The Top Hat Transform function is a grey-level image analysis tool that allows extracting peaks and valleys in a non-uniform background. This function can be applied onto a grey-level Digital Elevation Model (DEM). It is herein applied to quantify the volume of recent incised material in a mountainous Pyrenean watershed. Grey-level Closing operation applied to the Present-Day DEM gives a new image called paleo DEM. The Black Top Hat function consists in the subtraction of the paleo DEM with the Present-Day DEM. It gives a new DEM representing all valleys whose sizes range between the size of the structuring element and the null value as no threshold is used. The calculation of the incised volume is directly derived from the subtraction between the two DEM's. The geological significance of the quantitative results is discussed.