During the Ice Age the thick northern icesheet covered northern Holland, and extended across the present North Sea, into England. Upon its retreat it left sandy glaciofluvial deposits with many glacial ridges, which now cover eastern Holland north of the Rhine (A332). At the same time the swiftflowing Rhine and Maas (Meuse) laid down the sands which now form the uplands in the provinces of Brabant and Limburg south of the Rhine. These provinces, as is clear in A333, can be called uplands only in contrast to the lower western part of the country. Because of their loose, dry soils, they were the home of the first men who followed the retreat of the ice. Meanwhile in the shallow depression which now forms the North Sea, the rivers at first flowed northward, probably uniting with others from England and southern Scandinavia to form a single great stream.
In time, however, the rise of the ocean because of the melting of the ice and the sinking of the land permitted the sea to encroach more and more. The process of drowning the land culminated only after the sea had broken the land connection between England and France. As a result the sandy uplands became the coastline of the continent. Along the shore, however, from the protruding Nose of Calais (Nez de Calais) northeastward the winds and currents formed a sandy bar, indicated by the heavily dotted strip along the coast, with a broad closed lagoon behind it. This extended as far as the low morainic upland of Denmark. The bar in time became a narrow zone of sand dunes and was broken in many places to give an outlet to the deltaic branches of the Scheldt, Maas, and Rhine rivers.
The next stage was the gradual filling of large portions of the lagoon, partly with clay brought by ocean currents and rivers, and partly by peat, the result of lagoon vegetation. In this way the lagoon was transformed into a swamp, half land, half water, which appears in A333 as the area below sealevel. At later times storms attacked the sandy bar and broke through it at many places, flooding the swamps beyond and forming inland basins of salt water.
The present map of Holland still shows evidence of this process, in the form of the disconnected character of the dune islands of the northern or Frisian coast, as well as in the central basin of the Zuider Zee, and the ragged coastline of Zeeland in the far southwest.