INTRODUCTION
Meant in terms of probability of good archaeological results, during an excavation, the archaeological potential of an area is something to be evaluated by considering two main components: (i) the probability that the area – or parts of it – did really host, in ancient times, buildings or other marks of the human presence and (ii) the probability that, nowadays, those signs are still there, protected in the underground. While the former is a matter for archaeologists and historians, the latter may require contributions from Earth scientists; not only to sense geophysically the possible buried ruins, but also to preliminary subdivide the area in sectors that – having recorded different geomorphological and sedimentary evolutions since the ancient period of interest, have different preservation potential.
A zoning of this kind is proposed here for the area of Agerola (Fig. 1) and particularly for remains of the Roman period. For that area, the study puts also attention on some older formations and palaeosols (see section III) that are to be recalled to archaeologists because the usable geo-materials that contain and because, in the future, they could reveal layers of prehistorical interest.
Although no archaeological campaign was ever taken in Agerola, a number of fortuitous findings occurred there since the 19th century [1-4], suggest that the place was frequented during the whole Bronze Age (probably by semi-nomadic shepherds), while a village with its necropolis rose there during the Iron Age. Moreover, in Roman times, the basin floor was punctuated by farms, some of which of the villa rustica type. Therefore it seems that Agerola has an appreciable archaeological potential that could reveal new interesting data about early phases of human settlement in the mountains facing the Tyrrhenian coast of S. Italy.
II – APPROACH AND METHODS
For the present study, searching and analyzing outcrops were the main sources of data, but to overcome limitations imposed by the widespread soil cover, I used also logs of drillings and penetration tests, taken from previous geotechnical reports. The interpretation of SPT data started from the test sites falling near to places of drillings and/or outcrops, allowing correlations between stratigraphical units and corresponding value-ranges and curve shapes in SPT (Fig. 4). After such calibrations it was possible to gain litho-stratigraphical and stratimetrical information even for places with only SPT data available.
The uppermost terms of the local stratigraphy, which record the eruption of Vesuvius in A. D. 79 and following events, influence much the archaeological potential for Roman times. For it an attempt to generalize the data collected at control points was carried out. To the purpose, the observed lateral variations of stratigraphy (which depend largely on post-eruption erosion and re-deposition) were spatially analysed to understand how they depend on local morphometry and geomorphology. Once understood, such causal links became keys to extend areally the stratigraphical information (from the points of control to the areas delineated on geomorphological basis).
III -THE PYROCLASTIC COVER: GENERAL STRATIGRAPHY AND ASSOCIATE USABLE GEO-MATERIALS
Within the Agerola basin, over the substrate rocks lies a multi-layer cover of pyroclastic origin which is 10 to 18 m thick on the terraces of the basin floor and thins out rapidly moving toward the surrounding hillslopes (Fig. 5). Wherever the slope angle exceeds some 30°, bare rock prevails and terms of the pyroclastic cover are found mainly inside karstic pits and pockets.
The bulk of the cover at issue derives from the accumulation of pyroclastic coming from the volcanoes of the nearby Campana Plain (Fig. 1). The top part of the sequence is made of the products of A. D. 79 eruption of Mt. Vesuvius (WP unit; see # IV) which generally constitute the parent material of the present soil cover, an exception to this rule are steep slopes, where the WP cover is removed by erosion and the soil develops on older pyroclastic units. The eruption units occurring in YP (in alternation with palaeosols and reworked materials) probably belong to the Late Pleistocene. For the youngest of them (which is >1 m thick) an age greater than 18 ka is proposed, based on the fact that none of the youngest eruptions of the nearby volcanoes (excepted that of A. D. 79) dispersed its fall out toward the study area .
As to RC, an origin from deep and long lasted weathering of pyroclastics is proved by the presence of sanidine crystals and halloysite . The frequent presence of a coarse sandy fraction suggest that the parent pyroclastic material was supplied mainly by volcanoes of the Campana Plain. The age of RC is still uncertain. It could fall in the first part of Late Pleistocene and/or in the Middle Pleistocene.
The fact that ruins of ancient furnaces were found in Agerola, immediately below the pumices of A.D. 79  strongly suggests that the clay in RC was used in Roman times to produce kitchenware and bricks.
Merely hypothetical is – on the contrary – the use in ancient times of another geo-material worthy to signal to archaeologists: a thin crust of limonite (yellow ochre) and other Fe and Mn oxides, which occurs at the basal contact of RC on the carbonate bedrock. While ignored by recent geological literature and maps, it was cored and studied during the Italian Autarky . The metallic content was found to be high enough, but no mine was ever open in Agerola because the ore body was too thin.
IV -THE WP UNIT AND THE DEPTH OF ROMAN PALAEO-LANDSURFACE
Very important to the WP for Roman times, is the eruption of Vesuvius in A. D. 79. As well known, that explosive eruption dispersed its fall out toward SSE , that is almost the direction from Vesuvius crater to Agerola.
Based on the new measurements made at points of minimum post-eruption disturbance, the amount of fall out was about 200 cm on the southernmost portion of Agerola. For the N part of the basin, where the A. D. 79 cover appears extensively thinned by erosion, an original thickness of at least 230 cm was estimated taking into account the thickness gradient shown in the isopachous map of Sigurdsson et al. . As desumable from some descriptions of ancient excavations in Agerola [1, 4], the weight of such cover caused the sudden collapse of most roofs, while conditions were created for the preservation of daily life objects inside the buried ruins.
In Agerola, as in other parts of the range , after the A. D. 79 eruption, the steepest slopes experienced many decades of both accelerated erosion and sliding of the pumice cover. Most of the removed material flowed to the sea, while part of it accumulated on the terraces (Wpb).
Nowadays, at point where Roman buildings existed, the possibility that taller or lower ruins still remain underground varies from case to case, depending on the present thickness of the pumice cover. For this reason, the study area was zoned as shown in the map of Figure 9 by using the procedure discussed in Section II.
V FINAL REMARKS
In addition to a description of the whole sequence of pyroclastic units covering the gentler portions of the area, this research has produced a map of the Agerola basin proposing a zoning based on presence and thickness of the pumice cover related to the A. D. 79 eruption of Vesuvius.
Although preliminary and improvable, this zoning can help future archaeological researches by indicating in which sectors the underground has higher potential of remains preservation and which sectors are more promising for surface archaeological surveying (see Figure 10).
Moreover, the map could help defining well commensurated measures of heritage protection ,both in terms of consistence and in terms of spatial distribution.
It is also important to remark that, for a matter of scale, the proposed zoning does not consider the thickness variations that occur at the site scale wherever a hillslope is terraced for agricultural purposes. Said terraces, which are very common and ancient in the area, were often obtained by transferring ground from the upper to the lower half of each plot. The consequence is that near the outer edge of a terrace the Roman palaeo-landsurface rests deeper than near the inner edge. Depending on how steep was the pre-existing natural slope and how large are the plots realized on it, the above difference varies between about 0.5 and 1.5 m: enough, in some areas of “a” and “c” classes, to create cases in which the Roman palaeo-landsurface has come to the surface or at least to the plowing zone.
However, the influence of artificial terraces on near-surface stratigraphy (and, consequently, on archaeological potential) is clearly a matter that future investigation will have to analyse case by case at site scale. Both for this particular need and – more in general- for improving the “first approximation” map presented here, many more points of controlled stratigraphy should be added in the future.
Preferably they should consist in new drillings and test pits, but also the less expensive penetration tests can be very useful, because the experience made during the present study demonstrates that the transition from the A. D. 79 pumices to the underlying Roman palaeosol has a clear penetrometric signature: number of blows falling from 5 -10 to 0 -2 (Fig – 11).
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