As soon as the land has attained the required height, the dam is built sufficiently high, and also strong enough, to answer the purposes of a dike and to withstand the force of the largest tidal waves. In constructing these dams, enormous rafts are made on the shore and then floated to the works, where they are weighted by stones and sunk in the required position. Within a few weeks large quantities of silt and mud accumulate, and the whole forms an exceedingly tough and strong elevation under the water; the currents grow weaker, and deposits are lodged also outside the dam, the base of which is of course of great width. The Zuider Sea is one of the strongest evidences we have of the power of the sea over the land. Its formation commenced as far back as the twelfth century, prior to which it was only an inland lake. On December 14, 1287, during a terrific storm, the sea broke through the dividing shore line and widened the lake into a wide bay (Southern Sea, Dutch, Zuider Sea) of the North Sea; 80,000 persons lost their lives on that occasion. The same storm also did enormous damages in other localities. This was, however, not the first occasion on which the sea had made inroads into the coast lands. Before the works of destruction commenced a narrow isthmus connected Great Britain with the Continent. The North Sea was then--comparatively speaking--calm; vast chains of sandy downs ran parallel to the coast, and stretched from this isthmus to the coast of Jutland; they were of considerable height, those on the west coast of Schleswig attaining an altitude of 200 feet. Behind these downs enormous swamps are formed, in which the rivers, with few exceptions, disappeared; but the deposits they brought down formed those rich agricultural lands now known locally as "Marschland." The destruction of the shores commenced from the date that the narrow isthmus above referred to was carried away by the tidal waves which broke the English Channel during westerly gales. Traces, found far inland, show that this catastrophe occurred when the locality was inhabited, in fact a legend, in circulation to this day, relates that an English queen, to revenge herself on a Danish king, had the dam which connected England with France pierced, and so destroyed Denmark. When the Romans appeared on the scene the work of destruction was in progress, the chain of downs had been broken, and its place taken by many islands, far larger and more numerous than at present.