Connection to the Earth Science Curriculum
Basalt – a volcanic rock, dark in color; forms when lava flows onto the surface of the Earth and cools quickly creating a fine grain
Conglomerate – a sedimentary rock; made up of a variety of pebbles cemented together by fine particles
Rhyolite – a volcanic rock, lighter in color than basalt; forms when lava flows onto the surface of the Earth and cools quickly creating a fine grain
Volcanic – any region, rocks, or minerals influenced by volcanoes
Date VisitedJuly 12, 2011
Horseshoe Harbor Beach
Horseshoe Harbor is located on the northern shore at the tip of the Keweenaw Peninsula. This pristine beach is a prime location for understanding the Earth’s past, and how the Earth constantly changes to create its present.
Difficulty: 4.5 (This site is only accessible by a trail head located on a two-track dirt road.)
Terrain: 4.0 (Mildly hilly hike, approximately .5 mi, terrain includes rocks and roots on dirt path.)
1. Before you leave check, the batteries in your GPS.
2. Be sure you know how to use your GPS to enter and label waypoint coordinates and navigate to waypoints.
3. Answer the logging questions below.
Location of Horseshoe Harbor – 47°28.340’ N, 087°48.077’ W.
Background and History Information
Upon arrival at the beach, your first glimpse is the beauty of Lake Superior. On either side of the beach, you’ll also notice large Copper Harbor Conglomerate rock formations jutting into the water, providing some protection from the amazing power of the wind and waves that can be created on this large lake. The protection is limited though, and you’ll notice as you walk along the beach that Horseshoe Harbor does not have the beautiful white sand found on much of the Michigan shorelines; this shoreline is different.
Figure 1 A view from the trail at Horseshoe Harbor, with the Copper Harbor conglomerate structures jutting into the water. The sheer size of Lake Superior dominates the background. Photo by A. Ernstes
The origin of the rock formation here happened approximately 1 billion years ago when a rift valley was formed in the center of the North American continent. This caused many sequences of lava flows, which is why we now find volcanic basalts and rhyolites here. However, eruptions were not consistent over time, and this allowed for periods of erosion to occur between lava flows. Repetition of these sequences combined with tremendous pressure from the layers piling atop one another formed the conglomerate structures now visible at Horseshoe Harbor.
These formations were revealed through series of faults and fractures in the northern Michigan region. A continental collision caused the Copper Harbor Conglomerates to be squeezed upward, causing them to become exposed both along the northern shore of the Keweenaw, and at Isle Royale.
Tour of Horseshoe Harbor
To reach Horseshoe Harbor, begin in Copper Harbor and travel east on Highway 41. Continue east until you reach 47°27.757’ N, 087°49.359’ W. At this point you will be turning north onto a two-track dirt road. Continue along this two-track road until you reach the trailhead located at 47°28.153’ N, 087°48.224’ W. At this point, park your vehicle and proceed along the trail until you reach 47°28.340’ N, 087°48.077’ W. (Insect repellent is recommended for this hike.)
Figure 2 Horseshoe Harbor is located on the northern shore of the Keweenaw Peninsula. Note the reddish brown color of the rhyolite pebbles on the beach. Photo by A. Ernstes
When you reach the beach, make an observation of the shoreline. You’ll notice that it is unlike the white sand beaches so typically found on Michigan shorelines.
Logging Q1: Describe the pebbles/cobbles found here. (Size, shape, colors, makeup etc.) Compare/contrast those found near the shoreline to those found near the tree line.
Logging Q2: What natural process(es) could have caused the pebbles and cobbles to be deposited here and arranged in such a way?
Now, notice the large conglomerate structures found on either side of the beach. These are the structures formed many years ago through sequences of volcanic activity separated by weathering and erosion. Initially, about a billion years ago, lava poured onto the Earth’s surface from openings created by a Mid-continent rift. The lava filled the valley created by the rift, and cooled rather quickly forming the basalt and rhyolite found here. During a time of little or no volcanic activity, weathering and erosion from the edge of the volcanic rift along with ancient mountains led to deposition of sediments in the rift valley. These two processes alternated over time. The immense weight caused the entire area to sink, and become cemented together. Finally, a collision of continental plates caused the entire rock formation to be tilted and revealed the structures that you see here today at Horseshoe Harbor. They are formed mainly of igneous rock, with a high percentage of densely formed basalt and rhyolite. Cementing the conglomerate structure is a small percentage of sedimentary rock such as shale and sandstone which are generally not as strong and durable as igneous formations.
Logging Q3: Take a closer look at one of the conglomerate formations on either side of the beach. What is the connection between these conglomerate structures to the pebbles that now exist on the shoreline?
Bonus Logging Question: Make note of any exceptions to the pebbles/cobbles generally found on the beach. Make a claim and include evidence about how these rocks came to rest on this beach.
Figure 3 The Michigan chapter of the Nature Conservancy is dedicated to preserving this location. Photo by A. Ernstes
This land is owned by the Michigan Chapter of the Nature Conservancy. It is only open during summer months as the dirt road and parking area are not maintained during the winter.
References and Citations
Robinson, S. (2001). Is this an agate?: An illustrated guide to Lake Superior’s beach stones Michigan. Hancock, Mich: Book Concerns Printers.
Schaetzl, Randall J., Joe T. Darden, and Danita S. Brandt. Michigan Geography and Geology. New York: Custom, 2009. Print.
United States Geological Survey. Copper Harbor conglomerate on Isle Royale. Retrieved July 25, 2011 from http://www.nps.gov/history/history/online_books/geology/publications/pp/754-B/sec2.htm