Mill Bluff is one of many isolated, rocky, castle-like hills that rise abruptly from the surrounding plain. A formation of this kind is properly called a mesa if large and a butte if small. In this driftless area, mesas and buttes escaped glacial activity and retain a hard cap on top that resists weathering. Wind and water erode the softer sides until the weight of the overhanging cap causes it to fall. As the mounds shrink, the capping is eventually removed, and the mound becomes a conical hill that gradually blends with the plain. Mill Bluff took its name from a sawmill operated there in the early days of settlement.