For the Harpstedt-Nienburger culture as little map
green (scandinavia): Nordic group
light green: hausurnen-culture
green (between Oder river and san river: Pomerellian faceurn-culture
In the early and older pre-roman iron age the HNC buried their deads in urns on great urn-fields, later they burned their deads on pires and covered this with small grave hills.
During the 3rd and 2nd century forts were build, especially in the south, perhaps as reaction to the celtic oppida. In this time there is a strong celtic influence on the HNC.
During the middle and younger pre-roman iron age the influence of the Jastorf-culture increased, especially in the North-east of Lower Saxony. Between Elbe and Weser river we can find the Nienburg group and west of the Weser we have the Ems-Hunte-group. The coastal area is an own group, influenced by groups from the North. The Weser- mountain region is a link between these northern" cultures and the La-tene.