Located northwest of Sterling, Flattop Butte is a rock outcrop that was used extensively by prehistoric peoples as a source of stone for tools. The butte has a Chadron Formation capstone that is the only major bedrock source of high-quality stone between central Kansas/Nebraska and the Rocky Mountain foothills. Known as Flattop chalcedony, this stone has been found at sites across a wide swath of the Great Plains, allowing archaeologists to reconstruct prehistoric patterns of migration and trade.

Flattop chalcedony is a White River Group silicate, a type of stone commonly used for tools on the Great Plains. The only other two major sources of White River Group silicates are Table Mountain, Wyoming, and White River Badlands, South Dakota. Flattop chalcedony ranges from opaque white to translucent lavender in color, with some flecks of pink and blue. It has excellent flaking qualities, making it perfect for shaping into sharp points and tools.

In 1976 Sally T. Greiser became interested in Flattop chalcedony—which was known to have been used from the Paleo-Indian period (before 6000 BCE) throughout prehistoric times—and performed a study of the butte. Greiser identified more than 200 depressions on top of the butte that were prehistoric quarries. She found that the butte’s surface was littered with prehistoric flakes, cores, hammerstones, and finished tools. Quarrying was apparently done by digging to the caprock and extracting blocks of stone using sticks, bone tools, and wedges.

Because it can be so easily traced to a single source, Flattop chalcedony has helped archaeologists reconstruct the movements of prehistoric peoples who left behind few other remains. For example, lithic tool caches from the central Great Plains have revealed that White River Group silicates such as Flattop chalcedony were the most commonly used lithic tool materials in the region during the Clovis period (around 11,000 BCE). Clovis tools made from Flattop chalcedony have been found more than 300 miles from Flattop Butte, with an average distance of about 105 miles away to the east or southeast. According to archaeologist Steven R. Holen, this distribution of Flattop chalcedony indicates that Clovis people knew Flattop Butte well and made it a regular stop on migratory routes that extended up to 350 miles. Holen has suggested that Clovis bands may have gathered at the butte to make tools in late summer or early fall, allowing different bands to exchange goods, information, and mates.