About 14,000 years ago, near the end of the last Ice Age, the area now known as Lincoln County lay along the eastern edge of a glacier that covered most of the county and reshaped the landscape. Immense torrents of glacial meltwater carved the deep, wide river valley that now holds the narrower Big Sioux River. As the glacier scraped across the land, it carried rocks of many sizes and left them in huge piles when it melted, creating sand and gravel later quarried for construction and road projects. Tiny, nutrient-rich rock fragments carried by the glacier also formed the rich soils that support eastern South Dakota's agricultural industry. The Blood Run villages across the river stood atop deep glacial deposits of gravel and sand, and when quarrying destroyed some village areas, the state of Iowa purchased land and took steps to preserve the remaining cultural areas. Glaciers also scattered very large rocks called erratics across the landscape, and some of these boulders were used for ceremonial purposes; by repeated hammering and grinding in one spot, pits were left across the surface of one erratic on the Iowa side of Blood Run.