Lake of the Isles, and Cedar Lake, known together as the "Chain of Lakes". Lake Nokomis and Lake Hiawatha are to the east. Connected by bike, running and walking paths, Minneapolis lakes are used for swimming, fishing, picnics and boating. Waterfalls The area now occupied by the Twin Cities generally consisted of a 155 foot (47 m) thick layer of St. Peter Sandstone, under a 16 foot (5 m) thick layer of shale, under a 35 foot (11 m) thick layer of Platteville limestone. These layers were the result of an Ordovician Period sea which covered east-central Minnesota 500 million years ago. The hard limestone cap was formed from fossilized shell fish. About 20,000 years ago, the area was covered by the Superior Lobe of the Laurentide Ice Sheet, which left the St. Croix moraine on the Twin Cities as it receded. Later the Grantsburg Sublobe of the Des Moines Lobe also covered the area. Under these vast layers of ice, tunnel valleys were formed, cutting through the limestone layer with tremendous force, to release ice meltwater and glacier effluence. The result was a series of troughs in the limestone, which were filled by glacial till and outwash deposit as the glaciers receded. Sometimes the sediment would be mixed with huge chunks of ice, which would leave voids in the soil. These voids created basins for the Twin Cities Lakes, such as Harriet and Lake Calhoun. Connecting the city lakes in several north-south arteries are gorges cut through the bedrock, but filled with sand and sediment. When River Warren Falls receded past the confluence of the much smaller Upper Mississippi River, a new waterfall was created where that river entered the much-lower glacial River Warren. The new falls also receded upstream on the Mississippi, carving the Mississippi River gorge, as it migrated eight miles (12875 m) over 9,600 years to where Louis Hennepin first saw it and named St. Anthony Falls in 1680. Due to its value as a power source, this waterfall determined the location of Minneapolis. One branch of the river coming from the west, Minnehaha Creek receded only a few hundred yards from one of the channels of the Mississippi. Minnehaha Falls remains as a picturesque and informative relic of River Warren Falls, and the limestone-over-sandstone construction is readily apparent in its small gorge. Notes References Minneapolis–Saint Paul