How Michigan was built

The western Upper Peninsula sits on very old rock from the Precambrian and Cambrian. The eastern Upper Peninsula and the whole Lower Peninsula sit on younger layered rock from the Ordovician to the Pennsylvanian.⁸

Most of the Upper Peninsula sits on Precambrian rock, the southern extension of the Canadian Shield. Nearly all of Michigan's metal ores, including iron, copper and silver, are found in rocks of that age.⁸

Michigan's oldest known rocks are about 3,600 million years old (3.6 billion). They come from the Watersmeet area of the Upper Peninsula.¹

About 1.9 billion years ago a broad, shallow sea covered the region, and the famous banded iron formations of the Marquette and Gogebic ranges formed in it.²

The classic Lake Superior banded iron formations, including the Marquette, Gogebic, Mesabi and Gunflint iron ranges, formed in a broad, shallow sea about 1.9 billion years ago, after about 1.88 billion years ago.² Hematite and specularite are found on the Precambrian iron ranges of the western Upper Peninsula.¹³

Later these rocks were squeezed, folded, broken and invaded by granite during the Penokean orogeny. Then the land wore away for about 650 million years.¹

• Banded iron formation
• Hematite

On the time scale

1. 4,031 to 2,500 Ma

   Archean Eon

   Small continents and pieces of ocean crust were pressed together to form the Superior Province by about 2,600 million years ago. Michigan's oldest known rocks, about 3,600 million years old, are from the Watersmeet area of the Upper Peninsula.¹

2. 2,500 to 1,600 Ma

   Paleoproterozoic Era

   Mountain building called the Penokean orogeny began about 1,880 million years ago. A broad, shallow sea formed, and the Marquette and Gogebic iron formations formed in it about 1.9 billion years ago.² From about 1,750 million years ago the bedrock sat above sea level and wore away for a very long time.¹ Puddingstone's source rock, the Lorrain Formation in Ontario, is 2,200 to 2,450 million years old.³

North America began to tear apart. A rift about 2,000 km long opened between about 1,109 and 1,087 million years ago and filled with lava flows. A new ocean nearly formed before the rifting stopped.⁴

Hot fluids later left pure native copper inside the Keweenaw lava flows and conglomerates, between 1,060 and 1,047 million years ago.⁵¹ EGLE calls the Keweenaw the most important place in the world for pure native copper.¹³

A rift about 2,000 km long split the continent about 1,109 to 1,087 million years ago, pouring out mostly plateau basalts on land. A new ocean basin nearly formed before the rifting stopped.⁴

Native copper sits in lava flows full of mineral-filled gas bubbles (amygdaloidal basalts) and in conglomerates of the Portage Lake Volcanics, and formed between 1,060 and 1,047 million years ago.⁵ More than 5 million tons of refined copper came from Keweenaw native copper deposits from 1845 until 1968.¹³

• Basalt lava flows
• Native copper
• Lake Superior agate
• Chlorastrolite

On the time scale

1. 1,600 to 1,000 Ma

   Mesoproterozoic Era

   The Midcontinent Rift was active about 1,109 to 1,087 million years ago⁴ and filled with up to 25 km of mostly basalt volcanic rock.¹ Native copper formed in the Keweenaw between 1,060 and 1,047 million years ago.⁵

2. 1,000 to 538.8 Ma

   Neoproterozoic Era

   A quiet time. The land was a bare, worn-flat surface with no life except bacteria and algae.¹ The Jacobsville Sandstone is the youngest of the area's Precambrian rocks. It is about 960 million years old (scientists still debate the exact age).⁶⁷

From about 500 to 300 million years ago, shallow seas came and went. They left up to 14,000 feet (4,300 m) of sandstone, shale, limestone, dolostone and salt in the Michigan Basin.¹ EGLE compares the layers to a stack of shallow nested spoons.⁸

In the late Cambrian, sand on the shores and in those shallow seas became the sandstone of the Pictured Rocks area.¹

The Michigan Basin is shaped like a stack of shallow nested spoons.⁸ It covers the entire Lower Peninsula and about half of the Upper Peninsula, and its rocks get younger toward the center.¹¹

• Sandstone
• Fossil limestone
• Petoskey stone

On the time scale

1. 538.8 to 486.85 Ma

   Cambrian Period

   Seas spread over the old bedrock, and sand on the shores and shallow sea floor became the sandstones of the Pictured Rocks area.¹

2. 486.85 to 443.1 Ma

   Ordovician Period

   The land took turns being under shallow seas and dry land.¹ Sandstone, shale, dolomite and limestone formed.⁸

3. 443.1 to 419.62 Ma

   Silurian Period

   Limestone, reef limestone, dolomite, shale and evaporites such as rock salt and rock gypsum formed.⁸

Under the Lower Peninsula, the Silurian salt beds of the Salina Group hold about 25,000 cubic kilometers of rock salt.¹⁶

Detroit has a salt mine that has worked 317 meters (about 1,040 feet) down since it began producing salt in 1910. It is Michigan's only salt mine.¹⁶

For many years, Michigan was the leading salt-producing state in the United States.¹⁶

See the halite (rock salt) card

In the Devonian, Michigan sat near the equator under a shallow, tropical sea. The coral Hexagonaria built reefs there alongside other corals, brachiopods, sea lilies, trilobites, bryozoans, sponges and armor-plated fish.⁹

Its fossils are Petoskey stones, from the Middle Devonian, 393 to 382 million years ago.¹⁰

For millions of years, vast, shallow tropical seas covered the state, and almost all the fossils on Michigan beaches come from them. Corals are among the most common because they built huge reefs.¹¹

• Petoskey stone
• Charlevoix stone
• Crinoids
• Brachiopods
• Horn corals
• Sandstone
• Fossil limestone

On the time scale

1. 419.62 to 358.86 Ma

   Devonian Period

   Michigan sat near the equator under a shallow, tropical sea, where Hexagonaria built reefs.⁹ Petoskey stones date from the Middle Devonian, 393 to 382 million years ago.¹⁰

2. 358.86 to 323.4 Ma

   Mississippian Period

   Shale, sandstone, limestone and rock gypsum formed.⁸ Horn corals are found in Michigan rocks from the Ordovician up to this period.¹⁰

3. 323.4 to 298.9 Ma

   Pennsylvanian Period

   Not all sea: some of the youngest basin rocks are sandstones from stream channels, shales from floodplains and coal from swamps.⁸ Michigan's youngest bedrock is more than 300 million years old.¹¹

Rocks from about 300 million to about 2.5 million years ago are missing from the Michigan Basin. Geologists call this gap the Lost Interval.¹

Even Michigan's youngest bedrock is more than 300 million years old.¹¹

On the time scale

1. about 300 to 2.5 Ma

   The Lost Interval

   Rocks from this long stretch are missing from the Michigan Basin, so geologists call it the Lost Interval.¹ The missing record covers the Permian, Triassic, Early and Middle Jurassic, Cretaceous, Paleogene and most of the Neogene.¹²

The Ice Age began 2.58 million years ago.¹⁷ Again and again, ice sheets up to one mile thick pushed down from the north and carved out the basins of the Great Lakes.¹

At one point the entire state was covered by ice. Rocks that glaciers carried away from home are called glacial erratics.¹³

Most stones moved by glaciers traveled only a short way, typically less than about 10 miles. Some, though, were carried hundreds of kilometers.¹

Rocks moved by glaciers are called glacial erratics. Puddingstone is quartzite with bright red jasper pebbles. Yooperlites are sodalite that glows under a black light, and they actually come from Canada, near Marathon, Ontario.¹³

• Glacial erratics
• Lake Superior agate
• Puddingstone
• Float copper

On the time scale

1. 2.58 to 0.0117 Ma

   Pleistocene Epoch

   Ice sheets up to one mile thick advanced again and again. The last one lasted from about 55,000 to 10,000 years ago and reached its farthest point about 20,000 years ago.¹

The last glaciers left the Great Lakes basin about 10,000 years ago.¹

Now the waves do the sorting. EGLE suggests hunting where glacial moraines meet a Great Lake, and heading out after a storm or high waves.¹³¹¹

On the time scale

1. 0.0117 (11,700 years ago) to today Ma

   Holocene Epoch

   The last glaciers left the Great Lakes basin about 10,000 years ago.¹ Waves and storms keep reworking glacial gravel along the shore.¹³