The coastal settlement in Middle Mesolithic (MM) Southeast Norway is generally described as a dynamic system of small, short-lived sites and large sites that were visited repeatedly over a long time. It has been suggested that in this period – compared to the preceding Early Mesolithic (EM) – an increased attraction towards the large archipelagos along the Norwegian coast, and their broad spectrum of both marine and terrestrial food sources, took place. This has, however, been debated. An important background for the discussion is the documented population influx and the abrupt shift in tool technology around 8300 cal BCE. The question remains whether this great social and technological upheaval was followed by a transformation or break in the environmental knowledge and changes in the ways of living, such as new human–environment relations, settlement systems, and mobility patterns. We propose that the continuous process of enculturation of the land and environment would have played a decisive role in the societal development. In this article, we therefore investigate details of site construction and mobility patterns, as well as the marking of the land by the establishment of settlement sites and exploitation of lithic raw materials on a larger scale. We concentrate our investigation on the on-site activity, site location, lithic raw material economy, and technological organisation, which brings renewed opportunities for discussing enculturation processes in the period. Our results document a diverse settlement pattern, as well as diachronic variation, suggesting an internal duality within the sociocultural development process in the area. The article presents the new results from the InterCity Vestfold project, situated by the Oslofjord west coast. Our data comprise a total of eight excavated and 69 surveyed sites from 4 municipalities. A strong isostatic land uplift caused continuous and rapid changes in the shoreline configuration and its nearby environments, in the Oslofjord area throughout the Mesolithic. To enable the discussion of human–environment relations, we therefore present a new relative sea level (RSL) curve, supplying detailed data on the shoreline development in the project area.