Geologic Formations

Chapel Rock

Craig Blacklock © photo

The geologic formations of Pictured Rocks National Lakeshore are most spectacularly represented by the 50-200 ft. sandstone cliffs that extend for more than 15 miles along the shoreline. Sea caves, arches, blowholes, turrets, stone spires, and other features like the famous Chapel Rock have been sculpted from these cliffs over the centuries by unceasing waves and weather.

A Tapestry of Geologic Layers
Geologic history recorded in the sedimentary rocks and surficial deposits of Pictured Rocks National Lakeshore is limited to two widely separated intervals of geologic time, the Late Precambrian, Cambrian, and Early Ordovician Periods (500-800 million years before present), and the Late Quaternary Period (two million years before present to the present).

During the Cambrian and Early Ordovician periods, sediments were deposited in the shallow seas and near-shore deltas that covered what is now northern Michigan. These deposits became the sandstone layers that are exposed within the lakeshore. Except for their exposure near Lake Superior, these layers are presently covered by a veneer of Quaternary glacial drift.

Bedrock is best observed in the western one-third of the park where cliffs rise up from Lake Superior. These extend along the lake about from Munising to Beaver Basin. For a short distance inland from the escarpment, bedrock is occasionally exposed.

The Jacobsville Formation, of Late Precambrian age, is the oldest formation exposed in the lakeshore. It is a river/lake deposited, feldspar-rich quartz sandstone, deep red in color with white mottling. Only the top few feet of this formation rise above lake level within the lakeshore; east of Hurricane River Campground, at Au Sable Point, and the gorge at Sable Falls. Due to its attractive color, Jacobsville sandstone was quarried on nearby Grand Island for building stone in the late 19th century.

The Mid to Late Cambrian, light grey to white Munising Formation lies unconformably above the Jacobsville. The Munising Formation probably represents a complex shoreline/shallow water environment that was influenced by river, wave, tidal, and wind processes. The Munising is divided into three members: the basal conglomerate, the hard Chapel Rock sandstone (characterized by large, sweeping cross beds), and the crumbly Miners Castle sandstone.

Capping the easily eroded Miners Castle Member of the Munising Formation in the western half of the Pictured Rocks, is the resistant Early Ordovician Au Train Formation. The Au Train Formation is a light brown to white dolomitic sandstone that lies above the distinctive caprock above the lip of Munising and Bridalveil Falls. This harder sandstone is responsible for the presence of so many of the area's waterfalls, as it resists erosion better than other formations.

 

Open Transcript 

Transcript

Hi my name is ranger Zach and welcome to Pictured Rocks National Lakeshore. Today I wanted to show you one of the unique features of Pictured Rocks which helps to create this beautiful landscape that people have been drawn to for centuries. These photos show the towering Pictured Rocks cliffs as seen from a boat on Lake Superior. Notice the bright blue waters of Lake Superior as it meets with the crumbling sandstone formations of the 100 foot cliffs. The cliffs make up about 15 miles of shoreline in the western half of the park. Visitors who explore this landscape are drawn to rock formations such as Lovers Leap which forms a large archway over Lake Superior. Grand Portal Point, which is the highest point on the cliffs standing over 200 feet tall and Chapel Rock which forms a pillar of sandstone holding an old white pine tree that is only alive because of the root system reaching back to the mainland. Erosion is what formed these cliffs. While many of these cliff features are hundreds or thousands of years old, every year  wind, waves, and ice from Lake Superior weathers the shoreline and alters this landscape, which means that every year visitors experience a park that is slightly different than the previous year. For instance, here is a photo of the miner's  castle rock formation in 2005. Notice this rocky cliff stands high above Lake Superior with two rock pillars sitting on top of it. This next photo shows what a difference a year can make. Notice how the Miner's Castle rock formation is now missing one of the two rock pillars. This massive piece of stone fell into the lake as  a result of ongoing erosion from Lake Superior. While there are many more impressive rock formations to view, I would like to show you one more unique feature that makes this park so famous. Visitors may notice bright streaks of colors that run down the cliffs. These videos show streaks of color such as brown, orange, white, black, and green that almost look like somebody dumped buckets of paint all over the rocks. These colors are the result of mineral staining. There is groundwater that is constantly leaking out from the porous sandstone. This groundwater has minerals present in abundance as the water falls down the cliffs the  minerals meet with the air and stain the rock. Different minerals will create different colors. These photos display some of the colorful parts of the Pictured Rocks clips, which are most noticeable during sunset. The orange and brown staining is caused by iron, the white stains are caused by limonite, black stains are caused by manganese, and although not as abundant as the other colors, blues and greens are caused by copper. While these features are best seen by boat from Lake Superior, there is plenty of opportunities to see some of the cliffs from the North Country Trail and remember although the forces of nature are constantly reshaping the landscape here at Pictured Rocks, we must all do our part to help preserve this special place. Pictured Rocks sees close to a million visitors every single year and they speed up the process of erosion if they act carelessly by carving into rocks, walking in enclosed areas, or breaking apart rock formations. We encourage you to visit but remember to protect your park. For more videos and information about other areas of Pictured Rocks please visit www.nps.gov/piro   

Duration:

4 minutes, 13 seconds

Explore the unique cliffs of Pictured Rocks and discover how some of these famous features formed!

 

Multi-colored cliffs of Pictured Rocks National Lakeshore

Craig Blacklock © photo

The name "Pictured Rocks" comes from the streaks of mineral stain that decorate the cliffs. Stunning colors occur when groundwater oozes out of cracks and trickles down the rock face. Iron (red and orange), copper (blue and green), manganese (brown and black), and limonite (white) are among the most common color-producing minerals.

The best way to see the geologic layers and colors of the Pictured Rocks cliffs is from the water. Late afternoon or early evening sunlight brings out the richest colors.

 

What about Fossils?
Fossils are completely absent from the Jacobsville Formation and uncommon in the Munising Formation. Fragments of trilobites have been found in the Miners Castle member and 26 taxa of conodonts (ancient relatives of jawless fish) in the upper Munising Formation and the lower Au Train Formation. The Au Train also contains Middle Ordovician cephalopod and gastropod fossils.

Sand Dunes
Another major geologic feature of the lakeshore are the Grand Sable Dunes on the park's east end. Constantly moving and shifting, these dynamic dunes sit atop a high bluff of ancient glacial stone and rubble.

 

For more information ...

Grand Sable Dunes Research Natural Area

NPS Geology website