Abstract:

The present research was motivated by the need to date rock-art engravings discovered in the Aptian Alps mountain chain (northwestern Tuscany, Italy). The engravings, which were made on the horizontal surfaces of marble, are contour-line etchings of billhooks (Italian “pennato”), whose chronology and meanings are still unknown. In this paper we present a computer-aided mathematical simulation of the effects of natural erosion processes on an ancient linear etching engraved on a flat horizontal marble surface. The main goal is to demonstrate that such engravings may still be visible after centuries-long exposure to natural erosion. The mathematical core of the simulation program is the widely used Monte Carlo method, based on continuous repetition of microscopic stochastic events whose probability laws are known. Formulating such laws for the task at hand requires knowing the speeds of the two main mechanisms involved in the erosion of limestone: freeze-thaw cycles and chemical dissolution. By applying the procedure developed, it has been possible to observe the evolution over time of the engravings3 cross sections during a time span of about 2000 years and thereby evaluate the values of the depth and width of the grooves forming the engravings. Analysis of these values using an estimation of the average annual rainfall has enabled formulating an algorithm to calculate the absolute dating of the engravings. The results of a first experimental application of the present method on the so-called ‘Billhook Step” (Mount Gabberi,ApuanAlps) are presented and discussed, including the influence of both random and systematic errors.

Keawords:

Absolute Dating, Rock Art, Historical, Marble

Introduction

In the last fifteen years several rock-art sites have been discovered in the mountain chain of the Apuan Alps (the famous white marble mountains) along the northwestern coast of Tuscany. These rock-art sites bear figurative engravings depicting billhooks (Italian “pennato” or “roncola”, a sort of curved hand knife used for woodcutting) as their most often reoccurring subject. The billhook engravings, which were etched out life-size, though in contour only, and are extremely worn, are found mainly on horizontal flat marble surfaces at high positions overlooking the lower peaks. Recent studies (Bagnoli, Panicucci and Viegi 2005) have come to the conclusion that the chronology of these markings is highly uncertain because of the lack of comparisons with other sites elsewhere in Italy (with the exception of two billhook representations in the southern part of Trentino (Riva del Garda and Arco, Italy).