GIS as a tool to detect flat erosional surfaces in coastal areas: a case study in North Spain

The delimitation of flat surfaces, such as marine or continental terraces, can be easily done if they are wellpreserved by using classic techniques such as fieldwork and photointerpretation. However, subsequent landscapeerosion can modify their initial morphology, hindering their recognition. This paper presents a methodologydesigned to identify and delineate flat erosional surfaces (known as rasas) in a sector of 1,228km2 in theCantabrian coast (eastern Asturias, N Spain). The spatial distribution of rasas was quantitatively established bycomparing the Digital Terrain Model (DTM) with previously available information about flat surfaces alreadymapped. From a lithological point of view, rasas are modelled on Ordovician quartzite (9.7km2 of 1,228) mainlybetween 132–232m above sea level (a.s.l.) altitude and on Carboniferous limestone (2.9km2 of 1,228) mainlybetween 24–69m a.s.l. The use of Geographic Information Systems (GIS) combined with a quantitative analysisof the relief (using the hypsometric method) allowed us to develop a predictive approach for flat erosionalsurface detection. The methodology has been tested and validated in areas in which there were no erosionalsurfaces previously mapped. The results reveal new surfaces modelled on Ordovician quartzite (0.43km2), ataltitudes ranging from 200 to 250m, on Jurassic mixed formations (35.9km2) at 150–175m altitude, and onEocene limestone (1.1km2) at 110–120m altitude.