Generally in radiocarbon dating, the younger alkaline extract of a sample of peat or organic soil is considered to contain the infiltrated humic substances, whereas the older residue supposedly consists of the actual autogenous organic matter. In several examples of vegetation horizons (fossil A₀/A₁-soil horizons occurring in younger Holocene marine deposits), in which the carbon content is generally very low (less than one percent), the alkaline extracts also showed lower ¹⁴C ages than the residues. However, in the Friesland coastal area of Tergracht-Birdaard, the extract ages were consistent with results on peat in the same profile and agree with local geological evidence, whereas the residue ages are too high. Therefore, the hypothesis is put forward that the larger part of the humic substances has actually been formed <i>in situ</i> by humification of fresh organic material. The residues, which are very small fractions of the total soil samples, then have to contain a relatively old component causing the aging effect. This is in agreement with the fact that samples of recently deposited mud show a considerable apparent ¹⁴C age. More detailed results of the Schildmeer area, which are compared to general geological and archaeological evidence of the Groningen coastal area, confirm the above hypothesis. Regarding the history of settlement in this area, the data obtained suggest that the raising of the so-called second generation of <i>terps</i> (dwelling mounds) is related to a tardy sub-phase of the Dunkerque I-B transgression.