From about 31,000 to 12,000 years ago, the Missouri River Valley served as a major channel for large volumes of meltwater and released sediment. As massive ancient glaciers moved across the land, they ground the underlying bedrock into fine-grained sand and clay and closely packed grains of quartz. In summer, warming weather melted the glaciers, and the meltwater carried these fine materials and deposited them across the floodplains. In winter, the waters receded and the flats dried during extended droughts. Loose silt, or loess, left by the floodwaters was lifted by strong westerly winds and spread across the state, becoming rich agricultural soil. Over thousands of years, heavier and coarser silt settled close to the floodplain while finer silt traveled farther inland, and erosion later carved the accumulated soils into the narrow ridges and steep sideslopes set back from the river. The irregular Loess Hills form one of Iowa’s most distinctive landscapes and one of the rare places in the world where deposits are abundant enough to create their own landform; only Shaanxi, China, has other deep loess-type hills. The Loess Hills extend the full 200 miles of Iowa’s Missouri River Valley, with local loess soils ranging from 60 to 200 feet thick. In western Iowa these deposits are porous, lightweight, and easily eroded, making the Loess Hills the state’s most fragile landscape. The park’s variable topography supports varied vegetation, with yucca on the high, dry exposed slopes and mosses and broad-leafed, shade-loving plants in the deep, moist ravines. Sioux Quartzite, used in many C.C.C. park projects, is the oldest exposed bedrock in Iowa and can be seen in Lyon County north of Stone State Park.