The Loess Hills of western Iowa are a rare landscape of towering loess soil dunes more than 200 feet deep, found in comparable form only there and in China's Yellow River valley. Extending in a narrow band two to ten miles wide along Iowa's western border by the Missouri River valley, they were formed from wind-blown silt created by glacial grinding during the Illinoian and Wisconsinan glacial periods, with most deposits accumulating between 18,000 and 150,000 years ago. Meltwater carried the silt downriver in summer, and in winter slower water left it on the flood plain, where strong winds redeposited it along the valley's eastern edge. Over tens of thousands of years, erosion shaped the steep terrain, irregular peaks, saddles, and distinctive catstep terraces. The rugged landscape and sharp local differences in climate and soil created a desert-like habitat for rare native plant and animal communities, many found in Iowa only in the Loess Hills. As the climate warmed, vegetation gradually stabilized the dunes, shifting from coniferous forest to deciduous forest to prairie. Native Americans had little environmental impact on the region, but after European cultures arrived in the 19th century, farming practices suppressed the wildfires that had supported prairie dominance, allowing trees to spread again across the hillsides and giving the hills their present appearance. The Loess Hills Scenic Byway is a signed road system through the region, with a 220-mile paved main route and 15 excursion routes linking scenic, historic, natural, and recreational sites.