In its traverse of 2400 miles from Lake Itasca to the Gulf of Mexico, the Mississippi River falls 1475 feet, nearly two-thirds of which is within or along the eastern side of Minnesota. For 300 miles from Minneapolis, its course lies between rocky bluffs bounding a valley from one to six miles in width. The gorge has existed throughout the Pleistocene period of geologic time, during which it has served as a drainage channel for meltwaters from the glaciers of the Great Ice Ages. The melting of the ice at the end of each glacial epoch produced torrential floods which scoured the valley 200 feet below the present river surface. During the inter-glacial stages, after the ice had melted, the river, greatly reduced in volume and no longer able to transport sand and gravel, filled its valley to the present level. The history of the Mississippi River during the last million years, while Minnesota and Wisconsin were undergoing repeated glaciation, is characterized by corresponding cycles of erosion and channel filling, the latter being the phase in which it is now engaged.