About 13,000 years ago, glacial melt water that had collected in Glacial Lake Agassiz burst through a natural earthen dike and created a huge torrent that carved the Minnesota River Valley as it appears today. Along parts of the valley, the scouring force of the ancient Glacial River Warren cut so deeply that 3.6 billion-year-old metamorphic gneiss bedrock was exposed. These gneiss outcrops are scattered along the valley floor from Ortonville to Mankato and today support unique plant and animal communities that persist in a harsh environment. The Glacial River Warren was not the first river to follow this path, but its great volume left a landform still recognizable from satellite images, with a valley almost two miles wide in places and 130 feet deep in others. Today the slower Minnesota River winds through exposed knobs of ancient gneiss from Browns Valley to its confluence with the Mississippi River at Fort Snelling in Mendota, Minnesota. Some of the oldest exposed rocks known on earth occur in the Minnesota River valley, and geologists estimate that some bedrock in this area formed more than three billion years ago during the Precambrian Era. In Memorial Park, throughout Granite Falls, and nearby in the Blue Devil Valley Scientific and Natural Area, outcrops of 3.6 billion-year-old gneiss can be seen. Gneiss is a banded metamorphic rock formed when granite and other rocks were subjected to intense heat and pressure deep beneath the earth's surface over long periods of time. The word comes from an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material. Rocks in the upper Minnesota River Valley that show minerals in distinct bands are probably gneiss. These rocks are among the oldest known exposed rocks in North America, and only bedrock recently identified in Greenland is currently known to be older. The gneiss outcroppings in this area provide rare habitat for distinctive plant and animal communities. The five-lined skink depends on open rock outcroppings with nearby hardwood communities, and the area in and around Memorial Park is one of the few places in Minnesota where it is found, though encroaching woody vegetation is reducing its habitat. Lichen, a symbiosis of fungus and algae, live on these barren surfaces, can dissolve rock, begin soil formation, survive severe cold, and remain dormant for long periods. On the ancient gneiss outcroppings in the Minnesota River Valley, the Plains Prickly Pear Cactus, the Brittle Prickly Pear Cactus, and the Ball Cactus have thrived for hundreds of years. From its beginnings in the plains to its meeting with the Mississippi River, the Minnesota River and its valley preserve a record of ancient history and ecological diversity in the Upper Midwest.