The Ancient Shoreline location for this EarthCache is found on the Scoville Point Lava Flow. This lava flow is one of the 100 flows that make up the Portage Lake Volcanics Formation that create the northeast end of Isle Royale. The paleoshoreline that will be the cache site is a location that correlates to the Nipissing stage of the Isle Royale shoreline. The Nipissing stage occurred about 5,000 years ago and is the most recent postglacial lake shoreline stage of the Lake Superior basin. The lakeshore level during this time was at an elevation of 605 feet above sea level, which is quite close to the current elevation of Lake Superior at 602 feet above sea level. Due to the crustal process of isostatic depression and rebounding the Nipissing shoreline has rebounded and is now higher than the current Lake Superior shoreline. While at this EarthCache the visitor will hypothesize, based on their observations, why it would be considered to be a former beach and part of the Nipissing shoreline. They will also measure the distance between the current and ancient shoreline. And estimate the height of the basalt cliff adjacent to the path. In addition to this location it is suggested the visitor hike to “Suzy’s Cave”, which is approximately 1.8 miles from the park’s visitor center. The cave is a documented Nipissing shoreline feature with a current elevation of 640 feet above sea level. The visitor will be asked to estimate the height of the opening of the cave and the height of basalt outcropping that the cave is in.
Earth Science/Geologic Vocabulary
Holocene Epoch: This Epoch is the most recent 10,000 years within the Quaternary Period which is divided into the Holocene and Pleistocene Epochs. The Pleistocene Epoch started about 2.6 million years ago. The Quaternary Period is the most recent division within the Cenozoic Era.
(Blewitt, 2009, p. 2)
Glaciation: “an event during which the ice accumulated in northeastern Canada, advanced southward into the present Great Lakes region, and then completely melted away.” (Pictured Rocks Resource Report)
Wisconsin glaciation: Began about 79,000 years ago, reached its furthest extent about 23,900 years ago, then disappeared completely about 8,000 years ago. This glaciation period occurred during the late Pleistocene and early Holocene Epochs (Pictured Rocks Resource Report)
Nipissing Lake Stage: “slowly rising lake levels controlled largely by crustal uplift resulted about 5,000 years ago in the Nipissing lake stage at 605 feet above sea level”. (The Geologic Story of Isle Royale National Park)
Isostatic Rebound: rising of the earth’s crust in response to the removal of the weight of glacial ice.
Postglacial: after the retreat of the glaciers.
The shorelines of Isle Royale have been altered since the retreat of the last glacier. Look at the image below to compare the ancient shorelines and the changing shape of Isle Royale.
(Huber, 1983, p. 51)
The Nipissing stage occurred about 5,000 years ago and is the most recent postglacial lake shoreline stage of the Lake Superior basin. The lakeshore level during this time was at an elevation of 605 feet above sea level, which is quite close to the current elevation of Lake Superior at 602 feet above sea level. Due to the crustal process of isostatic depression and rebounding the Nipissing shoreline has rebounded and is now higher than the current Lake Superior shoreline. Isostatic rebound is a process directly related to the amount of time and the mass of the glacial ice that sat over Isle Royale. Isle Royale was covered with glacial ice during the Pleistocene Epoch and was not free of ice until the end of the Wisconsin Glaciation about 8,000 years ago. About 11,000 years ago the last mass of ice, which was about 2 miles thick, was sitting on top of Isle Royale pushing it down. After the ice retreated the land started to bounce, or rebound back. Isostatic rebound is still occurring today at an average rate of about 1 inch per century. Since the glaciers retreated in a northeasterly direction the glacier sat longer on the northeast end of the island so more rebound in the northeast is occurring then on the southeast end of the island. “Nipissing phase shorelines appear in many places all around the coasts of Lake Superior, Michigan and Huron, and are often the first prominent shoreline above the present lake level”. (Schaetzl, 2009, p. 187)
Logging Questions: While at this cache site answer the following questions:
What is the estimated distance from the “Ancient Shoreline” (cache location) to the current Lake Superior shoreline located just south of the cache? As you make your way to the current shoreline be sure to walk on hard surfaces to minimize your impact to the vegetation. Pictured below is the current Lake Superior shoreline south of cache site.
What is the height of the basalt cliff adjacent to the ancient shoreline?
What are two reasons why this cache site would be considered an ancient beach and part of the Nipissing shoreline?
Once you have finished enjoying this Nipissing shoreline about 1.8 miles from the visitor center is “Suzy’s Cave”. At the visitory center you can obtain a trail map for directions to the cave. This geologic feature is a documented Nipissing Shoreline location with an elevation of 640 feet. Hike to the cave to answer the following questions:
What is the height of the cave opening?
What is the height of the basalt cliff that the cave is in?
An even older shoreline found on the island is the Minong shoreline. A feature left here on the northeast end of the island is a sea stack called Monument Rock. You can find this geologic feature on your hike up to “Lookout Louis”.
Blewett, W. (2009). . Pictured Rocks Resource Report, Understanding Ancient Shorelines in the National Parklands of the Great Lakes. Washington D.C.: National Parks Service.
Huber, N. K. (1983). The Geologic Story of Isle Royale National Park. Houghton, MI: Isle Royale Natural History Association.
Lewandoski, M. (2012, June). Isle Royale Photos [Photograph; jpeg].
Olson, J. (2012). [Personal interview by the author].
Rose, W., Ph.D. (2012, June). [Personal interview by the author].
Schaetzl, R. (Ed.). (2009). Michigan Geography and Geology. New York, NY: Custom.
The “Ancient Beach” visitor will observe an ancient shoreline and compare this shoreline elevation and distance to the current shoreline. Investigating these differences help the visitor to understand the change in the shoreline location due to the process of isostatic rebound.
Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet. Scientists have been collecting data around the Lake Superior shoreline to determine the locations of ancient, postglacial shorelines.
Big Idea 2: Earth is 4.6 billion years old. Part of the field identification of the Nipissing shoreline is the rounded shape and location of the rocks.
Big Idea 4: Earth is continuously changing. Ancient Isle Royale shorelines are much higher than the existing shoreline mainly due to the process of isostatic rebound.
Lakeshore elevations have not drastically changed since the glaciers.
All effects from the glaciers were done shortly after the glaciers receded.
Human alterations to the Great Lakes have dramatically changed the lakeshore elevation.
Earth Science Standards:
E5.3C Relate major events in the history of the Earth to the geologic time scale, including formation of the Earth, formation of an oxygen atmosphere, rise of life, Cretaceous-Tertiary (K-T) and Permian extinctions, and Pleistocene ice age.
E3.3C Describe the motion history of geologic features.