During the Continental period (about 14.000 to 9.000 years ago) this area was dry land, but during the Atlantic period (9.000-6.000 years ago) the sea level rose, and we know that a fresh water lake developed at Felsted Kog. In contrast to the northern coastal areas in the geopark, Nissum Fjord has not been influenced by marked changes in relative sea level since the Atlantic period because the rates of isostatic uplift and eustatic sea level rise have kept in balance with each other.
At some stage the freshwater lake at Felsted Kog became part of a fjord that subsequently became cut-off from the open sea by the construction of spits that merged to form an isthmus and enclose a lagoon (site 37 The West Coast). The age of this process is not known, but in the light of the enormous amounts of material that are transported along the west coast it seems likely that the lagoon formed not long after it became a fjord. The coastal forms in parts of Nissum Fjord indicate some action by waves. The coastal cliffs at Nørre Fjand, Nørhede and Felsted are part of Skovbjerg hill island. These are now inactive and lie behind a stretch of tidal meadowland. At Nørre Fjand and Fjandø there are well-developed spits and beach ridges. The coasts of the fjord are generally characterized by a low energy environment and there are extensive areas of tidal meadows (see site 36 Skibsted Fjord - Kås Bredning for more details) (Figure 38-2). Marshland has developed in Felsted Kog and elsewhere as a result of the addition of fine-grained material, largely supplied by the river, which are deposited during periods of high water levels in the fjord. Together with the fine-grained deposits from the freshwater lake mentioned above, the tidal meadows and marshlands comprise highly fertile areas.
Nissum Fjord is a valuable biological location because of the extensive areas of grazed tidal meadows, reeds and shallow water that attract large flocks of birds. Nissum Fjord is a Natura 2000 habitat and bird protection site, as well as a Ramsar area and a reserve for migrating birds. Many of the tidal meadows have also been preserved.