Coming back to the physical background, how much of Holland is now below sealevel because it consists of the old bottom of the lagoon converted into fertile fields and grasslands. Exceptionally low tides make it possible at fairly frequent intervals to drain much of this low area by means of canals; at high tide closed locks keep the ocean out. The relation between polders, dunes, dikes, rivers, and the sandy eastern region where the low places between dikes, including the one marked "reclaimed lake," are polders. The isles of Zeeland in the southwest were reclaimed by means of dikes in this way, as were the provinces called North and South Holland which lie along the coast from Rotterdam northward. Recently the last and greatest task of reclamation, that of drying up the Zuider Zee, has been attempted with the result that 50,000 acres of good land have already been added to Holland, and a large dike across the northern end of the Zuider Zee will help to reclaim far more by and by.
On the north coast east of the Zuider Zee most of Friesland and the northern part of Groningen belong to the same type of once swampy, half-drained or flooded country. Numerous lakes in this region still remain undrained because their peaty bottoms are not well adapted to agriculture. In summer these Frisian lakes are much used for the sport of sailing. When they freeze during severe winters they are so much used for the national sport of skating that industry is almost crippled for the short time that the ice remains good. Leeuwarden, the capital of Friesland, now located in the center of the province, grew up as a seaport, on the shore of a former inland basin that now no longer exists.
This historical treatment of the physical background is necessary to explain the present geography. Now we are ready to discuss the three natural divisions -- the dunes, the lowland, and the upland.