Miller Nature Sanctuary was shaped over millions of years by flowing water and glaciers, which created its natural arches, slump blocks, waterfalls, and the soils that support some of Ohio's rarest plants and one of southern Ohio's best spring wildflower displays. About 430 million years ago, this part of Ohio lay beneath a warm, shallow sea, where sediments formed the bedrock layers known as Peebles Dolomite, the Lilley Formation, and the Bisher Formation. Peebles Dolomite, the uppermost visible layer, is the prominent bedrock from which many Ohio natural arches and bridges arise, contains abundant fossils, and forms both Miller Natural Bridge and Miller Arch, while Bisher Dolomite, the lowest exposed layer, shows crossbedding that records sediment flow and tidal action and forms both waterfalls along the Falls Trail. The sanctuary also supports rare native plants, including shooting-star, which grows in dry, open woods and limestone soils and is pollinated by bumblebees through buzz pollination, and Sullivantia, a delicate wildflower discovered in 1839 by Eliza and William Starling Sullivant within a few miles of the preserve that grows in moist, shaded dolomite crevices and ledges. Between 300,000 and 14,000 years ago, glaciers advanced and retreated across this land, and after a glacier halted 1 mile to the southeast, meltwater and a draining lake diverted by an esker began Rocky Fork Gorge; water from later glaciations then shaped the arches, natural bridges, and slump blocks seen there today. Rocky Fork Gorge also influenced human life in the area, serving as a passage for travel and supporting a busy milltown in the 1800s along Rocky Fork Creek. In 1855 Captain David M. Barrett, a descendant of early 1800s pioneers, businessman, entrepreneur, Civil War company commander in the 88th Ohio regiment, and local politician, purchased many local mills, rebuilt many of them, and constructed others, creating a family mill operation that eventually included about 1,000 acres. Barrett's Millhouse, the family mansion, is the only surviving part of that operation, while the grist mill and adjoining covered bridge built in the 1860s stood for more than 110 years before fire destroyed them in the 1980s.