Since it is extensive from near the Schuylkill River in the NE to the Lancaster County border in the SW, its terrain varies considerably. Most is drained to the Schuylkill except for the SW end which is in the Susquehanna watershed and drains via the Conestoga River. While much of the north is low-lying, much of the SW is mountainous. The Cacoosing Creek forms the natural NW boundary and flows into the Tulpehocken Creek, which in turn forms the natural NE boundary.