Jacob Fowler, a fur trapper and trader, passed the point of Red Rocks on October 19, 1821, about 600 yards from the river, and his journal entry appears to be the first recorded mention of Pawnee Rock. Although Fowler believed the red rock was a volcanic formation, Pawnee Rock is sedimentary Dakota Sandstone. About 100 million years ago, sandy and clay sediments were deposited along the edge of a vast sea that covered much of central North America, and this Dakota Formation was later buried beneath other sea deposits. The chemical makeup of the sediments determined their color, texture, and density, with high iron concentrations hardening some areas. Over long periods, erosion uncovered these sediments, and water draining toward the Arkansas River stripped away softer clay, silt, and sand, exposing the harder sandstone formations of Pawnee Rock high above the surrounding plain.