Geology of Pictured Rocks National Lakeshore
Learn more about the geology of Pictured Rocks National Lakeshore.
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.
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.
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.
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
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.
What gives the cliffs at Pictured Rocks their characteristic colors?
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.
