The northeastern corner of the county tends to be hummocky, while in the south and east it generally is smoother and unaffected by stream dissection. Along streams, slopes are gently undulating to very steep and vertical limestone bedrock is exposed. Nearly level and gently sloping soils in the county are on till plains dissected by numerous drainageways. The majority of streams flow southwest. The primary drainageways are the Flatrock River, Clifty Creek, Fall Fork, Wyaloosing Creek, Sand Creek, Salt Creek, Cobbs Fork, Panther Creek, and Vernon Fork Muscatatuck River.
Salt Creek drains the eastern row of townships and flows east.
The native vegetation in Decatur County was mainly deciduous trees. In general, well drained upland soils supported sugar maple, beech, hickory, red oak, and white oak. Poorly drained upland soils supported pin oak, beech, and ash. These pages were designed and written by D avid L eas 1999 David A. Leas - all rights reserved .