The stream length-gradient (SL) index is widely used in geomorphological studies aimed at detecting knickzones, which are extensive along-stream deviations from the typical concave-up shape assumed for stream longitudinal profiles at steady-state conditions. In particular, SL was practical for identifying anomalous gradients along bedrock stream channels in mountainous catchments. This work presents the GIS toolbox SLiX designed to extract values of the SL index, starting from Digital Elevation Models (DEMs). SLiX is also suitable for the spatial analysis of the SL values, allowing for the identification of landscape portions where anomalous high values of SL occur and, consequently, those catchment sectors where stream channels show peaks in the erosional dynamic. The SLiX main outputs are (i) point shapefiles containing, among stream channels attributes, the extracted values of SL along the stream network analyzed, and (ii) SL anomaly maps in GeoTIFF format, computed through the Hotspot and Cluster Analysis (HCA), that permit the detection of the catchment sectors where the major SL anomalies occur and consequently the principal knickzones. The application of the proposed tool within an experimental catchment located in the Northern Apennines of Italy demonstrated the proper functionality and the potential of its use for different geomorphological and environmental studies. The accurate and cost-effective detection of anomalous changes in stream gradient ensured by SLiX is of great interest and can be useful for studies aimed at unravelling the Earth processes responsible of their formation (e.g., active hillslope processes, such as landslides directly interacting with the streambed, presence of geological structures, and meander cut-off). The applications of SLiX have clear implications for preliminary analyses, at a regional scale in different morpho-climatic contexts, for the hydrological management of river basins and/or to prevent geological hazards related to the fluvial erosional dynamics.

The local and extra-regional (national and transnational) stones mainly used as building and ornamental materials in the historic centre of Urbino (UNESCO World Heritage List) were unravelled through a detailed geological and petrographic study. The types of building stones used in the past for the development of an urban centre were mostly affected by the availability of suitable geomaterials in the surrounding areas. For this reason, the stones found in the historical buildings of Urbino generally come from the local sedimentary formations (mostly limestones) belonging to the Umbria–Marche–Romagna SuccessionAuct., which crops out in the Northern Marche Apennines. Only some ornamental highly prized stones used for monuments and decorations come from both extra-regional Italian areas (Alps, other sectors of the Northern Apennines) and foreign countries (France, Egypt). A brief description of the Northern Marche geology was also reported to exactly match the local provenance of the stones, so highlighting the relationship between the territory and the architecture of Urbino. Because of obvious conservation reasons, no samples were collected from buildings or monuments and only autoptic observations, together with a detailed historical and bibliographic research, were carried out to identify the different materials and the provenance areas. Besides the availability of the local sedimentary rocks, we emphasised how the choice of the building and ornamental stones could have been also influenced by the historic period and artistic style, aesthetic features, economic and social importance of the building and/or monument and the relationship to some distinguished personality (e.g., Pope Clemente XI). An open-air stone itinerary across significant places (10 stops and additional sites and monuments in the urban area) is finally proposed for the best fruition of the geological and cultural heritage of Urbino, also aimed at geotourism development.

This study is addressed at the cultural heritage of the UNESCO historical centre of Urbino (Italy) through the focus on a very peculiar building and ornamental carbonate porous (spongy) stone also found in the opus quadratum Roman dry walls. For these rocks, the mathematician and historian Bernardino Baldi (16th century AD) and the mineralogist Francesco Rodolico (middle of the 20th century AD) introduced, respectively, the popular terms of Tufo spugnoso or Pietra Spugna. Physical observations and stable isotope data (δ¹³C and δ¹⁸O) of these rocks allowed, for the first time, their classification as calcareous tufas, thus contributing to the valorization of the stone heritage of the city. This carbonate lithotype was formed by the chemical precipitation of CaCO₃, driven by the CO₂ degassing of supersaturated calcium-bicarbonate-rich waters, coupled with the passive encrustations of organic material in continental environments. Radiocarbon analyses dated these stones mostly between 9100 and 4700 yr. BP when a maximum growth of these carbonate continental deposits occurred in Mediterranean regions and northern Europe, i.e., during the Holocene Atlantic climatic optimum. Work is still in progress on a perched springline of calcareous tufas found along the Metauro Valley (a few km from Urbino), being good candidates for provenance, at least for those blocks exploited by the Romans and successively reused in the architectural framework of Urbino.