ABSTRACT At an archaeological excavation site in central Belgium, we found whitish soil material interspersing a clay illuviation horizon under a Roman road. Starting from this case, we will illustrate how insights into soil formation and soil geography are relevant for understanding landscape evolution and archaeology. We do this by focusing on the ‘Abc’ soil types, which are silt-loam soils that are well-drained and have a mottled and discontinuous clay illuviation horizon. In Belgium, these soils are, almost exclusively, found under ancient forests. To explain their formation, two hypotheses have been proposed. A first assumes that chemical weathering leads to the degradation of the clay illuviation horizon, a process enhanced by the acidifying effect of forest vegetation. A second hypothesis explains their morphology as relict features from periglacial phenomena. We further review how views on their formation were reflected in Soil Taxonomy (Glossudalfs), the FAO legend of the soil map of the world ... : 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 ...