This research aims to characterize the geomorphological aspects of the areas where dark superficial horizons occur in relation to mountainous region. These kind of horizons present high organic matter contents and according to the Brasilian Soil Classification System they are called "humic" and "proeminent" A horizons since they have low base saturation values (V<50%). For this, a small watershed with about 10 Km2 - Manga Larga river basin - was chosen in Petropolis municipality, in Rio de Janeiro State. The locals of the soils with dark A horizon identified during the fieldwork were assigned in topographic map and aerial photographs. Laboratory analyses have shown that the organic matter contents reach 11,06%, decreasing with depth, that can be 1,60 meters. The basin present a great altitudinal variation, from 700 to 1400 meters with a very steeply sloping relief, being 20 to 45% the slope category dominant. There are rock walls that outline the local lithology composed by granites and granodiorite gneiss. The hipsometric, slope gradient and geomorphological features maps elaborated for this area show that the "humic" and "proeminent" horizons can be found since 800 meters of altitude in moderately steeply sloping relief (8 to 20% slope category) and in concave morphologic features (hollows), normally with an apparent slope rupture, being the upperslope lightly steeper than the downslope. The results point out that, in spite of the rainy tropical climate, it is possible to find these specific types of horizons if there is a set of favorable conditions to the organic matter accumulation.