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CA Beautiful Lakeshore |
LAKE SUPERIOR - PICTURED ROCKS Launch: Munising MI Town Ramp
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PICTURED ROCKS NATIONAL LAKESHORE
-----Cruising on Lake Superior is a daunting experience. We thought we would probably visit Pictured Rocks from land, but the weather cooperated beautifully. We launched at the Munising Town Ramp and headed up the coast - and what a beautiful coast - America’s first National Lakeshore, set aside in 1966 as Pictured Rocks National Lakeshore and administered by the National Park Service.
-----"We go along the coast, most delightful and wonderful said the first European to visit this coast , Pierre Esprit Radisson, in 1658. "Nature has made it pleasant to the eye, the spirit and the belly." In 1820, Henry Schoolcraft described the shoreline as boasting "some of the most sublime and commanding views in nature." And so it remains to this day.
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A Marvelous Sight
Ancient Geology
-----A beautiful cliff is, to a geologist, not only a wonderful sight to the eye but a challenge to the mind as well. Usually the thought is summed up in the simple early word of a young child, "Why?" I suppose we have never gone beyond the simplistic (?) challenge of trying to understand the reason for things around us. Those who have studied these rocks to determine the clues to their origin write that the sandstone comprising the cliffs are from an early time period on earth when simple life was first evolving into greater complexity - a time from 800 to 500 million years ago.
-----Imagine you are at this spot, a little over 550 million years ago. What a different sight! You are in sweltering heat, since you are only a few degrees south of the equator. You are surrounded by a sandy, barren, rock-strewn plain as far as you can see in three directions. Only to the southwest is there topography. There, in the distance, is a highland of rocky mountains - about as high as the Appalachians of today. Streams carry sand, and in flood, rocks down the plain stretching northward to the horizon from the mountains. Not a blade of grass or a single shoot of greenery - no tracks from land animals. In the oceans, life was teeming, but the material that comprises the base of the Pictured Rocks (Jacobsville sandstone )was deposited before evolution had produced visible land plants or animals. The equatorial region was hot and humid, so the iron in those ancient sediments rusted a bright red creating the red sandstones we see today as the bottom layer of the cliff on the west-facing side of Grand Island and along the cliffs near Munising Streams, in flood, scoured that plain for millions of years depositing and stripping off layers of sediment.
Ancient Sediment
-----Then, about 520 million years ago, a shallow sea flooded across this area. The earliest sediments deposited by the sea contain pebbles and cobbles from the underlying flood deposits, and comprise a conglomerate today. As the sea deepened, sands washed off the surrounding highlands and were deposited off the coast. Marine animals flourished in this warm shallow environment.
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The Conglomerate at the Bottom; The Sandstone Above
-----When looking at the red and white mottling, it is natural to question, "why?" Was the sandstone once all red? ... all white? ... Or layers of red and then white? Geologists who have studied this area extensively believe that it was deposited as a red sandstone, stained by oxidized iron. Later movement of ground water through the beds (especially along fractures and the bedding planes) leached out the iron. Notice the near-vertical fracture in the photo above. Water moving along this fracture has removed the red coloration from both sides of the fractures indicating an answer to the 'why?"
-----The red ancient sand was extensively quarried in the 1900's for building stone. Many buildings in Munising and Marquette are constructed or decoratively faced with the sandstone.
-----Near the end of this interval the first footprints of animals were found on our planet; their tracks and burrows can be found in the sandstone. During floods, rivers ran off the continent and carried sand and gravel into shoreline deltas and and some freshwater lakes formed behind the shoreline dunes. The former highlands were reduced by erosion and only the highest areas remained as islands.
-----The top of the cliffs is composed of a well-cemented light-brown sandstone, a fine capstone resisting erosion. It acts as the lip of the many waterfalls in the area.
The Capstone Light-colored Layer (Lip of the Falls) Above the Ancient Red Sediment
Recent Geology
-----The geology of the area is divided between the very old and the new. The old rocks make up the cliffs along the shore - much is near a billion years old. But above the hard, old remnants, there is a cover of sand, gravel and boulders - the tracks of ice!
-----A huge ice mass encroached on the Great Lakes country, from the north, four times - and melted away four times (fortunately, or we would live on an iceboat). Each advance wiped away virtually all the evidence of earlier advances. The last major thrust to the south ended about 12,000 years ago with the last pulse ending only 10,000 years ago in upper Michigan.
-----The melting of that last ice mass, about a mile thick, created huge rivers that deposited millions of tons of sediment over Michigan. River sediment covers the southern portion of the National Lakeshore, but most has been removed from the shoreline and cliff area by recent rain and streams.
-----Some chunks of ice, covered by the sediment that had been within the icemass, remained intact within the sheet of sand and gravel outwash covering the land. These buried ice hunks slowly melted and the overlying detritus sank into the holes left behind. Meltwater or rainwater often filled these depressions creating a landscape with many small 'kettle' lakes.
-----Green is the Seashore and AlI Those Blue Lakes - Glacial Kettles
-----Ice is heavy - any of us with ice chests on our boats are reminded of that every time we resupply our ice from a store. The weight of a mile-thick ice sheet is humongous. Since the rock crust we live on 'floats' on a soft, plastic hot mantle below, the weight of the ice depresses the land surface. When the ice melted, the outlet channels to the east near North Bay, Ontario, were still depressed and water drained the Great Lakes country dry. Pictured Rocks was not a lakeshore 9,500 years ago - it was dry land.
-----With the ice weight gone, the land began to rebound. About 5,000 years ago, the land had rebounded and water from the lake country could no longer drain easterly through the North Bay outlet. This was the birth of the Great Lakes. Superior's lake level was then about 40 feet above today's level. Slowly the draining rivers cut down through the glacial gravel or found new, lower courses to follow, and water level in the lake fell to the present stand. Periods of still-stand cut beaches above present levels - preserved as benches in the gravel above today's shore.
The Shore
-----Let's simply kick back and take a cruise along this beautiful treasure - our National Lakeshore.
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El Paddling
(08/10)
