Minnesota Rocks

Minnesota has a rich geologic history

Ancient Rocks 3.6 billion years ago. Morton Gneiss (pronounced “nice”) is one of the oldest rocks on Earth. Outcrops of gneiss can be found just an hour west of the Twin Cities in the town of Morton.

 

 

 

One Billion Years ago. Volcanoes on the North Shore of Lake Superior. Many of the rocks we are going to study are from that era, including:  basalt and Lake Superior Agate.

 

Basalt lava flows north of Duluth from volcanoes one billion years ago.

 

450 million years ago, there was an inland sea that covered much of North America. The Twin Cities was underwater as evidenced by ocean fossils found in St. Paul. A few of the rocks we are going to study were formed during this time period including:  Shale and limestone.

 

Prehistoric squid

 

Trilobite

 

 

15,000 years ago  Glacial Period  – The glaciers moved only a few feet a year and carried  many of the rocks we find today from Northern Minnesota to the Twin Cities area. Also when the glaciers melted it formed most of the lakes we see in Minnesota.

Glacial ice

Glacial Till. When the glaciers melted it left hills of rocks and sand that had been pulverized inside of the glacier. If you have a hill near your home, it was probably formed in this manner.

 

Glacial Lakes  were formed when the glaciers melted. The weight of the ice caused the earth to be pushed down,  forming what is called an ice block lake.  (Lake Minnetonka in photo below)

 

Glacial erratics are large rocks that were carried on top of the glacier and did not get ground up  inside of the glacier. When the glacier melted, the rocks dropped to the ground.
If you have seen a large rock in your neighborhood, it is probably a glacial erratic.

Glacial rocks are everywhere in your neighborhood environment. What is meant by “glacial rocks”? How did the rocks get in your neighborhood? What are the names and stories of some of the glacial rocks? Using the guide below, see what types of rocks you can find in your neighborhood.

15,000 years ago a huge glacier covered most of Minnesota, Wisconsin and other states in the Northern US. As the glacier moved along it gouged out many types of rocks and carried them south moving at a speed of only a foot a day.

Rocks that were inside of the glacier got ground up into smaller stones. If you have seen a large boulder in your neighborhood, it is called a “glacial erratic”. It was not inside the glacier, but rode on top of the glacier and was not ground up.

When the Earth began to warm, the glaciers melted and the rocks inside came out, leaving what is called “glacial till”. Glacial till has many many sizes of rocks in it, from large boulders to sand. Many of the hills in our neighborhoods are made of glacial till. People mined the hills and sold the rocks for landscaping, road material, and concrete.

 

 

Types of Glacial Rocks

Sandstone
Sedimentary Rock
Formed from wind-blown sand
that fell into shallow sea
400 million years ago

 

         Basalt
Igneous/volcanic
1 billion years old
lava from North Shore
Carried by glacier

 

 

Morton Gneiss
Pronounced “nice”
Metamorphic rock
3.5 billion years old
One of oldest rocks on Earth
Used in buildings. Not a glacial rock.

 

 

 

 

Shale
Sedimentary Rock
Formed in shallow inland seas
1.9 Billion years ago
Found in Northern Minnesota
Carried in glacier

 

 

Granite
Igneous Rocks
1.2 to 2.5 billion years old
Formed deep beneath the earth
Carried by glaciers from St. Cloud

 

 

 

Chert
Sedimentary Rock
Formed from shells of microscopic algae
Used to make fire (flint)/arrow points
2 billion years old

 

 

Porphyry
Igneous/volcanic
1 billion years old
Found in lava flows
and roots of volcanoes
Carried by glacier

 

 

Mary Ellen Jasper
Sedimentary Rock
Formed from fossilized algae
2 Billion years old
Carried by glacier from Iron Range

 

 

Lake Superior Agate
Formed from the weathering of igneous rock
1 billion years old     MN state rock
Carried by glacier from North Shore
of Lake Superior

 

 

 

Limestone
Sedimentary Rock
Formed in shallow warm oceans
450 million years ago
Can have some fossils

 

 

Milky Quartz
Igneous or Metamorphic Rocks
400 + million years ago
Quartz is used to make glass
Quartz is a common mineral

 

 

 

Ryolite
Igneous/volcanic
1 billion years old
Lava from North Shore
Carried by glacier

 

 

 

To download the Glacial Rocks Study Card 1: Click here

To download the Glacial Rocks Study Card 2: