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PIRO: Trough Cross-Bedding in the Jacobsville Form

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Hidden : 7/2/2012
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Geocache Description:

The Jacobsville Formation is the oldest exposed bedrock in Pictured National Lakeshore Park.  It is easily identifiable by its bold red color mottled with white.  Over the years it has been quarried in the area and used in the construction of buildings in this area.  It is exposed along Lake Superior’s south shore for about 150 miles and is usually just under or slightly above the water plane of Lake Superior.

PIRO: Trough Cross-Bedding in the Jacobsville Formation East of the Hurricane River

Terrain: You will be walking on sand.  The stable fly can come out in hordes in the summer time.  They usually come out on days that are hot and humid.  It would be wise to pick a cooler day or make sure you have appropriate clothing.  Once you arrive at the Jacobsville Formation the wet rocks can be slippery so use caution and watch your step.

Starting from the city of Munising, Michigan you take H58 East about 38 miles until you see a sign for Hurricane River Campground.  This will take approximately 45-50 minutes.  Turn left and park in the day use parking area near the beach.  Follow the stairs down to the beach.  Turn to your right and proceed to find the next waypoint.
Figure 1. This is Hurricane River.  When you descend the stairs to the beach this river will be on your left.
Photograph by: C. Bolduc

Cache Description: 
Jacobsville sandstone can be identified by its dark red color.  This coloring is caused by ferric iron oxide or hematite. This sandstone substance was  “eroded from Precambrian iron formations located in a highland source region to the south.” (Blewett, p.19)  Jacobsville sandstone also has white streaks found in most outcrops.  These white streaks are a result of leaching along fractures and cross-beds.  The Jacobsville Formation is fluvial or created by river deposits.  “The geologic history of Pictured Rocks National Lakeshore begins with the deposition of the Jacobsville sandstone as early as a billion years ago during the later part of the Proterozoic Eon.”  (Blewett, p 38)  The braided streams flowed into the mid-continental rift carrying with them deposits from highlands.  Each trough bed indicates where a stream once flowed.  It is thought that this is how the Jacobsville Formation was formed.  This is also why the Jacobsville sediments found in the Formation are thought to be mostly terrestrial or formed from land deposits rather than marine deposits, which come from ancient seas. 
Figure 2. This is what you will first see when you have reached the trough cross-bedding.
 Photograph by: C. Bolduc
When you look at the trough cross-beds you will notice that they differ in the way that they are shaped.  The trough cross-beds are curved or formed in a scoop shape.  By looking and measuring the angles of the cross-beds, geologists are able to determine which direction the streams flowed.  It was determined this flow pattern followed a northward direction.  As time passed and these streams flowed into the Superior Basin, Lake Superior’s Jacobsville bedrock was eventually formed with heat and pressure.  These conclusions are reinforced by the fact that Jacobsville sandstone runs along the southern shoreline of Lake Superior and is thought to make up most of the bottom of the lake.  Jacobsville sandstone is not present on the northern shore of Lake Superior.  “The Jacobsville Formation formed millions of years ago before life emerged from the oceans to live on land.” (Bruff, Gregg)
   :Screen shot 2012-06-25 at 1.32.26 PM.png
Which was based on Hamblin, 1958
Logging Questions: 
LoggingQ1: Observe the shape and color of the formation.  What do you see?
LoggingQ2: What did the angle of the trough beds help scientists determine?
Logging Q3: What causes the white coloring in the Jacobsville Sandstone?
Logging Q4: When you reach the first trough bed about how many feet is it from the tree line to the end of the Formation?  Estimate by using one normal step as approximately three feet.  Use caution on the wet rocks, they can be slippery!
                Figure 3:  Here is an example of a small stream channel.
he direction of the flow starts near the  lower right-hand corner and flows toward Lake Superior.
Picture by: C. Bolduc
Figure 4:  Here is another example of a small stream channel.  The water flowed in the same direction as the arrow.
Picture by Gregg Bruff
Essential Question:
What can changes in rock formations tell us? 
Earth Science Literacy Principles:
Earth Science Literacy Big Idea 4: Earth is continuously changing.
  • Big Idea 4.7
Landscapes result from the dynamic interplay between processes that form and uplift new crust and processes that destroy and depress the crust. This interplay is affected bygravitydensity differences, plate tectonicsclimatewater, the actions of living organisms, and the resistance of Earth materials to weathering and erosion.
  • Big Idea 4.8Weathered and unstable rock materials erode from some parts of Earth’s surface and are deposited in others. Under the influence of gravity, rocks fall downhill. Water, ice, and air carry eroded sediments to lower elevations, and ultimately to the ocean.
  • Big Idea 4.9Shorelines move back and forth across continents, depositing sediments that become the surface rocks of the land. Through dynamic processes of plate tectonics and glaciation, Earth’s sea level rises and falls by up to hundreds of meters. This fluctuation causes shorelines to advance and recede by hundreds of kilometers. The upper rock layers of most continents formed when rising sea levels repeatedly flooded the interiors of continents.
Retrieved From:

Water has played a massive role in shaping our Earth’s landscape over millions of years.  The Jacobsville Formation is an excellent example of how fluvial deposits, or sediments deposited by rivers, can influence the shape of the landscape over time.
Common Earth Science Misconceptions:
  1. The Earth has always been pretty much the way it is now.
  2. Rivers always follow the same path.
  3. Only water flows in streams and rivers.
Retrieved From:
Many people believe that rivers are merely directed by the Earth and follow a path already laid out.  The troughs show a different reality, where rivers affect the rock that it flows through and does not always follow the same path year after year.
Michigan State Science Content Expectations:
  • E.SE.E.2 Surface Changes-The surface of Earth changes. Some changes are due to slow processes, such as erosion and weathering; and some changes are due to rapid processes, such as landslides, volcanic eruptions, and earthquakes.
  • E.SE.M.4 Rock Formation- Rocks and rock formations bear evidence of the minerals, materials, temperature/pressure conditions, and forces that created them.
Retrieved From:

The trough cross-bedding in the Jacobsville Formation shows the change the rock has incurred due to the influence of water and compression/solidification over time.
Quarry: an open pit mine used to extract gravel or other kinds of rock
Trough Cross-beds:  layers of sediment that are deposited at an angle.  They have lower surfaces that are curved or scoop shaped
Fluvial: Deposits made by a river
Access Information:
PIRO:  Pictured Rocks National Lakeshore Park permission granted by Jim Northrup and Gregg Bruff
Date Visited: June 22, 2012
  • Blewett, W. L. (2012). Geology and landscape of Michigan's Pictured Rocks National Lakeshore and vicinity. Detroit, Mich: Wayne State University Press.
  • Bruff, Gregg. (Personal Communication, June 2012).
  • Lakeshore Geology: Pictured Rocks National Lakeshore National Park Service U.S. Department of the Interior:

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