Catalogus

The transformation from an open “tide-dominated” to a closed “wave-dominated” coast, which occurred in the Subboreal period, and the concomitant change in barrier movement from transgressive to regressive around 5000 yrs B.P. are the main events in the development of the coast of Holland up to the Middle Ages. The morphology of the pre-transgressional surface gives a shoreline in the late Atlantic which consists of two protruding headlands separated by a large tidal basin.

The southern headland is the alluvial plain of the Rhine and the Meuse, the northern headland constitutes the moraines of the Texel High. The tidal basin in between is connected to the North Sea by a large number of inlets. The rate of sea-level rise at that time (1 m/century) outran the supply of sediment to the tidal basin. After 6000 yrs B.P. the rate of sea-level rise decreased gradually, whereas the rate of sediment supply remained constant. This led to a gradual decrease in the tidal prisms of the inlets as the tidal basin was filled in with sediment. Shortly before 5000 yrs B.P. the first channels silted up and closed; the last tidal channel disappeared around 3300 yrs B.P., leaving two inlets along the coast which were both connected to the river Rhine. The closure of the first inlets occurred at the same time as the barrier began prograding because (1) the rate of sea-level rise diminished considerably, (2) not all the sand supplied by longshore and cross-shore transport disappeared into the tidal basin, but instead could be used for barrier progradation; and (3) ebb-tidal deltas of the closed inlets provided major sand sources.