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Eagle Harbor Lake Shore Traps

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Hidden : 1/12/2012
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Geocache Description:

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Connection to the Earth Science Curriculum

Essential Lessons:

Visitors to Eagle Harbor Light Station will gain an understanding that the rocks and other materials there serve as a record of the region’s geologic history, including the role of the mid-continental rift in the formation of the Lake Superior Syncline.   

Earth Science Literacy Principles-

Big Idea 2:  Earth is 4.6 billion years old.

Big Idea 4:  Earth is continuously changing.

Common misconceptions 

  • Earth has always been the way it is right now.
  • All rocks and planets were formed at the same time.
  • All rocks are more or less the same.

 Michigan State Science Content Expectations Addressed:

Third Grade:
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.03.22 Identify and describe natural causes of change in the Earth’s surface (erosion, glaciers, volcanoes, landslides, and earthquakes).

Sixth Grade:
E.SE.M.4 Rock Formation- Rocks and rock formations bear evidence of the minerals, materials, temperature/pressure conditions, and forces that created them.
.SE.06.41 Compare and contrast the formation of rock types (igneous, metamorphic, and sedimentary) and demonstrate the similarities

E.SE.M.5 Plate Tectonics- The lithospheric plates of the Earth constantly move, resulting in major geological events, such as earthquakes, volcanic eruptions, and mountain building.
E.SE.06.51 Explain plate tectonic movement and how the lithospheric plates move centimeters each year.
E.SE.06.52 Demonstrate how major geological events (earthquakes, volcanic eruptions, mountain building) result from these plate motions.
E.SE.06.53 Describe layers of the Earth as a lithosphere (crust and upper

E.ST.M.4 Geologic Time- Earth processes seen today (erosion, mountain building, and glacier movement) make possible the measurement of geologic time through methods such as observing rock sequences and using fossils to correlate the sequences at various locations.
.ST.06.41 Explain how Earth processes (erosion, mountain building, and glacier movement) are used for the measurement of geologic time through observing rock layers.
.ST.06.42 Describe how fossils provide important evidence of how life and environmental conditions have changed.


Fresnel lens- A large lens invented by Augustin-Jean Fresnel which was crafted from smaller lenses.  It was lighter, thinner, and flatter than conventional lenses and made lighthouses visible over greater distances. 

syncline- a fold in rocks, where younger rocks are found near the center

clinometer/ inclinometer – a tool used to measure angles and slopes

compression when a set of stresses or forces is directed at a rock mass

basalt a hard black rock formed from slowly-cooled magma

basalticmagma – molten material from the upper mantle, which cools to form hard, black volcanic rock

conglomerates sedimentary rocks that are formed when larger, round fragments are cemented together to form new rock

dip the measure of the slope or tilt of rock layers

basalticflow tops – the top layers of lava flows, which are often porous and less dense

floodbasalts – basalts formed in large eruptions when deep layers of lava spread over great distances

Optional Activity:
Visit the Lighthouse, Museums and Exhibits

The lighthouse is open seven days a week in June, September and October from noon to 5 pm, and in July and August from 10 am to 5 pm. There is a $5.00 admission fee for adults.  Children are free.  The admission covers a hosted tour of the lighthouse, and entrance to the Keweenaw Shipwreck Museum, the Commercial Fishing Museum, and an exhibit of copper mining artifacts.  


The lighthouse and all of the buildings on site are maintained by the Keweenaw County Historical Society, which gave their permission for the creation of this EarthCache.


Eagle HarborLake Shore Traps

Perched at the northern edge of the Keweenaw Peninsula, this red brick lighthouse continues to guide ships safely across the harbor and nearby waters of Lake Superior.  The rocky shore near the lighthouse provides visitors with an opportunity to view Lake Shore Traps and the syncline formed by the Mid-Continental rift found in Lake Superior’s basin.

Figure 1: Eagle Harbor Lighthouse and Lake Shore Traps.

Materials Needed for your Visit

The information provided, GPS, topographic map of the area, a compass, a ruler, and a homemade clinometer (see directions provided)


  1. Check the battery level in your GPS and make sure you know how to use it to enter and label waypoints and navigate to a waypoint.
  2. Assemble your clinometer.
  3. Navigate to each of the stops.
  4. Read the background material and answer the questions associated with each stop. 
Coordinates: N 47°27.588’ W 88°09.614’

The History of Eagle Harbor Light Station

The lighthouse was built at Eagle Harbor to help mariners navigate the treacherous waters of Lake Superior and to mark this rocky point along the Keweenaw Peninsula.  It was first constructed in 1851 as a wooden structure with a Fresnel lens lit by a sperm oil flame.  When the structure began to crumble under the harsh weather conditions along the shore, the current lighthouse was constructed next to it.  The new lighthouse boasted a two story keeper’s home and a 44-foot hexagonal light tower with walls 18 inches thick.  The Fresnel lens was replaced by an aircraft-beacon-type lamp in 1962, and the lighthouse was automated in 1980.  It remains operational today and is visible a full 29 miles from shore.  While the grounds and buildings are maintained by the Keweenaw County Historical Society, the U.S. Coast Guard is responsible for the routine maintenance of the light itself (“Eagle Harbor Light Station”, 2011).


Figure 2: Eagle Harbor Lighthouse.

The Formation of Lake Superior

Lake Superior’s history differs greatly from that of the other great lakes.  About 1.1 billion years ago, the North American continent attempted to break apart along what became known as the Mid-Continental Rift. This is one of the deepest rifts in the world (Linder, 2006) and its curving path above Michigan’s Upper Peninsula helped define the shape of Lake Superior (Miller, n.d.). 


File:Lake Superior NASA.jpg
Figure 3: Lake Michigan.  Satellite Image.


Figure 4: Mid-Continent Rift. The Rift (in orange) extends from Detroit to the Midwest.

 Continental rifting has occurred repeatedly in the history of the earth.  It leads to the formation of ocean basins and the fracturing of continents.  In essence, the earth’s crust cracks and begins to spread apart.


Figure 5: Continental Drifting.  Magma pushes up from the upper mantle as the two plates spread apart.

 A more recent example of this can be seen in the active East African Rift zone, where the African plate is breaking apart into two new tectonic plates, the Nubian and Somalian plates.


Figure 6: East African Rift.  This rift is a developing divergent plate boundary.

 As the crust spread apart in the Mid-Continental Rift, basaltic magma rose to the surface and cooled.  This happened repeatedly as the rift expanded, erupting layer upon layer of lava on the surface.  In the time between flows, sediments were deposited on top of these layers.  The rifting activity stopped abruptly about 1 billion years ago.  As the dense layers of basalt cooled, they caused the rift valley to sink (Miller, n.d.). Sediments continued to fill this valley.  Later, compression (pressure coming from all sides) of this region caused uplift, forcing the layers of basalt at the edges of the rift upward until they were almost vertical in some places, forming what is known as the Lake Superior Syncline.  The rocks on the shore of the Keweenaw Peninsula dip toward the north, while the rocks on the southern edge of Isle Royale dip southward, marking the edges of the syncline (Rose, n.d.). Examining the rocks in either location allows visitors to glimpse some of the layers of the syncline.


Figure 7: Midcontinent Rift System. Layers of lava pooled on top of one another, causing the heavy center to sink.  Later, compression applied to the region caused further uplift of the edges, forming what is known as the Lake Superior Syncline.

Lake ShoreTraps  

As the exposed layers of the Lake Superior Syncline have weathered over time, they have created Lake Shore Traps in some areas.  The word “trap” originated from a Swedish term used to describe stacked lava flows that have weathered to form a stair-step pattern (St. James, 2010).  The traps visible along the shoreline at Eagle Harbor were formed by the final lava flows of the Mid-Continental Rift which occurred 1087 million years ago, burying the conglomerate under 31 different flows with a maximum thickness of 600 meters. The resulting layers of flood basalts and conglomerates of the exposed Lake Superior Syncline have weathered differentially  to form this “stair-step” pattern.  Softer materials, usually the basaltic flow tops have eroded away.  These become the bays in the harbor, while the more resistant rock remains to become points or ridges (Rose, n.d).  Some of these ridges of resistant basalt are visible, but others create submerged reefs in Eagle Harbor and along the shore of Lake Superior, which make navigation in these waters extremely dangerous.  The Lake Shore Traps seen along Eagle Harbor are similar to the famous West Indian Deccan Traps.   


Figure 8: Eagle Harbor Light Station.  The lighthouse is seen here, along with the Lake Shore Traps that shelter Eagle Harbor and are responsible for the jagged profile of the coastline.


Figure 9: Lake Shore Traps.  These traps, remnants of the final lava flows that formed the Lake Superior Syncline, jut out into Lake Superior making navigation in these areas treacherous.

Tour of Eagle Harbor Lake Shore Traps

Tour of Eagle Harbor Light Station

Starting Point:  Lighthouse Rd. off of North St. Park in lot at N 47°27.588’ W 88°9.614’. Proceed on foot to the Platform at Stop 1 while staying on established trails.

Stop 1: N47°27.599’ W88°9.602’ –  overlooking Northward Dipping Flood Basalts of the Lake Superior Syncline

Several layers of the flood basalts which comprise the Lake Superior syncline are visible here.   They dip, or tilt, northward into the water due to the sinking of the dense basalts into the rift zone in the basin of Lake Superior and because of the compression applied to the syncline. 

Logging Q1: Gaze out into the lake a sketch your general location along the Lake Superior Syncline.  Which everyday object(s) does the syncline resemble?  Look closely at the rock around you and describe it.  What is its color and composition?  Is it all basalt, or can you see other rock types in the layers?  Finally, use your clinometer to measure the dip of these layers.    

Stop 2: N47°27.564’ W88°9.525 – Lake Shore Traps

Eagle Harbor is sheltered by the trap lavas, which stretch out like long, thin fingers into the water along the shore. The more resistant layers of the basalt flows are all that remain.  Softer materials have been worn away by waves and glaciers over time, leaving the bays and pools between the ridges or points of the traps.

 Logging Q2: Look at the traps along the shore.  Can you see any pattern or alignment among them?  Describe the color and consistency of rock of which they are comprised. Is it all the same?  How do you think these geological features impacted navigation in the waters in and around Eagle Harbor?   Why is the presence of a lighthouse at this location of vital importance?


Making and Using your own Clinometer
(Directions provided by Mark Klawiter)

A clinometer (also known as an inclinometer) is a tool used to measure the dip (tilt or slope) of a geological feature. It’s easy and inexpensive to make your own.

Materials Needed:  a protractor with a hole in it, string, and a metal nut

  Tie one end of the string through the hole in the protractor and the other end to the nut.

Readingthe clinometer:  Hold the clinometer upside down so that the flat end is horizontal to the ground.  This shows a slope of zero.  Notice that the string hangs straight down, measuring 90 degrees on the protractor.  When the slope is zero, your clinometer will show a measurement of 90 degrees.  To find the dip of an outcrop, match the angle of the flat side of the clinometer to the dip of the rock you want to measure.  The string will hang straight down, pointing to the new measurement on the protractor.  To find the dip, subtract 90 from the new measurement. 

 Example:  If the clinometer string points to 135 degrees on the protractor, subtract 90 to find the true dip, or 45 degrees.    



Figure 10: Clinometer Diagram.  The weight on the string causes it to hang straight down.


Figure 11: Reading the Clinometer. Align the flat base of the protractor with the rock layers you are measuring to find the dip.

(This EarthCache was created after visiting the Eagle Harbor on July 23, 2011.)



Clinometer . (2011).  Retrieved August 8, 2011, from Wikipedia Website:

Compression.  (2011).  Retrieved August 8, 2011, from Wikipedia Website:

 Conglomerates, (2011).  Retrieved August 8, 2011, from Wikipedia Website:

 East African Rift. (2011). Retrieved August 8, 2011, from Wikipedia Website:

 Fresnel lens. (2011). Retrieved August 8, 2011, from Wikipedia Website:

 Keweenaw County historical Society. (2011). Eagle Harbor Light Station [Brochure]. Eagle harbor, MI: Kordes, M.

 Lake Superior. (2011). Retrieved July 29, 2011, from Wikipedia Website:

 Linder, D.O., (2006). Simply superior: The world’s greatest lake. Retrieved July 29, 2011, from:

 Miller, J. (n.d.). An introduction to the geology of the north shore. Retrieved July 29, 2011, from:

Rose, B. (n.d.). Eagle Harbor. Retrieved July 28, 2011, from:

Schaetzl, R.J, Darden, J.T., & Brandt, D.S. (2009). Michigan geography and geology. New York, NY: Pearson Custom Publishing.

St. James, J. (2010). Lake shore traps. Retrieved July 27, 2011 from:


Aerial of Eagle Harbor [Photograph].Retrieved July 30, 2011, from:

 Clinometer [Diagram]. Retrieved July 30, 2011, from:

 Continental Drifting [Diagram]. Retrieved July 31, 2011, from:

 Eagle Harbor, MI. [Photograph]. Retrieved July 30, 2011, from:

Lake Superior [Satellite image]. Retrieved July 30, 2011, from:

Lake SuperiorSyncline [Diagram]. Retrieved July 31, 2011, from:

Leonard, K. (n.d.). Eagle HarborLighthouse [Photograph]. Retrieved July 30, 2011, from:

Mid-African Rift [Diagram]. Retrieved August 8, 2011, from:

Mid-Continent Rift [Diagram]. Retrieved July 30, 2011, from:

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