Rocky Falls reveals the Ozarks' turbulent geologic past through its reddish-brown rhyolite porphyry, which formed as molten rock deep within the earth and flowed onto the surface about 1.5 billion years ago, long before any living thing existed. Because this rhyolite is harder than surrounding dolomite, a type of limestone, streams tend to remain confined to cracks in the rock rather than eroding a wider valley, creating a shut-in where the harder rock constricts the stream; farther downstream, beyond the rhyolite, the valley widens and the stream spreads into a pool. Lichens, composed of algae and fungi, grow in the moist, humid conditions of the Ozarks and contribute to the weathering of sedimentary rocks, though they have very little erosional effect on durable rhyolite. Millions of years ago, volcanic lava flows were covered by warm shallow seas where life developed, and the shells of early sea creatures settled on the sea floor and later became cemented into softer sedimentary rock called limestone or dolomite, much of the exposed rock in the Ozarks. Over time, dolomite eroded and exposed the hard igneous rhyolite knobs at Rocky Falls. The durability of rhyolite also proved useful to early settlers at nearby Klepzig Mill, built in 1912 on Rocky Creek, where the stone was used for mill burrs to grind wheat into flour and corn into meal, reflecting the small water-powered mills that typified the early Ozark farmer's landscape.