The Mississippi River valley is a distinctive landscape feature of the central United States, shaped within a geologic framework of ancient sea floors and glacial flood waters and long attracting prehistoric and historic populations to its waterways, islands, and bluffs. In northeastern Iowa, the Mississippi carved a deep, steep-sided valley into sedimentary bedrock, and between 35 and 10 thousand years ago this valley carried enormous volumes of glacial meltwater and outwash sand and gravel from melting continental glaciers to the north and west. The level valley floor occupied by Guttenberg and Lock and Dam No. 10 is underlain by more than 150 feet of these sediments, showing deep scouring of the bedrock floor followed by partial filling of the valley. The high bluff on the west valley wall exposes layered limestone strata formed from lime-rich muds deposited in shallow tropical seas during Ordovician time about 440 million years ago. From here to the bottom of the hill is the most complete known exposure of the Galena Group rocks, a widespread sequence named for outcrops at Galena, Illinois, with a name tied to the lead ore mined in this district for at least the last 300 years. Fractures and crevices in these carbonate rocks were enlarged by seeping groundwater over geologic time, producing sinkholes, underground caverns, and springs along steep valley sides. Moist cool air moving through these subterranean passages creates isolated habitats for unusual plant assemblages more typical of northern climates. Along with its geologic and ecological significance, the valley also served as an important corridor for prehistoric and historic exploration, trade, settlement, and transportation.