About 13,000 years ago, as a warming climate melted the glaciers that encased this land, Glacial Lake Agassiz formed in the northwest corner of what is now Minnesota and eventually overflowed south across the continental divide, creating the ancient Glacial River Warren. This tremendous torrent, in places almost two miles wide and 130 feet deep, excavated millions of tons of earth and rock, gouged out a remarkable oversized valley across Minnesota, and drained into what is now the Mississippi River on its way to the Gulf of Mexico. After Glacial Lake Agassiz eventually drained as the remaining glaciers retreated farther north, the much smaller Minnesota River remained winding along the valley floor. The valley preserves evidence of these immense forces, including some of the world’s oldest exposed bedrock, potholes ground into solid rock by stones caught in the current, and rock outcrops that support unusual species such as prickly pear cactus, other cactus species, and the five-lined skink. From its beginnings in the plains to its confluence with the Mississippi River, the Minnesota River and its valley reveal ancient history and rich ecological diversity in the Upper Midwest.