The eastern-quarter has rolling hills and wide, level valley floors that were the result of unconsolidated glacial and alluvial deposits overlying the bedrock. The geologic structure is totally responsible for the drainage pattern in the western area, while the drainage in the eastern glacial area is controlled primarily by the topography of the underlying bedrock. Preglacial bedrock valleys, which are nearly filled without glacial debris, form natural drainage ways. Both areas are well drained. Surface bedrock of the basin area consists of sandstone and dolomite.
Unconsolidated deposits consisting of glacial drift and alluvium occur throughout the basin.
The glacial drift of the eastern one-fourth consists mainly of end moraine and thin gravel moraine. The Baraboo River descends at an approximate rate of 3 feet to the mile. The most prominent topographic feature of Sauk County is the Baraboo Bluffs, which attain a height in excess of 1,600 feet above sea level and tower approximately 800 feet above adjacent bottoms of river valleys. The Baraboo valley extends east and west through Sauk County with a large portion of the valley lying between the north and south ranges of the Baraboo Bluffs. Glacial drift consisting of sand and gravel is abundant in the area, with alluvial sand and gravel filling the Baraboo Valley.