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“Bite-Size Bits of Local, National, and Global History”
“Bite-Size Bits of Local, National, and Global History”
Grand Marais in Cook County, Minnesota — The American Midwest (Upper Plains)
 

Natural History of the Point

 
 
Natural History of the Point Marker image. Click for full size.
July 25, 2022
1. Natural History of the Point Marker
Inscription.  
Ancient Volcanoes
1.1 billion years ago, 500 million years before invertebrates appeared, a giant volcanic rift began to open where Lake Superior now lies. Huge lava flows erupted from the rift, spreading over hundreds of square miles of the landscape, some only inches thin, others many feet thick. The flows were periodically interrupted by explosive eruptions, showering boulders onto the land and water. The weight of the solidified lava bent and tipped the layers of rock in what must have been a series of earthquakes. Finally, the rift began to close, closing finally so firmly that islands were pushed up from the surrounding rock like watermelon seeds from between your fingers. The North Shore landscape we see today is rooted in these volcanoes.

Agates and Thomsonite
Some lava was full of gas bubbles. This cooled to form rock with gas pockets, called vesicular basalt. Over time, minerals were deposited in the vesicles. If the mineral was quartz, and came in layers, an agate was formed. Oxidized iron in the quartz gives it red colors. If the mineral was quartz with other contaminants, thomsonite
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forms with a ray pattern instead of layers. Other types of microcrystalline quartzes such as chalcedony, jasper, and the chert used for gun flints also come from these vesicles

Glaciers
The landscape created by the volcanoes was slowly covered with sand as the eroding Laurentian Mountains were washed into the basin. Then, only 10,000 years ago, glacial ice sheets a mile thick scraped off the sandstone, piling it near the Twin Cities. When the glaciers melted, the 1.1 billion year old landscape was at the surface again. This is one of the only places on Earth where you can stand on surface rock which is this ancient.

The "Point"... Or The "Tombolo"?
The Point is a unique geographic feature, a special kind of point called a tombolo. It is a point of land where a sand spit connects the mainland to an offshore island. Several of the North Shore's protected bays are formed by tombolos including the bay at Black Sand Beach north of Silver Bay and at Little Two Harbors by Split Rock lighthouse.

How to Build A Tombolo
Strong winds and waves washed sand and cobbles uphill on the beach, which then rolled back down. This slowly moved the sand and stones along the beach in what is called longshore drift.

When the longshore drift reached a headland, it was deposited and eventually formed a spit.
Natural History of the Point Marker <i>(right)</i>, from the north image. Click for full size.
July 25, 2022
2. Natural History of the Point Marker (right), from the north
The spit lengthened until it joined an island - which created the Point's tombolo.


Birds and Unique Plants
For such a small area, the Point is home to several unusual plants and many different species of birds. Sea ducks like the long tailed duck (formerly oldsquaw) and white winged and surf scoters are occasionally seen here on Superior's inland sea, and abundant warblers of many types haunt the trees.
The carnivorous butterwort grows here, along with other plants more common in other places. Please, do not pick plants, and stay on established footpaths to avoid crushing the flowers.

captions:
The characteristic shape of the Sawtooth Mountains is formed from the upturned edges of the solidified lava flows.
The Midcontinent Rift formed as Wisconsin, part of Inner Laurentia moved away from Minnesota on Laurentia. The formation of the Grenville Mountains closed the Rift again.
Thin lava flows cooled into dark gray basalt. Basalt will crack to form hexagonal columns.
Thomsonite shows a ray pattern instead of layers.
Lake Superior agate with typical bands of deposition layers and red color.
Grooves in the basalt were left by the glaciers as they passed.
Long tailed duck

 
Erected by the United States Department of Agriculture
Geology of the Point, on the trail to the Grand Marais Lighthouse image. Click for full size.
July 25, 2022
3. Geology of the Point, on the trail to the Grand Marais Lighthouse
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Topics. This historical marker is listed in this topic list: Natural Features.
 
Location. 47° 44.739′ N, 90° 19.953′ W. Marker is in Grand Marais, Minnesota, in Cook County. Marker can be reached from Broadway Avenue, ¼ mile south of Wisconsin Street, on the left when traveling south. Located at the beginning of the Grand Marais Breakwater Trail, southeast of the U.S. Coast Guard station. Touch for map. Marker is in this post office area: Grand Marais MN 55604, United States of America. Touch for directions.
 
Other nearby markers. At least 8 other markers are within 2 miles of this marker, measured as the crow flies. The Point Interpretive Site (here, next to this marker); Grand Marais Harbor (about 400 feet away, measured in a direct line); Pulpwood Rafting (about 700 feet away); a different marker also named The Grand Marais Harbor (approx. 0.3 miles away); Geology of Minnesota (approx. 0.3 miles away); Bally Blacksmith Shop (approx. 0.3 miles away); St. Francis Xavier Church (approx. 1.3 miles away); Chippewa City (approx. 1.3 miles away). Touch for a list and map of all markers in Grand Marais.
 
Geology of the Point image. Click for full size.
July 25, 2022
4. Geology of the Point
On the right, in the distance, the Sawtooth Mountains can be seen.
 
 
Credits. This page was last revised on August 5, 2022. It was originally submitted on August 4, 2022. This page has been viewed 135 times since then and 9 times this year. Photos:   1, 2, 3, 4. submitted on August 4, 2022.

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