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Lake Michigan Rock Picker's Guide
By Bruce Mueller Kevin Gauthier
The University of Michigan PressCopyright © 2006 Bruce Mueller and Kevin Gauthier
All right reserved.
Chapter OneLake Michigan's rocks
Type Stone Hardness
Basalt 6 Brachiopods* 3 Chain coral* 7/3 Chert 7 Cladapora* 3 Copper 3 Crinoids* 3 Diamonds 10 Favosite/ Charlevoix* 3 Feldspar 6 Fern Creek tillite 6.5-7 Frankfort green 5.5 Fulgurite 7 Greenstone (chlorastrolite) 6.5 Granite 6.5-7 Gold 2.5-3 Gowganda tillite 6.5-7 Horn coral* 3 Honey comb coral* 6.5-7 Jasper 7 Lake Superior agate 7 Leeland blue 5.5 Lightning stones 3.5-4 Marcacite 6-6.5 Misfits a range Moonstone 6 Pipe organ coral* 3 Petoskey stones* 3 (Michigan's state stone) Pudding stone 6.5-7 Quartz 7 Rhyolite porphyry 6-7 Sandstone 7 Stink stone 3 Trilobite* 3 Unikite 6.5-7
Rock Collecting Rules
Keep in mind you are sharing and using the same beaches with many other people. The parks and places mentioned in this book should be treated with respect by following their rules and regulations. The rules for collecting vary from state to state and from park to park. There are national parks, state parks, county parks, township parks, city parks, and there is private property. It's best to check in advance before collecting. In general the more important the park, national or state, the less likely collecting is to be allowed. Some park officials are more lenient than others. Leave a few less rocks for the waves to replenish and only foot prints behind.
For listing of parks see the following web sites:
www.michigandnr.com www.illiniosdnr.com www.wisconsondnr.com www.indianadnr.com www.parks.state.mi.us
How Lake Michigan Was Formed
Lake Michigan was created by the Wisconsin ice sheet, the last of the great ice sheets to move out of the north during recent time. One lobe of the ice sheet moved into a river valley that occupied a valley where the lake is today. The valley was there because the bedrock was soft shale.
As the glacier moved down the river valley, it drug hard rock along its base, especially granite, that it had picked up in Canada. To the north of the lake lie two million square miles of Canadian Shield; the ancient stable core around which the North American Continent was built. This glacier, which created Lake Michigan, was about a mile and a half thick. At its base the pressure of the overlying ice would have been well in excess of 200 tons per square foot. Both the rock that the glacier was dragging and the shale were ground to dust.
The glacier wasn't a bulldozer, it was a conveyor belt driven by sunlight on the northern hemisphere. Each cubic mile of ice carried a fraction of a cubic mile of rock south, and the glacial conveyor belt flowed south for about 63,000 years. As the glacier moved south it was eroding. The powdered rock, sand, gravel and boulders it carried were dropped to the south, east and west of the lake. As the glacier began to melt back, it deposited the stone it carried in the lake and on its shores.
Stones that the glacier had collected from the north, dropped these small treasures onto the beaches. Some of the rock found at Illinois Beach State Park may have come from the Arctic coast of Canada nearly 2000 miles away, and the glacier collected and deposited rock from everywhere in between. The glacier collected rocks for us from places we could never go and left them on beaches that are practically on our doorstep here in the Midwest. Because of glacial transport of rock from the Canadian Shield, Lake Michigan and the other Great Lakes have more varieties and colors of rock on their beaches than you are likely to find anywhere else on earth. All of this variety and color is part of the charm of Lake Michigan beaches.
Variety -16 Flavors in a Dixie Cup
On a very warm fourth of July, I was on Old Mission Peninsula off Bluff Road (near Traverse City, Michigan). The beach was sandy and very few rocks washed ashore as the beach faced south east. Prevailing winds can be a factor to rock exposure. That day I was in-charge of watching our children playing in the water - a very busy three-year old girl and a five-year old boy. Since it was such a hot day I decided to build a sand castle at the water's edge to keep my feet in the cool water. As I dug down, I hit a layer of very small rocks about dime size. The rocks were always there but I believe the current must have buried these rocks with about six inches of sand. Before I realized it, I was on a rock-hunting trip, one that took no effort to walk anywhere. Digging between my legs, I carefully studied each rock. That day I got a real feel for the variety of rocks that could be collected on Lake Michigan. In an hour I dug a one-foot circle, about a foot deep, filling only a Dixie cup. The best part about this rock trip was the carrying of the stones. A year later I decided to take a tally of the "pretty" rocks I found that day.
In that Dixie cup I had found: 4 red jasper 13 granite stones with colors varying from red, yellow, black, green, and pink 2 limestone (one was foot shaped) 1 basalt with green epidot lines 1 yellow granite with epidot lines running through 6 Petoskey stones 17 Favosite fossils 1 crinoid stem fossil (notice the eye ball resemblance) 1 branch coral 1 sand stone - burgundy in color 1 black basalt 2 feldspar 4 pieces of tan chert 1 flint chip (that looked like it could have come from working an arrow head) 18 transparent quartz (60% of the earths surface is quartz, much of which is sand) 3 Lake Superior agates (two chips and one whole nugget with distinctive eye banding) (See color photos 2, 3.)
All of this in a one foot circle which I had thought was only a sandy beach! When I found the rocks, my priorities changed very quickly. The kids ended up with a very small sand castle and I was reprimanded to keep a better eye on the kids next time.
Canadian rock is easy to recognize. You won't be able to scratch it with a knife blade. If it has flattened or elongated crystals or alternating dark and light bands of minerals, it is metamorphic. Otherwise, with a few negligible exceptions (such as chert), it is igneous. If the rock can be scratched with a knife (with the exception of chert), it is bedrock from the Upper Peninsula, or much more likely from the bedrock where you are or from just offshore.
Just Three Sources for Lake Michigan Rocks
It's hard to tell in advance whether a beach will be grassy, sandy or covered with stone; but it's relatively easy to know what the stone will be if it's a stony beach. There are just three sources for beach stone: The Canadian Shield, sedimentary rock from further north and local sedimentary rock. The Canadian Shield rock will be present in greater or lesser quantities on every stony beach; it's a given. If you know what the sedimentary rock to the north is and what the local bedrock is, you know in advance what types of rocks there will be at a particular beach.
A Trip around Lake Michigan-Understanding the Bedrock
Point # 1- Bedrock near Fort Michilimackinac, MI
To start a bedrock trip around the lake, a good place to begin is at the south end of the Big Mac Bridge at the top of the Lower Peninsula (point one on the map).
If we start there, we will save the best for last. If you stop at Mackinaw City and park under the bridge near Fort Michilimackinac and go down to the lake, the gravel you will find there is Devonian dolomite. This dolomite was shattered to a fine gravel called breccia when the underlying water-soluble rock dissolved away and the overlying dolomite collapsed into the void. All of this happened hundreds of millions of years ago and its probable collapses occurred repeatedly.
Most people who like to collect fossils like to collect large, splendid, well-preserved specimens; but there are no large specimens here, only gravel. The gravel does contain many fossils but with few exceptions, none is well preserved. While you're there under the bridge, Fort Michilimackinac is worth seeing.
Point # 2- Bedrock near St. Ignace, MI
On the other side of the bridge (at point 2 on the map), the rock is dolomite of Silurian age. It's called Niagaran dolomite because this is the layer over which Niagara Falls falls. There are generally few, if any, worthwhile fossils in dolomite. Sometimes there are interesting, even rare, fossils in dolomite, but the preservation leaves something to be desired. When you find other stony beaches in the Upper Peninsula, they will have stones similar to those found near Brevort. If you take Route 2 from the toll booth on the north end of the Big Mac Bridge, you will pass miles and miles of excellent sandy swimming beaches, which during the hot days of summer will be swarming with people swimming and getting a sun tan. Everyone just parks right along the road and goes down to the beach.
19.2 miles from the bridge toll booth, and about two miles east of a little town called Brevort, you will see a scenic overlook sign on the lake side of the road. At the end of the road there is a stairway leading down to the lake. This is an exceptionally nice beach. It is covered with glacial gravel, most of it from either the Silurian or Ordovician bedrock that lies to the north in the Upper Peninsula, and with colorful stones from Canada. Here there are many large fossil coral colonies.
There are at least five or six different coral species here. There is chert, some of it perhaps gunflint chert from Canada. There is banded chert here, turitella-like snail shells and hornblend schist. There is also Michigan moonstone, we found two low quality Lake Superior agates, mottled chert, chain coral, Favosites and the usual basalt, granite and quartz. If it's about lunch time you might eat in Brevort, as there are a couple of nice restaurants there. You can also find the very famous pasties all around this area and throughout the Upper Peninsula.
Point # 3 - Bedrock near Escanaba, MI
The rest of the Upper Peninsula (west to the shore of Green Bay, north of Escanaba) is mostly relatively barren Niagaran dolomite or sand.
Look for Cut River Park at Cut River Bridge. There is nice scenery and there are great trees here, but it's 231 steps by stairway down to the lake. At the lake you will find igneous, metamorphic and dolomite boulders too big to pick up. Driving south along the shore of Green Bay, the water is very shallow, dolomite bedrock is exposed almost everywhere except where it is covered by sand. There is little or no glacial gravel.
Driving north on the west shore of Door County, the bedrock continues dolomite, but here the water offshore is deeper and the prevailing wind blows onshore, generating better waves. Along this shore are places where the lake is accessible by road. The rock on the beach contains Canadian gravel, but not much. The rock here is primarily broken cobbles of dolomite washed clean by wave action. There are brachiopods and corals here along with many other fossils.
About 2 miles east of Dykesville on Route 57 there is a small park and boat launch. The road cuts through a cliff where great blocks of dolomite have fallen off. The surfaces of these blocks are covered with raised, irregular marks called fucoids. These are what remains of trails and tunnels on and in the mud of the sea floor created by unknown organisms.
Wisconsin's State Fossil The most commonly found Silurian trilobite is Calamene.
Point # 4 - Bedrock near Dykesville, WI
Niagaran Silurian dolomite is a very white dolomite, the same dolomite that is seen at Fayette on the Garden Peninsula. Driving to the point of the Door County Peninsula and south along its east side, it's all Niagaran dolomite, and the shores are nearly free of Canadian gravel. Not until you get to Algoma, near the base of the east side of the peninsula, do you see much Canadian gravel. Right in the middle of town there is glacial gravel and sand, which I suspect was hauled in by trucks to create a decent beach.
South of Algoma at Point Beach State Park there is much glacial gravel, but it is mostly buried by sand. To the south between Two Rivers and Manitowoc, the road parallels the lake and many parking places are provided. Here you will find stones from Canada along with fossils from the Upper Peninsula.
To the south, north of Cleveland, is Fisher Creek State Park. Here, there is a nice deposit of Canadian rock and rock from the Upper Peninsula. My wife and I found tillite (probably from near Norway, Michigan), two low quality Lake Superior agates and several septarian concretions, banded chert, porphyry, rhyolite and the usual igneous and metamorphic Canadian stones.
How do I know if I have a "good" agate?
Agates can be graded many different ways. Primarily the contrast between colors and intense banding will yield a higher dollar value. Also to be considered is the transparency or milkiness of the stone. How rare the type of agate is also can be a big factor. Buying bulk Lake Superior agates should set you back around $8-$20/lb. Most of the bulk agate sold comes from Minnesota gravel pits. The Gem Shop in Cedarburg, Wisconsin, has some very nice agates, and it is well worth a visit if you are traveling near this area.
Point # 5 - Bedrock near Sheboygan, WI
The bedrock here is Devonian dolomite. Just to the south is Kohler-Andrae State Park. There is little here other than sand on the beaches. At Harrington Beach State Park, north of Port Washington, dolomite was quarried and shipped south for use as a flux in the steel mills. A pier made of stone taken from the quarry was built out into the lake for loading the dolomite into ships. This stony pier remnant can still be seen to the south along the beach. The quarry itself is abandoned and filled with water.
Point # 6 - Bedrock near Milwaukee, WI
At Eastbrook Park, just off the lake on the Milwaukee River, there is Devonian rock with Devonian fossils, mostly bryozoans, crinoids and brachiopods. At Kenosha the beaches are about 100 feet wide and alternate between sand and gravel. Big diamonds were found in Wisconsin, so don't forget to keep an eye out!
21.25 carat diamond found near Milwaukee, Wisconsin! Diamonds are found in a path two hundred miles wide that runs from Iowa on the west to Ohio on the east. The largest diamonds have been found in Wisconsin near Milwaukee. The largest that I know of, a 21 and a quarter carat diamond, was found near Theresa, Wisconsin. One found near Eagle, Wisconsin, in the northwest part of the state, weighed almost 16 carats.
Illinois Beach State Park
Near Zion, Illinois, is Illinois Beach State Park. There are seven miles of beach here with an obsolete nuclear reactor in the middle. Unconfirmed rumors say it contains radioactive material. I hope they locked the door when they left. On this beach there is metaconglomerate, probably from near Norway, Michigan. There is also jasper from the U.P., banded quartzite, and rhyolite porphyry from the floor of Lake Superior, basalt with gas cavities filled with calcite, pebbles filled with crinoid stems, breccia (shattered rock glued together with calcite), chert, quartz, jasper, granite, local Niagaran dolomite, black basalt and phylite (a metamorphic rock containing mica). And there is porphyritic basalt and metamorphosed porphyritic basalt, along with other stones of lesser interest, as well as some God-only-knows-what-they-are stones.
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