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Oh, the Story At the 'Root' of This Ordinary Rock

LAKE SUPERIOR - APOSTLE ISLANDS NATIONAL LAKESHORE

 

"Nature, even when she is scant and thin outwardly, satisfies us still by the assurance of a certain generosity at the roots." - H. D. Thoreau

A GEOLOGIC RIFT

- Iowa DNR

The Midcontinent Rift

-----One of the most significant geologic features on the North American continent is hidden beneath up to 6 miles of younger sedimentary rocks. Our continent almost split in two! And the evidence is exposed generously along the shores of the Apostle Islands in Lake Superior.

-----Slightly more than one billion years ago a huge tear or rift, over a thousand miles in length, began to rupture our continent from today's Lake Superior to Oklahoma! The resulting deep trough between the continental fragments filled with enormous lava outpourings from the deep bounding fractures. Rivers poured into the rift valley carrying sandy continental detritus. Sediment-filled lakes occupied much of the trough. The underlying crust buckled down even farther under the immense weight of the lava and sediments. The deep subsidence of the rift downbowed the crust so far it was well on its way to becoming connected to the existing oceans and becoming a major ocean in its own right. No known rift of such dimensions as the Midcontinent Rift ever failed to become an ocean. Wisconsin and Minnesota would perhaps be as distant today as Oregon and China. As the rift widened, river-borne sediments continued to flood into the trench from the surrounding continental margins, These bedded and fine-grained sediments, now rock, are seen along the Wisconsin shores of Lake Superior. The lavas that poured from the fractures solidified into layered basalt beds, seen today farther to the west on Minnesota's Superior shores.

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Thin Rift-filling Sands Over One Billion Years Old

-----And then - it stopped! The forces, that had begun to rip apart a continent, ground to a halt. And then reversed! The continental margins pushed back toward each other. The lake sediments, river sand and gravels, and basalts in the deep rift were forced upward by the compression (such an uplifted block is termed by geologists a horst) and lifted a lofty mountain range, flanked by deep downbuckled basins on each side. Note the dark blue interior of the rift, labeled horst, in the diagram above (and the light-colored flanking areas termed basin).

-----The extent and enormity of this huge rift was largely unknown until geophysical techniques were developed in the last century. Now we can accurately trace the feature beneath the overlying sedimentary cover. These geomagnetic, seismic, and gravity maps have outlined the rift as one of the most prominent features of the continent. Lake Superior occupies a basin created near the northern end of the rift. Here - on the shores of Kitchi-gami - the evidence of an 'almost ocean' is exposed at the surface.

SPECIAL GEOLOGICAL FEATURES

-----About 12,000 years ago only one island (Oak) stood above the waters of the early Lake Superior (called Glacial Lake Duluth by geologists). A beach developed about 1,000 feet above the current lake level. Slowly, with isostatic rebound, after the weight of ice was gone, other islands rose above the water and earlier beaches were 'high and dry' above the present-day lake level. Lake level has been relatively static for the past 5,000 years.

Note the Notch Half Way Up The Distant Hill - That's The Ancient Wave Cut Terrace and Site of the Beach

Sandstone Cliffs

----Lake Superior's notorious storms create massive collision between water and land. Eventually the erosive force of water wins the battle, and the sandstone foundation of most of the islands is exposed on the island margins. Some of the cliffs rise 50 feet above the lake and make dramatic scenery for cruising boaters.

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Cliffs Eroded into Sandstone Architecture

Sea Caves

----Fractures (termed joints by geologists) formed in the brittle sandstone in response to pressure from the crustal movement. First the loading of over a mile of ice atop the sandstone, then the removal of that glacial weight, created or enlarged fractures. Storm waves sometimes take advantage of these weaknesses in the bedrock sandstone, enlarging the fractures or hollowing out sea caves. Winter ice, freezing in the fractures, also wedges out sand grains or hunks of sandstone.

-A Fracture Enlarged into a Sea Cave

----Cruising or kayaking from Little Sand Bay to Meyers beach, through Squaw Bay along the mainland, is a delight to the eyes. Some of the caves extend more than 50 feet into the cliff and can be entered (carefully) by kayaks or small boats. A visit to Devil's Island, offshore on a calm day, is also a visit to beautiful caves. Don't miss these sea caves - they are one of the highlight features of a visit to the National Lakeshore.

Sea Cave Big Enough to Shelter a C-dory

Sea Stacks

----Sometimes sections of sandstone cliff are isolated from land by wave erosion and sea stacks result. They are temporary features, since the next storm may destroy them.

A Stack in the Making

Arches and Windows

----The elusive fingers of erosion also remove rock fragments creating arches or windows where overlying rock maintains its integrity. The collapse of these features result in changing landscapes over time.

Hanging on to Shore - A Superior Column

Beaches

----The final erosive result of the reddish/brown sandstone is red sand grains. These concentrate at the head of coves or in the lee of islands into beautiful beaches. Where the water is shallow, summer sun warms the lakewater and swimming is delightful (albeit often a bit chilly).

Tombolos and Cuspate Forelands

----Sand is mobile. Waves shift the sand grains and in the lee between two islands sand often finds respite. When the sand connects the islands the resulting beach is referred to as a tombolo. Some of the islands, long joined by tombolos, have become 'one' with the tree-covered tombolo masking the original dual nature of the island. In the lee of some islands sand has accumulated into a pointed rounded beach - a cuspate foreland (geographers love names).

GEOLOGICAL MUSING

----Geology is mostly a study of the past, but speculation into the future is a natural byproduct of pondering history. Swinging on the hook in the lee of billion-year-old rock, as the moon rises slowly with the earth's spin, one can't help but wonder ...

----Of course, there are those who do their best pondering at dawn ...

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New Ideas With a New Day

(07//10)

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