Read an Excerpt
Lake Huron Rock Picker's Guide
By Bruce Mueller Kevin Gauthier
The University of Michigan PressCopyright © University of Michigan 2010
All right reserved.
Chapter OneLake Huron Geological History
Although beach composition varies greatly, about half of the stones on an average shoreline are local bedrock. All other rocks, except those trucked in for erosion control, have been brought in by glacial transport from one of two vast and divergent rock systems: the Michigan Basin System and the Canadian Shield System.
The Canadian Shield covers 2 million square miles and is composed primarily of granite that has melted its way up from below to form a wildly diverse landscape of rock. Largely assembled from miscellaneous bits and pieces brought in from elsewhere, this rock was scraped off as the seafloor was subducted below the Shield. A similar process is currently occurring in portions of California west of the San Andreas Fault and the Baja Peninsula. This area is moving with the Pacific seafloor plate toward the Alaskan Peninsula at a rate of 20 feet per hundred years. One day in the near future (as geologic time goes), they will be part of Alaska.
To further complicate the picture, much of the Canadian Shield has been folded to form mountain ranges, now largely eroded down to their metamorphosed roots. Canadian Shield rocks are almost always harder than a knife blade, and they are old—up to 4.28 billion years old—with granite being relatively younger and the basalts and metamorphic rock generally older.
In contrast, Michigan Basin rock is young, sedimentary, and almost always softer than a knife blade. The basin was formed during the earth's early history when an asteroid impact or down-warping caused by the rising of a distant mountain range created a basin in the earth's surface. Once created, the Basin began to fill with sediment brought in by rivers and marine invasions. The Basin filled with sediment from its outer edge inward. Seen from above with all soil and glacial debris removed, these layers form a targetlike pattern with the bull's eye, the youngest rock in the system, located at Midland, Michigan. Seen from below Midland in cross-section, the oldest layers are on the bottom with each overlying layer younger.
The outer edge of the Michigan Basin is marked by the Niagaran Escarpment (so-called because Niagara Falls flows over its edge along an entrance way to the Michigan Basin). Much of the Niagaran Escarpment can be seen on any map of the Great Lakes. The Escarpment was formed during the Silurian period when seawater moved across what is now New York State, along what is now the St. Lawrence River Valley, and into the Basin. This water flowed in just fast enough to replace evaporation, leading it to grow saltier as time went on. As water becomes more saline, its least soluble elements start being deposited. In this case, limestone, composed primarily of shell fragments from marine organisms as well as from evaporation, was deposited around the Basin's edges and across its floor. As the water continued to increase in salinity, magnesium replaced some of the calcium in the limestone to create a hard, resistant layer of dolomite (a calcium magnesium carbonate). This conversion to dolomite caused the rock to lose volume and become harder, more acid resistant, and more porous. The process also typically degraded any fossils present.
Marking the outer edge of the Michigan Basin, the Niagaran Escarpment encloses Michigan's Lower Peninsula, Lake Michigan with the exception of Green Bay, Lake Huron with the exception of Georgian Bay, the eastern portion of Michigan's Upper Peninsula, eastern Wisconsin, the northeast corner of Illinois, northern Indiana, northern Ohio, the western half of Lake Erie (where it has no surface expression), and southwestern Ontario. Physical expressions of the Escarpment can be seen throughout the region, starting with the Bruce Peninsula, which separates Lake Huron from Georgian Bay. Following the curve made by the Bruce Peninsula leads to a string of islands, among them Manitoulin Island and Drummond Island. Following the same curve across the Lower Peninsula leads to the Garden Peninsula and eventually to another string of islands and Wisconsin's Dorr County Peninsula. From here, the Escarpment forms the western edge of Lake Michigan through Chicago. Further evidence of the Escarpment can be found near Thornton, Illinois, a suburb of Chicago, where one of the world's largest quarries extracts material from what was once a reef on the Escarpment. The dolomite layer then passes through Indiana and Ohio and back to the Bruce Peninsula.
Just inside the Niagaran Escarpment are layers of younger, softer rock that were formed as the seawater in the Basin continued to grow saltier and deposited additional minerals. Among these was alabaster, today mined at Alabaster, Michigan, a major producer of the gypsum used in the manufacture of wallboard. (Note that other alabaster formerly mined below Grand Rapids is of a younger age.) Additionally, a layer of various salts accumulated, ranging from only about 500 feet thick around the Basin's edges to, with minor interruptions, around 2,000 feet thick near its center at Midland, Michigan. This layer contains table salt, bromine, calcium chloride, iodine, and other valuable chemicals that can be pumped from the ground as brine. It is for this reason that Dow Chemical started in Midland, where these salts are concentrated.
These soft layers along with a layer of soft shale known as the Antrim Shale were cut downward by the Wisconsin glacier to create Lakes Michigan and Huron, with the Niagaran Escarpment serving as the outer edge of each. In a very real sense these two lakes are mirror images. Lake Michigan lies along the west side of the basin and bows outward toward the west following the curve of the basin; Lake Huron lies along the east side of the basin and bows outward toward the east following the curve of the basin. Walk south from the Big Mac Bridge on either lake's shore, and you will encounter nearly the same rock layers in the same order.
One way to determine the age of bedrock within the Michigan Basin is by its position (see fig. 3). The closer the bedrock is to Midland, the younger it is. However, rock is sometimes transported from one place to another, unless it is bedrock. Its location is not always an appropriate way to determine age. Because rock in the Michigan Basin is sedimentary, there is often another way to easily determine its age: fossils within the rock. When shallow inland seas form as they did in the Michigan Basin, similar seas form in other locations worldwide. These seas absorbed warmth from the sun to create a warm stable climate, hospitable to whatever organisms were living in the sea at the time. When these seas began to withdraw, the climate became less stable. Marine organisms can be pushed to extinction by a temperature change of as little as 3 degrees, and so many organisms became extinct.
Out at sea, new organisms evolved. In the ocean, the fossil record of this constant evolution was erased as the seafloor vanished beneath the continents due to subduction—this was not the case, however, in shallow inland seas like the ones that covered Michigan. When the water returned to these seas, a vast array of newly evolved organisms came with it. In turn, these organisms eventually went extinct, some leaving records of themselves in the form of fossils, and were then replaced by still newer species. As the process continued, layers of fossils were laid down, like isolated scenes from a play spanning over 600 million years. Certain organisms will appear in one scene but be absent from the preceding and succeeding scenes. By getting to know a few of these players and at what point they entered the drama, we can immediately recognize the age of the bedrock in which they appear, regardless of whether or not it has been glacially transported. For example, halysites (well known as chain coral to any who have walked the beaches of Lake Michigan or Huron) inhabited the Niagaran Escarpment from the late Ordovician to the Middle Silurian. Because of this, we know rocks containing fossilized halysites must have been formed during those periods—that is, 461 million to 416 million years ago.
Fossils are the key to the age of sedimentary rocks. The book Index Fossils of North America, by Harvey W. Shimer and Robert Rakes Shrock, lists a tremendous number of fossils (maybe 8,000) and the time periods during which they lived. Though now out of print, the book can be found in many large libraries. Another source for information on fossils is Geology of Michigan, by John A. Dorr, Jr., and Donald F. Eschman (ISBN 978-0-472-08280-3).
Chapter TwoA Trip around the Lake
Anything east of the Mackinac Bridge might be considered Lake Huron, so the Big Mac is a logical place to start a trip around the lake. Across the bridge lies the Niagaran Escarpment. Below the bridge are seven miles of water, where the extremely soluble Silurian anhydrite and salt layers are close to the surface. A major inactive fault runs along the St. Lawrence River and passes under the bridge. In the past, when sea level was low, water draining out along the fault dissolved the layers of anhydrite and salt, undermining the overlying rock, causing collapse and shattering of the sediments above. Such collapses occurred repeatedly for hundreds of millions of years. The result: the Straits of Mackinac. Near the Straits you will see shattered rock, some of it reduced to the size of gravel, on either side of the bridge, along the Straits, and on Mackinac Island.
Points # 1–3 from the Bridge to Alpena
Devonian Bedrock, Limestone, and Dolomite
To the southeast of Cheboygan lies Black Lake. Here and along rivers in this area, Hexagonaria fossils can be found. These colonial corals, commonly called Petoskey stones, can weigh up to twenty pounds or more. The skeletons of these corals have filled with calcite from the outside, a process that closed off the corals' porous interiors. Usually only a thin shell can be polished on the exterior, but this shell can be very beautiful. See figure 4 for locations.
Fossilized corals from the Niagaran Escarpment and upper Lower Peninsula have been glacially transported to the Lake Huron shoreline, though more commonly to the northern parts of the lake where the source is closer. As previously mentioned, the conversion of limestone to dolomite that occurred where these corals originated seriously degraded fossils contained within the rock, but halysites (often called chain coral) and syringopora from the escarpment are notable exceptions. By some process, these two corals, along with some horn corals, were replaced with quartz before dolomitization could take place, leaving their extraordinarily intricate and fragile structures intact.
It's possible to extract these fossils from dolomite. Dolomite is slowly soluble in muriatic acid, which is available at any hardware store. By sanding dolomite fossils flat on the bottom and spraying this surface with lacquer to prevent solution, you can create a base for support. Next, immerse the fossil in water, and add acid from time to time. Always use care when using acid: work outdoors and neutralize remaining acid with limestone or crushed marble (which are available as landscaping supplies). Do NOT pour the acid down the drain.
It may take several days or more for solution of the dolomite. Once solution has occurred, remove the fossil carefully as it will be fragile until dry. The result will be a fossil such as can seldom be collected anywhere. Syringopora has cross members to hold its columns together and can, therefore, be removed entirely from its stony tomb. Sometimes horn corals can be exposed attached to the interiors of these tiny coral reefs.
US 23 from Nine Mile Point to Hammond runs close to the lake. Many beaches here are pure sand, but at Huron Shores Park and other beaches along the way to Hammond Bay State Park a variety of interesting specimens can be collected, including stromatoporoids, favosites and corals of several other species, brachiopods, cladopora, basalt, chert, puddingstone, and Gowganda tillite. Walk ten paces on one of these beaches, and you will find something astonishing. At Huron Shores in particular there is an unusual amount of quartzite, most likely brought down from the Gordon Lake sediments to the north in Canada; some of it is banded and of unusual color and beauty.
Points # 4–7 from Alpena to Harrisville
Devonian Shale and Limestone
Alpena is an exceptional place for fossil collecting. Many of the area's best fossil sites are described in The Complete Guide to Michigan Fossils, by Joseph Kchodl (ISBN 978-0-472-03149-8), which we recommend to anyone interested in collecting here.
The shale here is known as the Antrim Shale because it is exposed in Antrim County near Norwood, Michigan. Ranging in age from Devonian to Mississippian, the shale on rare occasions contains armor-plated Devonian fish. Large round stones called concretions (or kettle stones) that have grown within the shale as it was compressed can also be found here. Until 1994, the Antrim Shale was excavated at the Paxton Quarry near Alpena, Michigan, one of the few places where these concretions sometimes contain rare fossils of land plants from the late Devonian, the time of the first forests. The Jesse Besser Museum (phone: 989 356-2202) in Alpena displays some of the finest examples of these fossils.
In Alpena at the Lafarge Quarry (phone: 989-354-4171), limestone is quarried and mixed with slag from the Chicago area rather than the shale formerly used to manufacture concrete. Quarries where stone is blasted and loose blocks are left hanging can be dangerous, and their owners rarely conduct tours because of insurance considerations. On occasion, however, these quarries dump an amount of quarry rock in flat, level areas for the public to examine—sometimes very nice trilobites can be found here. If you would like to visit a quarry, it is advisable to call well in advance and make an appointment; they may let you in—the key word being may. If you do visit a quarry, always stay out of situations that appear unsafe. If you do not and are injured, blame no one but yourself.
One of the best known collecting sites in the Alpena area is Partridge Point—if you go, a hammer for splitting limestone layers will be helpful. Four miles south of Alpena on US 23, turn east on Partridge Point Rd S and head toward the lake. After driving 1.6 miles, you will see a two-track road to the right that, while probably drivable, may be more safely walked. A short walk up this road will reveal Devonian limestone bedrock that contains several types of trilobites, many brachiopods, very large horn corals, large favosites, and many other invertebrate fossils. Walking east along the shoreline, with luck you may find petrified wood toward and beyond the end of the point. This is petrified fern tree wood, identifiable by bark impressions on its exterior and notably missing the growth lines and radials found in other petrified woods. Much of the petrified wood that can be collected here is composed of opaque, amorphous quartz that is black in color, probably due to carbon in the tree trunk. Sigillaria, lepidodendron, and calamites were some of the common and very large fern trees, and they can be recognized by their bark.
In the coal-age swamps of the late Devonian, the Mississippian, and the Pennsylvanian, gelatin-like colonies of bacteria accumulated clay, calcium carbonate, and iron carbonate. When these colonies died, they would be buried and begin to dry. As this occurred, the soft stony material would crack and eventually be filled with white calcite and sometimes other minerals such as quartz, galena, and sphalerite. These can be found on Sulfur Island just offshore from Partridge Point. They are similar to the lightning stones found at South Haven, Michigan as discussed in the Lake Michigan Rock Picker's Guide (ISBN 978-0-472-03150-4).
From the top of Sturgeon Point Lighthouse, an exceptional pop-up can be seen. The Wisconsin glacier (which lasted from about 100,000 to 10,000 years ago) was so heavy that it caused the land to subside in a downward arc. When the glacier melted, the land began to rebound, and there was compression between one side of the arc and the other. The result: a fracture where one side of the land rose more than the other. This fracture has brought granite to the surface, resulting in the creation of a point that runs a half mile or more offshore, including an underwater segment. Granite boulders the size of houses can be seen underwater here. Further evidence of the underwater ridge is the wave action: waves slow down and are bent as they enter shallow water, breaking on both sides of this point and depositing gravel that is up to and beyond cobble sized. This gravel is Canadian Shield material, composed especially of puddingstone and Gowganda tillite as well as bedrock fossils, including trilobites and other marine invertebrates.
Excerpted from Lake Huron Rock Picker's Guide by Bruce Mueller Kevin Gauthier Copyright © University of Michigan by 2010. Excerpted by permission of The University of Michigan Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.