ABSTRACT: The ubiquitous urban Dark Earths composes a main challenge for urban archaeologists. Due to their homogeneous character they cannot be readily understood based on field data alone. Geoarchaeology (field study and micromorphology) has shown to be particularly well suited to tackle these layers, and to reveal their complex formation histories and the human activities and natural events involved. During the excavations of the site of Sint-Jozefs college in the centre of Aalst (Belgium) a thick dark earth was discovered underneath the remains of the rampart of the 11th century town wall. An interdisciplinary study, involving archaeology, geoarchaeology and phytolith analysis has been performed. It demonstrates that the Dark Earth layer has a long formation history involving pasture and crop growing, intimately mixed with soil processes such as bioturbation and colluviation. The identified activities confirm the rather rural character of the area until the 11th century AD. ; This article is part of a book edited at the occasion of the Geoarchaeological meeting of Bruges: Soils as records of Past and Present: the geoarchaeological approach. Focus on: is there time for fieldwork today? - Bruges (Belgium), 6 and 7.11.2019. Editors Judit Deák, Carole Ampe and Jari Hinsch Mikkelsen Technical editor Mariebelle Deceuninck English language reviewer Caroline Landsheere Graphic design Frederick Moyaert Printing and binding Die Keure, Bruges