PIRO: The “Painted” Pictured Rocks
Figure 1. Pictured Rocks Cliffs Photograph by: Beth Dondit
Why are the Pictured Rocks the “Pictured Rocks?” What caused the colors to form on the cliffs? How did it happen?
The Pictured Rocks cliffs span approximately 15 miles from the Beaver Basin west to Munising, MI. The cliffs are constantly being weathered and eroded by the powerful forces of the water, waves, wind and ice. A sea cave can form when a softer rock is affected by erosion. The Chapel Member of the Munising Formation of Sandstone is this softer layer. Waves, wind, etc. have slowly eroded “Rainbow Cave” over a long period of time. Inside this sea cave, you will see many colors along the rock walls/ceiling. The staining or nature’s painting, has left quite a canvas inside this cave. When groundwater seeps through porous rocks, the minerals in the groundwater also seep through, and leave behind colors to mark their presence. Green and blue colors come from copper; black from manganese, or tannins originating in the streams; orange and red come from iron; and white from limonite. Water follows the path of least resistance, and travels downwards (gravity). The water table here is higher than Lake Superior, thus water is moving to the lowest point possible. This is very evident inside the “Rainbow Cave.”
Figure 2. Natural Paintings on the Pictured Rocks Cliffs Photograph by: Beth Dondit
Logging Your Visit:
L1: At this location, what is the most prominent color in the cave? What mineral caused that coloration?
L2: What is the most unique color here? What mineral “painted” this color?
L3: What force caused this landform to form?
L4: What is causing the “rain” inside the cave? Where is it coming from?
Figure 3. “Rainbow Cave” in the distance Photograph by: Beth Dondit
Earth Science Literacy Principles Big Ideas:
Earth Science Literacy Big Idea 4- Earth is continually changing.
Earth materials take many different forms as they cycle through the geosphere.
As water and minerals travel through both the hydrological cycle/geosphere, they continually shape our Earth’s landscape. Evidence of this is easily viewed along the Pictured Rocks coast. Even more impressive from a kayak, viewing the features firsthand (beware of changing weather conditions, clapotis, and cracks/fractures, which could amount to falling rocks).
The mineral deposits/stains that paint the Pictured Rocks are very colorful and impressive. The minerals seep through the porous sandstone layers, with the groundwater, and leave their mark.
Common Earth Science Misconceptions:
Minerals are only found in rocks, as solids, and they don’t change forms.
Rocks are solid, hard surfaces that don’t change shape/form.
Rocks are hard, nonporous substances.
Michigan State Content Expectations:
Grade 3- E.SE.E.2- Surface Changes- The surface of the Earth changes. Some changes are due to slow
processes, such as erosion and weathering; and some are due to rapid processes, such as landslides, volcanic eruptions, and earthquakes.
Grade 6- E.SE.06.12- Earth’s Changing Surface- Explain how waves, wind, water, and glacier movement,
shape and reshape the land surface of the Earth by eroding rock in some areas and depositing sediments in other areas.
Clapotis- “confused seas.” This occurs when waves hit large cliffs, and bounce off, connecting with other waves, creating large, multi-directional wave patterns.
Blewett, William L. Geology and Landscape of Michigan’s Pictured Rocks National Lakeshore and Vicinity. Detroit, MI: Wayne State University Press,
National Park Service. “Pictured Rocks: Frequenly Asked Questions.” 11 June 2012. <http://www.nps.gov/piro/faqs.htm.>