Two new approaches to a study of the Early Upper Palaeolithic have been developed over the last twenty years in France. The first approach consists of a reassessment of ancient profile sections with a better understanding of cultural entities, while the second approach endeavours to apply a palethnographic approach to this period. Only a few sites have allowed this second type of approach, because of the limited size of the excavated areas or their inadequate preservation. Even when the deposits are suitable, lack of time and investment may not favour the extensive site studies required for a palethnographic reading of places and artefacts. The fortuitous discovery of the “les Bossats” open-air site at Ormesson, in the early 2000s, enabled us to pursue this ambition. Initially concerning the Gravettian occupation, the first excavation in 2009 rapidly led to the identification of a second occupation layer attributed to the Mousterian. The following excavation seasons revealed five other Palaeolithic layers with unequal surface coverages and different conservation states: another Mousterian layer (probably a palimpsest), a Châtelperronian layer, a Solutrean layer, a Badegoulian assemblage and, between Châtelperronian and Gravettian, an isolated fireplace without any associated artefacts. This Palaeolithic stratigraphy shows a continuous occupation of the place over approximately 30,000 years with regard to the particular configuration of the valley at this point.The main Mousterian layer, with a discoid technical tradition, is preserved over 800 m². A major activity area has been identified with more than 900 pieces of knapped flint, 2,406 pieces of colourant material and about 20 osseous remains registered within only two square metres. The presence of four large blocks of calcite concretion (weighing 10 to 20 kg) in and above this archaeological setting raises questions as to the use of these stones that must have been brought in by Neanderthals: were these seats, chopping blocks or architectural elements? The Mousterian layer is also renowned for its important collection of colourant rocks the number of which is unique in a Mousterian context. The 2,400 chips and pieces of ferruginous concretions have a total mass of only 1 kg but 51 objects show traces of intentional breakage, scraping and/or abrasion indicating the extraction of red powder rich in hematite. The function of these materials and powders remains unknown and we can only refer to the multiple pre-existing hypotheses, without a priori excluding a decorative use of colour. Châtelperronian people occupied a large sandy surface of approximately 150 m². Their flint debitage is exclusively directed toward the manufacturing of elongated products, blades and bladelets, while nothing attests independent flake production. The soft stone hammer blades, thin and regular, are the blanks for Châtelperronian knifes and burins. The numerous bladelets also correspond to first intention products even if only a few of them are retouched. We noticed some awkward debitage indicating the presence of inexperienced knappers. The “les Bossats” site therefore constituted a Châtelperronian campsite grouping together several generations of people, in which a number of activities took place, including flint debitage, animal carcass treatment and osseous material working.The Gravettian occupation shows only one archaeological concentration, with an area estimated at approximately 80 m². Although partly affected by post-depositional processes, this layer contains the original organisation around a central fireplace and bones are preserved. Among these, bison constituted the main game. The most striking feature of this locus lies in the floor arrangement with a stone pavement composed of small limestone aggregates, collected from the bottom of Ormesson valley. In addition to butchery activities and bone working, attested by more than 2,600 osseous remains and about twenty osseous fragments showing clear working traces, we highlighted the presence of non-domestic activities. Indeed 146 fossil shells, probably used for adornment, were collected in Lutetian deposits over a distance of 80 to 90 km. Moreover a human second molar, from a child between 8 and 12 years old, was discovered near the fireplace. The Solutrean layer shows several stone structures which contained archaeological remains resulting from several activities: shaping of laurel-leaf points, blade and flake debitage, production of common tools, animal carcass treatment. The Solutrean people collected almost 70 blocks of calcite concretions (ranging from 20 to 80 cm for the largest ones) from the bottom of the thalweg. The moved mass is actually estimated at around 900 kg, including blocks weighing more than 50 kg. These stones form three built structures while a fourth archaeological concentration shows a lighter arrangement. They might correspond to super-structure edging. The presence of reindeer osseous remains and the identification of birch within charcoals suggest rather cold climatic conditions possibly requiring the building of tents or other forms of shelters. Another striking feature of the Solutrean layer is the presence of numerous fragments of shaping flakes whose cortexes are engraved with parallel lines, crossed lines or sinuous lines, indicative of esthetical or symbolic purposes. After eight years of excavation, the “les Bossats” deposit is now a reference for the Early Upper Palaeolithic in the Paris basin and beyond in northern France, with 30 dates, detailed sedimentary and environmental settings and a palethnographic approach practicable for all these occupation layers.