Geology of the Twin Cities: Remains of Ancient Oceans


The rocks in this bluff at Lock and Dam #1 form the Mississippi River Gorge and were deposited ~450,000,000 years ago when oceans covered most of southeastern Minnesota. The lower white, grey and iron-stained rock unit is the St. Peter Sandstone which is the oldest since it is on the bottom. The very thin light green, grey rock unit on top of the St. Peter is the Glenwood Shale (silty clay) which causes the indent of the bluff and is often where vegetation takes hold. The Glenwood is very easily eroded, thus creating the often wide overhangs of the rock unit above, the Platteville Limestone. The Platteville is the very thick, layered, blocky and fractured grey rock unit, often with fossils in it, that is on the top. It is formed from the hard parts of once living organisms in the ancient sea and is less easily eroded so maintains a steep face. The Platteville can form large overhangs as the Glenwood Shale is eroded away that can break off without the underlying support of the Glenwood and this is very easy to see at Minnehaha Falls. This limestone collapse process has occurred frequently and has caused the falls to move backwards from the Mississippi River to its present position

Photo: Megan Longtine-Jones

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