Ten years later, the new bishop, Jan van Arkel, was able to retrieve the pledged territories. He also reformed administration by breaking Oversticht into the three sheriffdoms of Twente, Vollenhove, and Salland. In time they were demoted to bailiwicks ("drostambten"), and even though they were superseded by the sheriffdoms of IJsselmuiden, Diepenheim, Haaksbergen, and the sheriff of Hasselt was made high sheriff, the original three bailiwicks have nonetheless served as the basis for Oversticht's three-way division to this day. When the Archbishop of Utrecht sold his domains to Holy Roman Emperor Charles V in 1527, Salland became one of the three constituent parts of the new dominion of Overijssel, which in turn became one of the seven provinces of the Dutch Republic in 1581 (Drenthe became a dominion in its own right). The borders of Salland are not well defined, but the largest area which may be indicated by Salland, corresponding to the historical dominion, is bound to the west by the provincial border with Gelderland, which is partly formed by river IJssel; to the north by the Meppelerdiep stream and the provincial border with Drente, which partly consists of the Reest stream; to the east by the borders with the German state of Lower Saxony and the eastern Overijssel region of Twente; and to the south by the border with the Gelderland region of Achterhoek, part of which is formed by the Schipbeek stream, with other words Salland is all of Overijssel with the exception of Twente and the region known in Dutch as "Kop van Overijssel". Geologically, most of Salland's landscape consists of a lowland covered with river sediment. To the east, large moors have been drained to provide pastures. These moors, together with the hilly Holterberg region, were the natural border with Twente. On the Holterberg, stretches of the originally vast heathland have been preserved. In the northwest, reclaimed boglands used for peat digging have resulted in low-lying areas vulnerable to flooding, pre-1932 (in that year, the Zuiderzee was closed off from the North Sea to become the IJsselmeer lake). Salland is a low-lying area dominated by a number of rivers and streams, which have deposited rich layers of river clay.