Stratigraphic, paleontologic and sedimentologic study of an escarpment on the right bank of the Oise river at Saint-Leu-d'Esserent, France, permits the division of the lower Lutetian beds into three zones corresponding to R. Abrard's zones I, II, and III. Zone I is characterized by Nummulites lamarcki and Lenita patellaris. It consists of coarse sands representing tumultuous sedimentation and limestones with a constant fauna indicating calm depositional conditions. The microfauna is similar to the Aizy Cuisian but the frequencies are different--Miliolidae do not exist in zone I. The zone I-II passage is progressive, marked only by the appearance of Nummulites laevigatus, the decrease in N. lamarcki and the disappearance of Lenita patellaris. The two zones seem to belong to the same group--lower Lutetian. The zone II-III boundary is clear, corresponding to a major change in bathymetry, the seas having become quite shallow. Zone III is characterized by the disappearance of glauconite and N. laevigatus and by the rapid increase of the Miliolidae and Orbitolites complanatus. These facts suggest that part of Abrard's zone IV--basal upper Lutetian--is also present. The granulometry and heavy mineral content of the beds are presented in tabular form. A complete list of the microfauna is given.