## 4 Discussion  
### Highlight-Reflectance Transformation Imaging (H-RTI)  
The Reflectance Transformation Imaging technique has already been recognised as a valuable instrument for the examination of different types of objects in aerial conditions, like paintings [14], coins [17], papyrus [19], or large monuments [62], and further for subaqueous relicts [63]. RTI images are generally used to deepen the knowledge of objects already known for their specific ancient usage [20,21]. The methodology requires the use of basilar photographic instruments and open-source software, which make the technique particularly suitable to users of all levels. The capability of RTI images to facilitate interactive selection of the illumination direction of an object's surface enables researchers to identify traces and details that are not visible to the naked eye. Moreover, the resulting files can be shared and easily visualised by experts using the open-source software RTIViewer. This attitude may prevent archaeological objects from transport and excessive manipulation, which potentially lead to damages [21,64]. In the present study, the resulting images represent the initial element on which the hypothesis regarding the specific archaeological employment may be based. This is  
Figure 11: The primary outcrops and the main streams crossing them in northern Italy. achieved through the elaboration of a comprehensive framework of traces on the object surfaces. In addition, the introduction of a metric reference next to the object into the shooting frame allows reliable measurement of superficial traces in width and height while their depth may not be so accurate due to the Highlight-RTI method of photo acquisition that causes almost 10% error in profilometric measurements [65]. The interpretation method of RTI results elaborated by [PERSON] (2018-2019) [57] allows for the description and the temporal collocation of all the detected superficial traces, particularly through carbonate concretions as an essential proxy. In fact, they form naturally when artefacts are buried, as a result of the calcium carbonate percolation into soils and subsequent precipitation on rough surfaces. Surficial colour variations in both rocks are considered recent natural traces, resulting from the natural deposition on the porous matrix of iron and manganese oxides, which characterise the soils of Travo archaeological site [66]. This consideration is also corroborated by the evidence of fully coloured calcium carbonate concretions on the surface 2 of S1, on which iron oxides precipitate after the formation of concretions during the rock burial. The edges of both rocks have been regularised by distinct natural (for S1) and anthropic (for S2) events, and hemispherical concavities on their surfaces are labelled as ancient tapping marks. The rectangular shape and the several tapping marks on the plain Surface 2 of S1 lead to the hypothesis that the rock may have been used as a grindstone. In this regard, it may be considered a grindstone due to the medium to fine grain dimensions, which guarantee the right abrasive capacity. However, the tapping marks are not related to striae, which are typical features identifying a rock as a grindstone [67]. Moreover, the convexity of Surface 1 is not a specific feature of a grindstone even though deeper tapping marks are detected. Thus, all the evidence may result from deliberate human manufacture of the entire rock aimed at an as yet undetermined form of employment. The lower fractured edge may be caused by an ancient natural or intentional phenomenon. As regards sample S2, its edge regularisation is testified by the linear engravings along the edges, which may also be considered an effect of human rock manufacturing. The grooves found on the S2 Surface 1 are defined as hits due to their similar shape. They may be generated by the intentional beating of a specific instrument on the surface.