To the southeast, the land is mostly sandy. There are small rivers and brooks cutting through it. Examples for such rivers are Regge and Dinkel. In the northwest, the geology is mostly made of sediments from the "Overijsselse Vecht" and clay. The northern parts were once covered by "veen" (bogs) which separated the dryer and more arable south from Drenthe. These bogs have been exploited as fuel to a large degree. Only small patches survive today ("Engbertsdijksvenen" near Tubbergen, "Witteveen" (near Haaksbergen, and the Aamsveen (near Enschede). The extreme northwest is dominated by a system of lakes formed by former peat-mining, the Weerribben, which is a valuable wetland.