SciFri 6.24.05: Rocks of ages

A view of the east bank of the Mississippi River just below the Washington Avenue Bridge. The St. Peter sandstone is at the bottom, and above that, behind the children, is the yellowish Glenwood shale, which is often covered by vegetation. The highest layer is the Platteville limestone.

From sandy tropical beach to doormat for glaciers, the Twin Cities area has been through a lot during the last few hundred thousand millennia

By Deane Morrison

Published on June 24, 2005

If you think it's hot today, you should have been here 450 million years ago. Back then, the future Twin Cities area was near the equator, under a shallow tropical sea. Much more recently, our corner of the globe was subjected to the other extreme when glaciers ground in from the north and rearranged the landscape. It's been quite a journey, and the evidence for the region's rocky past is all right under our feet.

Take a look at the upper parts of the Mississippi River banks, and you'll see several layers of rock laid down during the Ordovician geological period, which began roughly 500 million years ago and lasted 50-80 million years. At river level is a layer of soft rock called the St. Peter sandstone, the remains of tropical shallows, sandy beaches, and dunes formed from the shiftings of an ancient seacoast. The rock is among the world's purest quartz sandstones, a source of glass for automobiles and quartz sands for filtering water and wines. The Cities are near the northern border of the sandstone, which continues south and west to Missouri and Kansas and east into Michigan.

Right above the sandstone lies a three-foot layer called the Glenwood shale, a pale rock formed from the mud of somewhat deeper waters than those that left the sandstone. The shale formed later in the Ordovician period, and, like the sandstone, the shale is physically weak and erodes easily.

Most prominent among the rocks is the uppermost part, the layers of Platteville limestone. Running about 35 to 40 feet thick, the limestone is built of deposits of lime mud and fossil shells that extend into Iowa and central Wisconsin. The horizontal layers are obvious, but you couldn't walk across the surface of the limestone without running into vertical cracks every four feet or so. The cracks break the limestone into a series of polygonal columns. The cracks also allow water to seep in, with sometimes spectacular results.

When it got to the area near where the downtown St. Paul airport is now, the Minnesota River formed a waterfall a mile and a half wide and perhaps as high as 200 feet.

You've probably heard how the St. Anthony Falls have retreated up the Mississippi Valley from the Fort Snelling area. When water penetrates the cracks in the limestone, it erodes the shale and sandstone underneath. When enough of the supporting rock has washed away, the limestone collapses-not inch by inch, but in big chunks, as the polygonal columns topple. Niagara Falls has retreated up the Niagara River some seven miles over 10,000 years by a similar process.

Above the Platteville limestone there's not much except glacial till. Glaciers have eroded the rocks that formed after the limestone, leaving nothing but the debris that settled out of melting ice blocks. But while they lasted, glaciers also formed lakes and rivers that shaped the terrain.

Retreating glaciers formed glacial Lake Agassiz, a huge expanse of ponded meltwater whose bottom included the present-day Red River Valley. Around 14,000 years ago, the lake burst through a land barrier in the Lac Qui Parle/Traverse Lake area of western Minnesota. The outflow carved the Minnesota River Valley. When it got to the area near where the downtown St. Paul airport is now, the Minnesota River formed a waterfall a mile and a half wide and perhaps as high as 200 feet, says Kent Kirkby, a lecturer in the University's Department of Geology and Geophysics.

The Mississippi River Valley upstream of Fort Snelling formed after the Minnesota River. The "father of waters" began draining northern lands about 10,000 to 11,000 years ago and was very young when the waterfall, having receded rapidly up the valley from near the St. Paul airport, arrived at the confluence of the two rivers. The waterfall split at the confluence, which is near Fort Snelling. The falls retreated faster in the Minnesota River Valley, which had a much bigger flow than the Mississippi Valley back then. The Minnesota River falls ran out of limestone bedrock and disappeared within a few hundred years. But it took 10,000 years for the Mississippi River falls to cover the eight miles to the St. Anthony area.

The St. Anthony Falls were a few hundred feet downstream of their present position when the first white settlers arrived. Had the settlers arrived today, the falls would have run out of limestone and disappeared. And without the falls, there probably would be no Minneapolis. In the 1880s, the Army Corps of Engineers stabilized the falls with concrete.

But the ice age that ended 10,000 years ago wasn't the only one. Several river valleys formed in the Twin Cities area during previous intervals between glaciations. Those valleys are now filled with sediment, but chains of lakes remain as relics. The Minneapolis chain of Cedar Lake, Lake of the Isles, Lake Calhoun, and Lake Harriet overlies one buried river valley. In St. Paul, another buried valley extends beneath McCarron's Lake, Lake Josephine, Bennett Lake, and Lake Johanna. Tributaries of that now-buried river valley used to run beneath Como Lake and Owasso Lake. Another big, buried river valley in the St. Paul area runs beneath Lake Phalen, Lake Gervais, Sunfish Lake, and Snail Lake.

"A lot of people don't realize that the only reason Minnesota and Wisconsin have so many lakes is that they were recently glaciated," says Kirkby.

To the south, the paths of these latter two large, buried river valleys bracket downtown St. Paul. The downtown area sits atop a bluff, between two lower areas that gently slope down to the Mississippi. These two slopes mark the places where the older, now buried river valleys cut through the bluffs. The city's location was chosen so it had easy access to the river for commerce, while the higher ground protected buildings from flooding.

South of the Cities, Lake Pepin formed where the Chippewa River meets the Mississippi. The Chippewa dumped sediment into the Mississippi, forming a delta that constricted the Mississippi's flow. The water backed up to form Lake Pepin, which once extended all the way to the St. Paul downtown airport site.

The Mississippi dumped sediment into the St. Croix River, causing water to pool upstream. The town of Stillwater--the first European-settled town in Minnesota--grew up on the banks of the calm water because logs floated in from the northern forests could be slowed down and sorted for the sawmills.

In the future, the Mississippi will cut deeper into the land, eventually draining Lake Itasca and eroding its way up to the continental divide--maybe even through it. Many other lakes will also be lost, either by rivers working their way across the land or by filling with sediment. Such is the way of all lakes.

"A lot of people don't realize that the only reason Minnesota and Wisconsin have so many lakes is that they were recently glaciated," says Kirkby.

So get out there and enjoy them now.

For a tour of the river bluffs with photos taken by Kirkby, see tour.