Roman historians wrote that people in Friesland were living on artificial mounds.
The terrain, Northern European delta soil, was near mean sea level. In its natural state, much of the land was only dry during the ebb-tide and then only relatively so.
These terpen, were sometimes extensive in area one was greater than 37 acres [15 hectares.] Height varied, depending on the tides, up to ten meters above the ground. Their production was vigorous in the region, for a period.
Archeology finds that after AD 1000, the residents of Friesland stopped building terpen. The area had been surrounded by dikes, and was dry.