Geology and Ecology of National Parks

Pictured Rocks National Lakeshore

Pictured Rocks National Lakeshore is located on the north shore of the Upper Peninsula of Michigan and is named for the colorful cliffs that line Lake Superior.

Geology of Pictured Rocks National Lakeshore

Sedimentary rock layers form bluffs along the shoreline of Lake Superior.

Sedimentary rock layers form bluffs along the shoreline of Lake Superior. NPS

(Public domain.)

PRECAMBRIAN  

Jacobsville Formation

The oldest rocks visible above water in Pictured Rocks National Lakeshore were deposited between 1 billion and 500 million years ago during the Precambrian period. The core of the North American continent began to split apart 1.1 billion years ago, forming the Midcontinent Rift System (also known as the Keweenawan Rift). The rift failed approximately 15-20 million years after the beginning of its formation, but not before leaving a basin now filled by Lake Superior. After the rift’s formation, sand and gravel were transported downstream, leveling out the southern shore of Lake Superior. Over time, the sand and gravel lithified, forming the Jacobsville Formation, a terrestrial rock unit with no fossils that formed during the Precambrian Era before land animals existed. The iron oxide in this formation imparts a characteristic red color. Only the very top of the formation is exposed above the water.

The Jacobsville formation is exposed just above the water and can be distinguished from the other formations by its red color

The Jacobsville formation is exposed just above the water and can be distinguished from the other formations by its red color. NPS

(Public domain.)

In the late 19th century, the Jacobsville Formation was mined because of its distinct color and durability. It is highly resistant to extreme temperatures, making it useful not only for enduring the large temperature fluctuations of the Midwest, but also for withstanding fires.

 

CAMBRIAN

Munising Formation

Most of the rocks exposed at Lake Superior belong to the Munising Formation, which has three members: a basal conglomerate, the Chapel Rock member, and the Miners Castle member. The Munising Formation ranges in color from light grey to pinkish grey and encompasses a wide range of depositional environments.

Bridalveil Falls is a great place to see the contact, or geologic boundary, between the Munising and Au Train Formations.

Bridalveil Falls is a great place to see the contact, or geologic boundary, between the Munising and Au Train Formations. NPS

(Public domain.)

The lowest layer is the Chapel Rock member, topped by the Miners Castle member. The Miners Castle member does not extend outward as far as either of the members it is in contact with because of its friable nature. On top, the harder Au Train formation extends out further than its underlying member.

 

Basal conglomerate

The basal conglomerate had a depositional environment very similar to the Jacobsville Formation. It was also formed by lakes and braided streams flowing north into the lowlands created by the midcontinental rift. The thickness ranges from about two to 15 feet, and it is composed mainly of rounded fragments of quartz, quartzite, and chert, with smaller amounts of basalt, slate, and granite.

 

Chapel Rock member

Above the basal conglomerate lies the Chapel Rock member of the Munising Formation. The Chapel Rock member ranges in thickness from 40 to 60 feet and is composed of light pink to brown medium-grained quartz sandstone. It has a huge variety of sedimentary structures, the most distinct being the large, sweeping crossbeds. One of the best places to view the crossbeds is at Chapel Rock, the type locality for the member. Other features include mud cracks, clay pellets, ripple marks, animal tracks, and clastic dikes.

 

 Miners Castle member  

The youngest part of the Munising Formation is the Miners Castle member, a crumbly pinkish-grey quartz sandstone approximately 140 feet thick. Because of how easily breakable it is, it forms a wide range of landforms across the park. Although fossils are generally rare in the Munising Formation, fragments of trilobites have been found in the Miners Castle member. The member is named for Miners Castle, one of the rock formations along Lake Superior.

Rockfalls are common within the Miners Castle member due to its friable nature. The most recent notable major rockfall in Pictured Rocks National Lakeshore was on April 13, 2006, when the northeast turret at Miners Castle collapsed. Local fishermen reported that most of the rock fell into Lake Superior.

Miner's Castle, before and after rockfall

Miners Castle in 2005, before the rockfall and again in 2007, shortly after the rockfall. 

(Credit: Gregg Bruff, NPS. Public domain.)

ORDOVICIAN

Au Train Formation

The Au Train Formation is found at the top of the cliffs and is a light brown to white dolomitic sandstone and spans three to nine meters thick. The Au Train Formation is harder and more resistant than most of the formations below, making it an ideal cap for the many waterfalls throughout the park. Unlike the other formations in Pictured Rocks National Lakeshore, the Au Train formation does contain fossils! Cephalopods, gastropods, and 26 taxa of conodonts (ancient relatives of jawless fish) can be found in the Au Train Formation.

 

PLEISTOCENE

Glacial deposits, dunes

Grand Sable Dunes

The Grand Sable dunes overlook Lake Superior at the eastern end of the park. NPS

(Public domain.)

Most of the landforms and surficial geology in Pictured Rocks consist of features deposited by recent glaciation including the Grand Sable Dunes Research and Natural Area. Between 6,000 and 4,000 years ago, the last ice sheets to cover North America were retreating. As they melted, their weight was relieved from the Earth’s crust, which caused a type of uplift known as isostatic rebound. That uplift formed a glacial lake, Lake Nipissing, that covered a large portion of the Midwest. As levels of Lake Nipissing rose from the uplift, the Grand Sable Banks were destabilized. Unconsolidated sand on the slopes of the banks was then blown by wind and deposited on the plateau to form the Grand Sable Dunes.

Sand dunes along the lake.

Sand dunes along the lake. NPS

(Public domain.)

What gives the cliffs at Pictured Rocks their characteristic colors?

Colorful mineral vertical stripes in rocks at Pictured Rocks National lakeshore.

Colorful vertical stripes can be seen on many of the rocks throughout the park. NPS

(Public domain.)

Pictured Rocks National Lakeshore is named for the colorful vertical stripes that adorn cliffs throughout the park. As groundwater seeps out of cracks in the rock, several elements and minerals are transported within the groundwater and are deposited as colorful stains. Red and orange stains are caused by iron, blue and green by copper, brown and black by manganese, and white by limonite. 

Colorful stripes decorate cliffs at Pictured Rocks National Lakeshore

Colorful stripes decorate the cliffs. NPS

(Public domain.)

Research in the park

Digital terrain model, or bare earth model, of Pictured Rocks National Lakeshore park along with the boundary of the p

Digital terrain model of the park, along with the park boundary highlighted in blue. USGS

(Public domain.)

 

References

Pictured Rocks National Lakeshore - NPS 

Geologic Resources Inventory Scoping Summary, Pictured Rocks National Lakeshore - NPS 

Geological Society of America Centennial Field Guide— North-Central Section, 1987 Pictured Rocks National Lakeshore, northern Michigan 

Grand Sable Dunes Research Natural Area - NPS 

Miners Castle - NPS 

National Park Service, US Geological Survey Partnership Program (usgs.gov)

Miners Castle Turret Collapses - NPS

USGS Upper Midwest Water Science Center

https://www.usgs.gov/media/images/pictured-rocks-national-lakeshore-piro-upper-peninsula-michigan

https://pubs.er.usgs.gov/publication/70160700

https://pubs.er.usgs.gov/publication/wri854103

https://ngmdb.usgs.gov/Geolex/UnitRefs/MunisingRefs_2907.html

https://pubs.er.usgs.gov/publication/1000870

https://mi.water.usgs.gov/projects/WQindex.html

https://mi.water.usgs.gov/reports/reports.html

https://www.usgs.gov/centers/umesc/science/consequences-forest-connectivity-beech-bark-disease-pictured-rocks-national?qt-science_center_objects=0#qt-science_center_objects

https://pubs.usgs.gov/wri/1985/4103/report.pdf

https://pubs.er.usgs.gov/publication/sir20175122

https://pubs.er.usgs.gov/publication/gf201