The Loess Hills of western Iowa are formed from light-weight, homogeneous, clayless, porous grains of feldspar, quartz, and mica that were created when receding glaciers ground rock into fine silt, meltwater and sediment rushed down the Missouri River Valley, and winds deposited the sediment into the hills seen today. Strengthened by a dry climate but endangered by erosion during excessive moisture, this Iowa formation is described as unique in the United States. The area experienced six glacial tills between 500,000 and 2.1 million years ago; the hills are 1 to 15 miles wide, up to 200 feet deep, and may erode at rates of 30-40 tons per acre annually. Loess can form bluffs that maintain a 90 degree angle, but when saturated these vertical bluffs slump as a unit and create cat steps, while prairie grasses and their deep roots help hold the steep slopes in place. Weathering also produces kindchens, a German term meaning small people, which are whitish, irregular, very hard, rock-like forms created when limestone-bearing water seeps down and attaches to plant roots. On July 16, 1804, while traveling up the Missouri River, William Clark recorded a "Bald-Pated Prairie," placing his party near the present-day Missouri-Iowa boundary close to Waubonsie State Park in the Loess Hills. The region also includes the Western Skies Scenic Byway, dedicated in 1998 as one of Iowa's first state-designated scenic byways, a 142-mile route through Harrison, Shelby, Audubon, and Guthrie Counties and 14 rural towns. Native American cultures prospered here approximately 4,000 years ago because Iowa's rich, dark soils supported abundant native prairie grasses, forbs, and wildlife, but around the 1800s European settlement brought plowing of the prairies and clearing of forests as agriculture became the region's primary economy. In Harrison County, native vegetation consisted largely of tallgrass prairies, with swamp grasses and sedges along the Boyer River valley and wetlands, and hardwoods including Bur Oak, Hackberry, American Elm, Silver Maple, and Green Ash. Conservation efforts today reintroduce native prairie species whose deep roots help them endure drought and seasonal flooding, flourish without special pesticides or chemicals, and recolonize places where they have been absent for hundreds of years, sometimes aided by prescribed fire to support native plants and control invasive non-native species.