Isle Royale National Park 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/037 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: 

Geologic Scoping field trip, Isle Royale NP 

ON THE COVER: 

Weathered and striated bedrock, Isle Royale NP 

NPS Photos 
Isle Royale National Park 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/037 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

June 2008 

U.S. Department of the Interior 

Washington, D.C.
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gre_publications) on the internet, or by sending a request to the address on the back cover. 

Please cite this publication as: 

Thornberry- Ehrlich, T. 2008. Isle Royale National Park Geologic Resource Evaluation Report. 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/037. National Park Service, Denver, 

Colorado. 

NPS D- 122, June 2008 
 ISRO Geologic Resource Evaluation Report iii

Table of Contents 

List of Figures .............................................................................................................. iv

Executive Summary ...................................................................................................... 1

Introduction ................................................................................................................... 2

Purpose of the Geologic Resource Evaluation Program ............................................................................................2

Geologic Setting .........................................................................................................................................................2

Geologic Issues............................................................................................................. 5

Submerged Geomorphology.......................................................................................................................................5

Ancient Shorelines......................................................................................................................................................5

Abandoned Mine Sites ...............................................................................................................................................6

Water Quality and Supply...........................................................................................................................................6

Other Geologic Issues ................................................................................................................................................7

Geologic Features and Processes............................................................................... 9

Geology and History Connections ..............................................................................................................................9

The Lake Superior Syncline and Isle Royale Fault.....................................................................................................9

Glacial Features .......................................................................................................................................................10

Isle Royale as a Natural Laboratory .........................................................................................................................10

Greenstones.............................................................................................................................................................11

Map Unit Properties .................................................................................................... 15

Map Unit Properties Table........................................................................................................................................16

Geologic History.......................................................................................................... 17

Archean through Proterozoic....................................................................................................................................17

Late Proterozoic through Early Cambrian.................................................................................................................18

The Missing Record..................................................................................................................................................18

Quaternary Period ....................................................................................................................................................18

Glossary....................................................................................................................... 23

References................................................................................................................... 25

Appendix A: Geologic Map Graphic .......................................................................... 27

Appendix B: Scoping Summary................................................................................. 29

Attachment 1: Geologic Resource Evaluation Products CD
 NPS Geologic Resources Division iv 

List of Figures 

Figure 1. Map of Isle Royale ..........................................................................................................................................4

Figure 2. Chlorastrolite specimens.................................................................................................................................8

Figure 3. Regional distribution of rock types and geologic structures of the Lake Superior basin................................12

Figure 4. Cross- sectional diagrammatic view of Isle Royale .......................................................................................12

Figure 5: Cross- sectional view of the Lake Superior basin .........................................................................................13

Figure 6. Diagrammatic view of a retreating glacier .....................................................................................................13

Figure 7. Map showing maximum extent of glacial ice.................................................................................................14

Figure 8: Geologic time scale.......................................................................................................................................20

Figure 9a-c. Evolution of the landscape in the Isle Royale area...................................................................................21

Figure 10a-c: Evolution of the landscape in the Isle Royale area continued................................................................22
 ISRO Geologic Resource Evaluation Report 1

Executive Summary 

This report accompanies the digital geologic map for Isle Royale National Park in Michigan, 

which the Geologic Resources Division produced in collaboration with its partners. It 

contains information relevant to resource management and scientific research. 

Isle Royale National Park preserves a remarkable 

wilderness area in northwest Lake Superior near the 

Canadian border. Its isolation and severe climate keeps 

the island relatively free from human influences, creating 

an ideal natural laboratory to study the complexity of a 

relatively pristine ecosystem. 

Understanding the geology of the northern Lake 

Superior region enhances understanding of the unique 

relationship between geology and the environment and 

the ecosystem of Isle Royale. The landscape of Isle 

Royale is reflected in the structure and type of 

underlying rocks and the geologic processes acting on 

them. The rock units present in Isle Royale consist of 

ancient thick lava flows interlayered with sedimentary 

rocks and overlain by conglomerates. The youngest 

geologic units on the island are surficial unconsolidated 

deposits, many of which are glacial in origin. Glaciation 

scoured the area leaving glacial grooves, fjord- like inlets, 

rocky fingers of land, and drowned valleys with 

abundant glacial deposits of till and erratics. The age gap 

between the consolidated bedrock and the 

unconsolidated surficial deposits is more than 570 

million years. 

The following issues identified at the Geologic Resource 

Evaluation (GRE) scoping meeting have geological 

significance and are important for resource management 

in the park. These are discussed in detail in the Geologic 

Issues section with recommendations for inventory, 

monitoring, and research. 

• Submerged geomorphology. 

Approximately two- thirds of the 890- square mile 

park is below the water level of Lake Superior. Many 

subaqueous habitats exist at Isle Royale National Park. 

Environments can change in just a few meters of water. 

Traditional surficial mapping does not include 

submerged geology and U.S. Geological Survey 

(USGS) maps extend to a depth of only 5 m (15 ft) 

offshore. Data requirements for better characterizing 

the submerged geomorphology of the park include 

lidar surveys, satellite imagery, multi- beam mapping, 

bathymetry, water quality and circulation, shoreline 

change data, and storm data. 

• Ancient Shorelines. 

When the last glaciers retreated from the area less than 

10,000 years ago, the fluctuation of the level of Lake 

Superior formed a series of paleoshorelines. 

Paleoshoreline features include beach deposits, wavecut cliffs, terraces and benches. Identifying and 

mapping ancient shorelines will provide important 

information for understanding the formation of these 

shorelines as well as indicating the locations of ancient 

human settlements and artifacts. 

• Abandoned Mine Sites. 

Historic mining, mainly for copper, occurred mostly in 

the 1800s. There is also evidence that there was some 

prehistoric mining by Native Americans. Unlike the 

Keweenaw Peninsula and other areas of the Great 

Lakes, Isle Royale does not have abundant deposits of 

copper. Native copper is widely distributed but there 

are no large lode deposits (Huber, 1983). About 80% of 

the park is unsurveyed for abandoned mine sites and 

there is also a need for bat surveys in the park. Two 

mines of note are the Island Mine and the Minong 

Mine, both of which were financial failures (Huber, 

1983). 

• Water Quality and Supply. 

Currently, all drinking water comes from Lake 

Superior. Although the park is interested in utilizing 

the groundwater supply for economic reasons, the 

characteristics of the groundwater aquifer system are 

not well known. Knowledge of how water travels 

through the subsurface is essential to predicting 

hydrologic response to inputs such as contaminants, 

and other wastes. The water table is high and there 

may be some contamination from human wastes as 

well as from petroleum spills and other toxic 

substances, especially at Rock Harbor, Mott Island, 

Amygdaloidal Island, and Windigo. 

• Other Issues. Other geologic issues discussed during 

scoping include: species isolation; chlorastrolite, the 

state gem; wetlands; and recreational demands. These 

issues are briefly summarized in this report. 
 NPS Geologic Resources Division 2 

Introduction 

The following section briefly describes the National Park Service Geologic Resource 

Evaluation Program and the regional geologic setting of Isle Royale National Park. 

Purpose of the Geologic Resource Evaluation Program 

The Geologic Resource Evaluation (GRE) Program is 

one of 12 inventories funded under the NPS Natural 

Resource Challenge designed to enhance baseline 

information available to park managers. The program 

carries out the geologic component of the inventory 

effort from the development of digital geologic maps to 

providing park staff with a geologic report tailored to a 

park’s specific geologic resource issues. The Geologic 

Resources Division of the Natural Resource Program 

Center administers this program. The GRE team relies 

heavily on partnerships with the U.S. Geological Survey, 

Colorado State University, state surveys, and others in 

developing GRE products. 

The goal of the GRE Program is to increase 

understanding of the geologic processes at work in parks 

and provide sound geologic information for use in park 

decision making. Sound park stewardship relies on 

understanding natural resources and their role in the 

ecosystem. Geology is the foundation of park 

ecosystems. The compilation and use of natural resource 

information by park managers is called for in section 204 

of the National Parks Omnibus Management Act of 1998 

and in NPS- 75, Natural Resources Inventory and 

Monitoring Guideline. 

To realize this goal, the GRE team is systematically 

working towards providing each of the identified 270 

natural area parks with a geologic scoping meeting, a 

digital geologic map, and a geologic report. These 

products support the stewardship of park resources and 

are designed for non- geoscientists. During scoping 

meetings the GRE team brings together park staff and 

geologic experts to review available geologic maps and 

discuss specific geologic issues, features, and processes. 

The GRE mapping team converts the geologic maps 

identified for park use at the scoping meeting into digital 

geologic data in accordance with their innovative 

Geographic Information Systems (GIS) Data Model. 

These digital data sets bring an exciting interactive 

dimension to traditional paper maps by providing 

geologic data for use in park GIS and facilitating the 

incorporation of geologic considerations into a wide 

range of resource management applications. The newest 

maps come complete with interactive help files. As a 

companion to the digital geologic maps, the GRE team 

prepares a park- specific geologic report that aids in use 

of the maps and provides park managers with an 

overview of park geology and geologic resource 

management issues. 

For additional information regarding the content of this 

report and up to date GRE contact information please 

refer to the Geologic Resource Evaluation Web site 

(http://www2.nature.nps.gov/geology/inventory/). 

Geologic Setting 

Isle Royale National Park protects 571,790 acres (2,300 

km2

; 890 mi2) of a group of islands in Lake Superior. Of 

this only 132,018 acres is land- based wilderness. Lake 

Superior is the largest freshwater lake in the world and is 

the coldest, deepest, and highest in elevation of any of 

the Great Lakes. At its widest point, Isle Royale National 

Park is 70 km (45 mi) long and 14 km (9 mi) wide. The 

park consists of one large island surrounded by 

approximately 400 smaller islands. This national park 

was authorized on March 3, 1931, established on April 3, 

1940, portions were designated a wilderness area on 

October 20, 1976, and became a national biosphere 

reserve in 1980. 

The main island is located more than 16 km (10 miles) 

from the Minnesota mainland, and more than 48 km (30 

miles) from the upper peninsula of Michigan. The park is 

only accessible by seaplane from Houghton, Michigan, 

or by boat ferry from Houghton or Copper Harbor, 

Michigan, or Grand Portage, Minnesota. This remote 

and isolated area harbors a unique environment that 

responds to natural influences with little human 

interference. Because it has been relatively protected 

from human development, heavy forests extend down to 

the shorelines, reflecting the conditions that must have 

existed throughout the Lake Superior region prior to 

settlement. 

The highest point on the main island at Isle Royale 

National Park is Mount Desor at 425 m (1,394 ft) in 

elevation. The lowest point is the water level elevation of 

Lake Superior at 183 m (600 ft). Mount Desor sits along 

Greenstone Ridge which forms the backbone of Isle 

Royale. The ridge is composed of ancient 

metamorphosed basaltic lava flows. These flows, named 

for their characteristic green hue, are part of one of the 

largest and thickest lava flows on earth. The Greenstone 

Flow at Isle Royale is up to 240 m (790 ft) thick and 

extends beneath the surface of Lake Superior, 

reappearing on the Keweenaw Peninsula of Michigan. 

During the last significant glacial maximum, 

approximately 11,000 years ago, thick ice sheets covered 

the Great Lakes area. Erosion by these glaciers and to a 

much lesser degree, their weight, formed huge basins and 

sculpted the landscape. Glacial scouring helped form 

parallel ridges and linear valleys, lakes, and harbors. 
 ISRO Geologic Resource Evaluation Report 3

A washboard- like pattern of linear ridges and parallel 

valleys, created by the tilted rock layers and the scouring 

erosive action of the last major glaciation, characterize 

this topography. These features align parallel in a 

northeast to southwest trend, affecting many facets of 

the environment at Isle Royale including animal and 

plant distribution and migration patterns, microclimates, 

wetland distribution, and human use. The most 

prominent ridges on the island are the Greenstone Ridge 

along Isle Royale’s center, Feldtmann Ridge, Minong 

Ridge, Red Oak Ridge, and Stanley Ridge (figure 1).The 

alignment of these linear, parallel features, as well as the 

islands’ shape and orientation indicate that the most 

recent glaciers covering the area flowed from northeast 

to southwest. 
 NPS Geologic Resources Division 4 

Figure 1. Map of Isle Royale and some surrounding islands with major ridges noted. Graphic by Trista L. Thornberry-Ehrlich (Colorado 

State University). 
 ISRO Geologic Resource Evaluation Report 5

Geologic Issues 

A Geologic Resource Evaluation scoping session was held for Isle Royale National Park 

on June 16, 2004, to discuss geologic resources, address the status of geologic mapping, 

and assess resource management issues and needs. The following section synthesizes the 

scoping results, in particular those issues that may require attention from managers. 

Submerged Geomorphology 

Since approximately two- thirds of the 2,300 km2 (890 

mi2

) park is below lake level, understanding the 

bathymetry and geology of the submerged areas is an 

important resource management issue. Many 

subaqueous habitat types exist at Isle Royale National 

Park and these different environments are strongly 

related to small changes in depth. Environments can 

change within meters of topographic relief. U.S. 

Geological Survey (USGS) maps extend to a depth of 

only 5 m (16 ft) offshore. Given the coverage of mapping 

at the park, traditional surficial maps are not sufficient 

for complex management decisions at Isle Royale. 

An interdisciplinary approach to mapping is necessary to 

produce a useful product for resource management. In 

addition to understanding the bathymetry and geology of 

the submerged areas, mapping these areas will provide 

information regarding fish habitats including shoals, 

gravel bars, and deeper areas. The isolation and pristine 

waters surrounding Isle Royale create an ideal setting to 

study fish populations and to understand geological and 

geomorphological relationships with fish spawning, 

distribution, and migration patterns. 

Useful data to collect for this effort include lidar surveys, 

satellite imagery, multi- beam mapping, bathymetry, 

water quality and circulation, shoreline change, pre- and 

post- storm comparisons, and lake water circulation data 

(waves, tides, currents, turbidity, temperature, sediment 

transport patterns, and species distributions). This 

ecosystem approach integrates biological, physical, 

cultural, and oceanographic variables. 

Inventory, monitoring, and research suggestions for 

submerged geomorphology include: 

• Promote sub- aqueous mapping projects. 

• Relate bathymetry to populations and distributions of 

fish and other aquatic species. 

• Map benthic habitats including sediment types and 

distribution patterns. 

Ancient Shorelines 

There are many shoreline types at Isle Royale including 

beaches, rocky shores, narrow fjord- like inlets, large 

bays, cliffs, paludal shores, inland lake shores, and 

wetlands. With historic fluctuating water levels, these 

environments are submerged or exposed. When the last 

glaciers retreated from the area, starting 10,000 years ago, 

a series of intermittent lakes formed, leaving 

paleoshoreline features on the landscape at Isle Royale. 

Developed lake levels include Lakes Washburn and 

Beaver Bay (~9,800- 9700 years ago), Lake Minong 

(~9,500 years ago), Lake Houghton (~8,000 years ago), 

and Lake Nipissing (~5,500- 4,700 years ago) (McRae et 

al. 1998; John Anderton, written comm. 2007). 

Isostatic rebound has now lifted these once horizontal 

lake planes with uplift increasing from southwest to 

northeast (McRae et al. 1998). Many paleoshoreline 

features are obscure and difficult to see. Identifying and 

mapping ancient shorelines will provide important 

information regarding the history of the relative rise and 

fall of the lake level in response to glaciation and postglacial changes. Preliminary studies using GIS and 

modeled shorelines have provided estimates of the 

elevation of various post- glacial shorelines around the 

island (McRae et al. 1998). 

This information may also be used to locate ancient 

human settlements and artifacts often concentrated 

along shorelines. The oldest known archeological sites 

are about 4,000 to 5,000 years old. There could be older 

sites (8,000 to 9,000 years old) along some Minong 

shorelines. Some have also been found at Thunder Bay, 

Ontario (Scoping participants, oral comm. 2004). Many 

of these and other unknown sites could be submerged. 

Submerged stumps have been found at Grand Island at 

over 60 m (200 ft) depth, these are believed to be 8,000 

years old (Scoping participants, oral comm. 2004). 

Native copper fragments could have been accessible in 

paleoshoreline deposits, due to winnowing by wave 

action along areas void of vegetation (McRae et al. 1998). 

USGS mapping by Huber in 1973 included some ancient 

shoreline mapping. Additionally, the NPS Geologic 

Resources Division, in cooperation with the Natural 

Resource Conservation Service (NRCS), initiated a Soil 

Resource Inventory project in 2005 which will include 

detailed soil mapping for the park and may provide some 

ancient shoreline information. This project has an 

estimated completion date of 2009. However, for 

resource management at Isle Royale, much remains to be 

completed and mapped in detail. Huber mapped the 

paleo- shorelines of post- glacial Lake Minong and 

Nipissing, published in USGS Professional Paper 754- A, 

finding more paleoshorelines in the southwest portion of 

the park than the northeast end. These areas have a high 

potential for preserving archeological resources. 

Following Huber’s mapping in 1973, a previously 

unrecognized event known as the Marquette advance 

changed geologists’ understanding of paleoshorelines in 
 NPS Geologic Resources Division 6 

the area. During the Marquette advance about 11,450 

years ago an ice sheet closed the eastern inlets of ancient 

glacial Lake Agassiz. This ice sheet generated significant 

glaciofluvial outwash that flowed across the Upper 

Peninsula of Michigan. Remapping paleoshorelines with 

this in mind could identify previously unrecognized 

geomorphologic features. 

Inventory, monitoring, and research suggestions for 

ancient shorelines include: 

• Study submerged inlets and paleoshorelines to 

determine patterns and dates of formation. 

• Study paleodrainage for Isle Royale. 

• Integrate information from the Soil Resources 

Inventory with other mapping products. 

Abandoned Mine Sites 

Native copper on Isle Royale may have attracted Native 

Americans to the area before European exploration. 

Although Isle Royale was never a significant source of 

copper, 14 abandoned mine sites (including prospects) 

have been inventoried. Historic mining occurred during 

three different periods: from 1840- 1850s, 1870s, and 

1890s. Though widespread, the deposits on Isle Royale 

were too sparse to prove economically viable for mining. 

High temperature fluids, percolating through fissures 

and conglomerates resulted in the precipitation of native 

copper in several of the geologic units at Isle Royale; 

however, such copper has not been found in major 

concentrations in the park. Copper ore is often 

composed of copper bearing minerals such as chalcocite 

(Cu2

S), chalcopyrite (CuFeS2), and cuprite (Cu2O), but 

native copper is rarely present in major concentrations. 

However, native copper deposits on the Keweenaw 

Peninsula of Michigan are among the most extensive in 

the world. These are present in two broad categories – 

lode vein and fissure filling deposits (Huber 1975). Lode 

deposits contributed about 98 percent of the total copper 

mined in the Native Copper district of Keweenaw 

(approximately 5,400,000 tons total). 

The Island mine, near the west end of Siskiwit Bay, was 

located on a conglomerate lode deposit. Most of the 

historic short- lived mines focused on fissure- filled 

deposits such as the Minong, Siskiwit, and other various 

smaller operations. Within these fissures, native copper 

occurs in nodules and irregular masses in highly altered 

parent rock. This alteration was concentrated in fracture 

zones a few inches to many feet wide. Several significant 

large masses were found at the Minong mine (the largest 

on the island on the southeastern flank of Pine Mountain 

near the Minong Ridge Trail) among the approximately 

250 tons of copper extracted there (Cannon and 

Woodruff 1999; Huber 1975). A few of these individual 

nodules weighed more than a ton (Huber 1975). 

In other locations at Isle Royale, veins of quartz, prehnite 

[Ca2

Al(AlSi3O10)(OH)], a layered silicate typically found as 

a secondary mineral in cavities, and calcite contain 

scattered grains of native copper. Native copper also 

occurs in amygdules in metamorphosed lava flows, 

especially in prehnite- bearing amygdules (Huber 1975). 

During the late 1980s, a survey of these historic mining 

sites in the park focused primarily on areas along trails 

and near campgrounds and identified 14 sites. However, 

approximately 80% of the park’s backcountry wilderness 

areas remain unsurveyed for abandoned mine location. 

Abandoned mine features may pose several concerns for 

resource management. Visitor safety is a constant 

concern wherever open shafts, pits, trenches, and 

associated waste rock occur. GRD records of Isle Royale 

National Park surveys in 1984 and 1989 indicate that most 

mine features accessible to the public have become 

overgrown and partially filled in or flooded. However, a 

formal hazard assessment recorded with the Abandoned 

Mine Lands inventory, maintained by GRD, is 

recommended as timing and funding permit. 

Inventory, monitoring, and research suggestions for 

abandoned mine sites include: 

• Map and create GIS layers of park mining sites 

• Perform studies relating historic mine features to 

biologic diversity (bats) at Isle Royale. 

• Complete a comprehensive survey of mine features at 

Isle Royale, determine if any historic mines pose 

visitor safety risks that may need remediation. 

• Study effects of historic mining features on the 

hydrogeologic system of the island. 

• Develop and promote more interpretive programs on 

historical exploration and extraction of mineral 

resources from Isle Royale. 

• Reclaim abandoned mine land sites. 

Water Quality and Supply 

Currently, drinking water is pumped from Lake 

Superior. In the past it has been expensive to pump, 

clean, and purify lake water. There are mineral seeps and 

springs that are critical to moose health; however, most 

are high in mineral content and would require treatment 

for human use. Water wells drilled into the underlying 

aquifer would be economically desirable, but the 

hydrogeologic characteristics for the park and 

groundwater aquifer system are not well understood. 

Understanding the hydrogeologic system at the park 

includes knowing how water is traveling through the 

subsurface for future well siting, and predicting the 

hydrologic response to inputs such as contaminants and 

wastes. The movement of nutrients and contaminants 

through the hydrogeologic system can be modeled by 

monitoring the composition of system inputs, such as 

rainfall, and outputs, such as streamflow. Other input 

sources include wind, surface runoff, groundwater 

transport, sewage outfalls, landfills, and fill dirt. Streams 

in effect integrate the surface runoff and groundwater 

flow of their watersheds. Thus, they provide a 

cumulative measure of the status of the watershed’s 

hydrologic system. Consistent measurement of these 
 ISRO Geologic Resource Evaluation Report 7

parameters is crucial to establishing baselines for 

comparison. 

The water table in the park is generally high. Disposal of 

human waste in shallow privies can contaminate both 

surface and groundwater. There have been oil spills from 

boats and facilities as well contamination from lead and 

other toxic substances on Passage Island, Rock Harbor, 

Mott Island, Amygdaloidal Island and Windigo. The 

presence of human wastes, residual PCBs, mercury, and 

other toxins are a major resource management concern 

for the park (Scoping participants, oral comm. 2004). 

A recent study of annual rainfall at Isle Royale 

discovered trace amounts of triazine herbicides such as 

atrazine, deethylatrazine, deisopropylatrazine, cyanazine, 

and associated metabolites precipitating into soils and 

water. The predominant westward wind direction 

carried these airborne contaminants from the upper 

Midwestern United States. Deep lakes at Isle Royale may 

have residence times for atrazine that exceed 10 years as 

opposed to the shallower lakes that promoted faster 

degradation of the contaminant (Thurman and 

Cromwell 2000). These contaminants are not yet a threat 

to the ecosystem, and frequent monitoring of the inland 

lake waters at Isle Royale would alert resource 

management to any changes. The pristine nature of the 

Isle Royale hydrogeologic system makes it ideal to study 

the nature and quantity of contaminants. 

In the mid- 1990’s, elevated mercury levels were found in 

game fish from six of the inland lakes at Isle Royale. This 

prompted a study to determine the sources of mercury 

contamination. Originally, bedrock geology was 

suspected of controlling the mercury concentration in 

the soil and surface water. However, further 

investigation revealed mercury concentrations were 

largely due to airborne deposition. The volcanic rocks 

had no detectable mercury, but some native copper 

mineralization had trace amounts (Cannon and 

Woodruff 1999; Woodruff et al. 2003). Distribution of 

high mercury concentrations appears to correlate 

strongly with the presence of organic carbon in the soils, 

possibly also related to forest fire history. Mercury 

attaches to organic carbon to form methyl mercury, 

which is very mobile. Soils burned in 1936 at Isle Royale 

have low carbon and mercury content compared to 

unburned areas. Further study may help improve 

understanding of the distribution of mercury in soils and 

surficial deposits and contamination of the aquatic 

system at Isle Royale (Cannon and Woodruff 2000). 

Inventory, monitoring, and research suggestions for 

related issues include: 

• Define the influences of bedrock and topography on 

local watersheds within Isle Royale National Park. 

• Map and quantify water in subterranean recharge 

zones. 

• Identify and map the near surface fracture system to 

help understand surface and groundwater flow 

regimes. 

• Create hydrogeologic models for the park to better 

manage the groundwater resource and predict the 

system’s response to contamination using wells for 

hydrogeologic characterization and monitoring. 

Other Geologic Issues 

Species Isolation 

There are many questions regarding species isolation on 

Isle Royale requiring geologic research. For example, 

when and how often has the island been isolated from 

the mainland? During extremely cold seasons, Lake 

Superior freezes over providing the opportunity for 

some animals to migrate over ice to the island. In the past 

Caribou and lynx were on the island, but today it is 

moose and wolf populations that have been the subject 

of predator- prey study for many years. This long record 

of data is invaluable to biologists. Isle Royale has 

approximately 20 species of mammals compared to 40 

present on the mainland. Biological studies have been 

ongoing at the island because of its isolation and relative 

lack of human influence. 

There are at least 72 species of state- listed plant species 

(excluding trees) on Isle Royale. These species thrive in 

the variety of environments including rock openings, 

rocky shorelines, clearings and beaches, dry bluffs, trails, 

boreal forest, rocky areas, mixed forests, aquatic areas, 

bogs, swamps, and burns among others (NPS 2005). 

Many of these species are rare or uncommon. There are 

plants native to the Pacific Northwest present on the 

island. Thus far, it has not been determined why these 

species are not present on the mainland. One possibility 

is the unique microclimate and the minimal human 

influence that exists on the island. 

The isolation at Isle Royale can lead to some biological 

problems for the species present there. Though large in 

area, the genetic diversity of the animal populations there 

is limited resulting in inbreeding and perpetuation of 

weaker genetic traits. The balance between predator and 

prey and their dependence on specific plant species on 

the islands can lead to periodic species population 

collapse. 

Inventory, monitoring, and research suggestions for 

species isolation include: 

• Study ancient lake levels and paleoshorelines to 

determine timing of highstands and lowstands. 

• Study paleoclimatic patterns to determine if species 

isolation correlates with glacial maximums. 

• Using genetic data and bathymetry data from Lake 

Superior, attempt to determine the approximate place 

of origin and direction of migration for the animal 

species present at Isle Royale. 

Chlorastrolite and Other Mineral Specimens 

In addition to the native copper deposits, the geologic 

units of Isle Royale contain many minerals of interest, 

including the state gem of Michigan, chlorastrolite, 

meaning “green star stone” (fig. 2). This mineral is 
 NPS Geologic Resources Division 8 

present as amygdules or vesicular fillings in certain 

altered lava flows. When weathered out, it appears on 

some local beaches as pea- sized pebbles. This green gem 

has a characteristic weathered mosaic or mottled pattern 

referred to as turtleback (Huber 1975). This gem is 

actually a variety of the mineral pumpellyite 

[Ca2

MgAl2(SiO4)(Si2O7)(OH)2·H2O] and is also known as 

“Isle Royale greenstone.” This is somewhat confusing 

since petrologically, greenstone refers to a rock type 

composed of the metamorphosed remnants of igneous 

basalt flows, and is common on Isle Royale. 

C.T. Jackson and J.D. Whitney discovered chlorastrolite 

on Isle Royale in 1847. Raphael Pumpelly later identified 

it as pumpellyite from a similar mineral first described on 

the Keweenaw Peninsula in 1825 (Huber 1975). The 

chlorastrolite variety of pumpellyite might be unique to 

Isle Royale. 

Other significant and interesting minerals on Isle Royale 

include barite, calcite, chlorite, datolite, epidote, 

laumontite, natrolite, prehnite, quartz (including several 

colors of agate), and rare thomsonite (a zeolite). 

Prehnite, appearing as small pink pebbles, occurs as 

amygdule fillings, crosscutting veins, fracture filling and 

as a replacement of other minerals (Huber 1983). 

Theft of chlorastrolite and other minerals by collectors is 

a resource management issue at Isle Royale. In 1997, at a 

remote site, park rangers found ten large Ziploc bags 

filled with over 300 pieces of datolite [a semi- precious 

gemstone - CaBSiO4

(OH)] that collectors had found in 

the shoreline sediments and submerged beaches of Lake 

Superior. According to the U.S. Attorney’s office and 

Department of the Interior lawyers, mineral collection 

laws afford protection under National Park Service 

regulations to submerged Lake Superior mineral 

resources. Thus, collection of any minerals in Isle Royale 

National Park is illegal, but no doubt, it continues. 

Visitors need to be informed of the prohibition against 

removing any rocks or minerals from the park. Also, 

increasing patrols, developing interpretive exhibits that 

discuss the unique minerals of the island and their 

formation, and investigating local private collections (if 

possible) may help to eliminate mineral theft. 

Wetlands 

In nearly every linear parallel valley, low- lying areas are 

host to wetlands (bogs, swamps, and small ponds). In 

general, at Isle Royale bogs are unforested and swamps 

support forests (John Anderton, written comm. 2007). 

Coring wetlands could provide a wealth of information 

on the sediment types and distribution, climate changes 

and patterns, cultural and land use history, and fire 

history of the island. There is some question as to how 

potential water well pumping for drinking water and 

water quality monitoring will affect the springs and 

wetland areas of the island. Mapping and monitoring 

water quality and soil characteristics at wetlands can 

create a baseline record of ecosystem health, useful in 

understanding future changes. 

Recreational Demands 

Two goals of the National Park Service are to preserve 

and protect the natural and cultural resources of each 

unit and to provide opportunities for visitors to enjoy 

those resources. Thousands of visitors come to Isle 

Royale National Park every year to enjoy the bounty of 

recreational wilderness opportunities. In 2007 there were 

15,973 visitors to the park. Visitor activities include 

hiking, motor boating, kayaking, sailing, canoeing, 

wildlife viewing, fishing, photography, camping, and 

picnicking (NPS 2005). Visitors are placing increasing 

demands on the natural resources of the park. Careful 

management is necessary to alleviate some of the 

problems associated with recreational demands and 

restore damaged areas such as old fire towers, overused 

trails, and picnic areas. Periodic site investigations by 

park staff will help identify areas of increased erosion 

and slope instability. 

Many trails, including the Greenstone Ridge Trail, wind 

through unique biologic and geologic environments in 

the park. Many of these are fragile and off- trail hiking 

accelerates their degradation. Associated with hiking, are 

backcountry camping and picnicking. The park attempts 

to concentrate the impacts of these forms of recreation 

in designated areas. Camping in non- designated areas 

increases the area of impact and places delicate 

ecosystems at risk. National Park Service wilderness 

regulations (in accord with the Wilderness Act) place 

strict restrictions on recreation at Isle Royale. 

Figure 2. Chlorastrolite specimens. The largest of these pebbles is approximately one centimeter in diameter. Photo by Roger Weller, 

Cochise College from http://www.cst.cmich.edu/users/dietr1rv/chlorastrolite.htm (accessed April 22, 2008) 
 ISRO Geologic Resource Evaluation Report 9

Geologic Features and Processes 

This section describes the most prominent and distinctive geologic features and processes 

in Isle Royale National Park. 

Geology and History Connections 

The geology and geologic setting of Isle Royale has 

heavily influenced its history. Interest in the mineral 

resources of Isle Royale began as early as 4,000 years ago. 

Three small pits near Scoville Point indicate that Native 

Americans found malleable native copper there, useful 

for making tools and implements to trade as far away as 

New York, Illinois, and Indiana. For more than 1,000 

years (mostly between 800 to 1600), the islands provided 

Native Americans with materials for tools, animals for 

food and clothing, and a variety of nuts, berries, and 

other plants (Edstrom 1994). They came to the island in 

the summer season, avoiding the area in the winter. It 

was likely a haven in the midst of Lake Superior’s stormy 

waters as well. By the 1840s, the only Native American 

presences at Isle Royale were a maple sugaring camp on 

Sugar Mountain and a seasonal fishing camp on Grace 

Island. 

One of the goals of the park is to preserve historical and 

archaeological artifacts and features in the area. 

Maintaining these features often means resisting natural 

geologic changes, which presents challenges to resource 

management. Geologic processes such as wave and wind 

action, chemical weathering, and slope creep are 

constantly changing the landscape of the park. Wind 

picks up fine- grained clay, silt, and sand, depositing 

them in small- scale dune- like features. Also, wind can 

expose artifacts by winnowing away smaller, obscuring 

particles. Coastal processes such as longshore drift, wave 

action, and beach formation constantly alter modern 

shoreline areas. Surface runoff removes mostly 

unconsolidated sediments and deposits them in streams 

and gullies, altering the historical context of the 

landscape. 

The Lake Superior Syncline and Isle Royale Fault 

The Lake Superior basin is the result of a combination of 

a failed continental rift, a geological syncline, and glacial 

erosion, which preferentially excavated the softer rocks 

from the middle of the syncline (Mineralogical Society of 

America (MSA) 2007). The failed continental rift in the 

middle of the North American craton (The 

Midcontinent Rift) was active approximately 1.1 billion 

years ago (Sexton and Henson 1991). The craton is 

composed of ancient continental crust that has remained 

more or less stable since the Proterozoic Eon. The 

approximate trace of the rift extends from northeastern 

Kansas through Iowa, Minnesota and under Lake 

Superior. Geophysical evidence suggest there are other 

segments of the rift that turn south through lower 

Michigan and perhaps an extension of the rift in 

Oklahoma. 

Normal fault- bounded half grabens characterize the rift 

system. As rifting proceeded locally, there were two 

graben- bounding normal faults on either side of the rift: 

the Keweenaw fault on the eastern side and the Isle 

Royale fault on the western edge (MSA 2007). The faults 

bounding the rift area may curve at depth, grading into 

gently dipping, large crustal detachments that extend 

into the lower crust of the basin, which thinned 

significantly during rifting (Cannon et al. 1987; Sexton 

and Henson 1991). 

The sliver of craton between the two bounding normal 

faults subsided forming a basin. As the craton extended 

and the basin down- warped, volcanic eruptions emitted 

thick basalt flows across the basin. Most of these 

eruptions were subaerial; however, a few pillow lava 

structures exposed on Isle Royale indicate some 

underwater eruption (MSA 2007). During quiescent 

periods between eruptions, erosion of the margins of the 

basin washed sediments into the basin atop the volcanic 

rocks. When volcanism proceeded again, basalt flows 

covered these sediments forming cyclic volcanicsedimentary layers in the basin. The rift basin 

accumulated an approximately 20 km (12 miles) thick 

sequence of volcanics and inter- flow sediments 

representing one of the deepest such assemblages on 

earth (Cannon et al. 1987). 

The weight of these sediments caused the basin to bow 

downwards into a synclinal structure. As the syncline 

subsided and more sediment and volcanic layers were 

deposited, their weight caused further subsidence 

(Edstrom 1994). This structure is referred to as the Lake 

Superior syncline or the Keweenawan basin (fig. 3). At 

Isle Royale, the entire rock sequence tilted southeast 

towards the axis of the basin and subsequent erosion has 

exposed the upturned units on end as individual layers 

(fig. 4). The dip of these units varies from less than 10° up 

to 50° (Huber 1973a). 

The Lake Superior syncline curves with the surface trace 

concave to the south following the shape of Lake 

Superior (Huber 1973a). Deformation along the Isle 

Royale and Keweenaw faults is responsible for 

steepening rock strata along the edges of the Lake 

Superior syncline, thus accentuating the synclinal 

structure of the basin (fig. 5) (Huber 1973a). This 

structure is ancient, with stratigraphic thickness of 

Precambrian age units increasing towards the synclinal 

axis. This indicates that this structure has been a defined 

a basin of deposition for a long time (Wolff and Huber 

1973; Dickas 1996). 
 NPS Geologic Resources Division 10 

About 30 million years after the continental rift formed, 

the region experienced compression by the northwest 

movement of the North American plate accompanying 

the Grenville orogenic event located several hundred 

kilometers southeast of Isle Royale. This event involved 

much of the continental crust in existence at that time. A 

northern landmass, Laurentia (present- day North 

America), collided with one or more southern 

landmasses and island arcs. This compression effectively 

stopped the rift. The Isle Royale fault, which runs 

beneath the surface of Lake Superior along the western 

edge of the island, was reactivated as a thrust fault during 

this compression. Its upthrown block is to the east of the 

fault’s map trace (Huber 1973a). The Keweenaw fault, 

which bisects the Keweenaw Peninsula, was also 

reactivated as a reverse fault. 

Glacial Features 

In contrast to the alpine glaciers that carved valleys and 

ridges in areas such as Glacier National Park, the glaciers 

at Isle Royale were massive, thick, ice sheets that covered 

millions of acres descending from northern Canada to a 

terminus south of Lake Superior (fig. 6). Pleistocene 

glaciation events are commonly divided into four major 

stages in North America named for the states where 

deposits of a particular period were first studied or 

where they are particularly well exposed. These are the 

Nebraskan, Kansas, Illinoian, and Wisconsinan. These 

major stages actually represent several advances and 

retreats – as many as 20 to 30 major glaciations during the 

Quaternary (John Anderton, written comm. 2007). 

Glacial ice covered Isle Royale during each major 

glaciation of the Pleistocene Epoch. Each event 

overprinted the earlier, leaving little trace. The last major 

event, part of the Wisconsin glaciation, occurred during 

the Marquette readvance, approximately 10,025 years ago 

(Huber 1973b; Woodruff et al. 2003). 

Glacial advance and retreat associated with major 

glaciations are not linear processes, but are rather 

incremental with smaller pulses of readvance and retreat. 

During the last major glacial advance at Isle Royale, the 

overriding glacial ice removed most of the pre- existing 

surficial debris. This last major glacial advance and 

retreat event left both depositional and erosional features 

on the landscape at Isle Royale (Huber 1973b). The glacial 

retreat event was relatively rapid, leaving a minor amount 

of glacial debris on the central and eastern portions of 

the island. Glacial till filled some valleys and smaller 

depressions, and glacial erratics (large rocks transported 

on floating ice) are scattered across the island. Till is 

abundant on the southwest end of the island and mantles 

most of the bedrock there. Deposits of glacial debris 

along the ice- margins of the glacier occur on Isle Royale 

(fig. 7) (Huber 1973b). 

Glaciers are powerful agents of erosion, capable of 

profoundly altering the landscape as they advance and 

retreat. Glaciers erode primarily by two distinct 

processes, plucking and abrasion (Dyson 1966). In 

plucking, the glacier actually quarries out distinct blocks 

of rock, incorporates these blocks within the ice and 

transports them as the glacier moves forward. On Isle 

Royale, the glaciers moved in a trend coincident with the 

prevailing geologic structure, accentuating the preexisting valley and ridge topography. This process carved 

deep ravines through fractured and altered bedrock, 

especially in the northwest end of the island. Glacial 

abrasion on Isle Royale is responsible for the rounding, 

striating, and polishing of the many exposed rock 

outcrops. Striations reveal that ice movement in the 

northern part of the island was northeast to southwest; 

and, across the southern part of the island ice movement 

was more east to west and oblique to the bedrock unit 

trends (Woodruff et al. 2003). As with any weathering 

process, weaker units typically erode away preferentially 

relative to stronger units. On Isle Royale, exposures of 

resistant units dominate and only a few weathered and 

heavily fractured outcrops persist (Huber 1973b). 

The ice sheet finally retreated from the Superior Basin 

between 10,700 to 8,900 years ago creating a thick 

sequence of glacial lake deposits that records, almost 

annually, differences in water levels of ancient Lake 

Agassiz and sedimentation rates (Breckenridge 2004). 

Lake Agassiz drained eastward into the Lake Superior 

basin through outlets in the Nipigon basin and Thunder 

Bay areas, bringing with it distinctive gray lacustrine clays 

that regionally mark the event (Woodruff et al. 2003). A 

series of lakes including Lake Minong and Lake 

Nipissing formed in the Lake Superior basin after the 

retreat of the ice terminus as described in the ancient 

shorelines part of the geologic issues section above. The 

constantly changing water levels of these lakes are 

recorded as wave- cut benches, cliffs, and beach deposits 

at Isle Royale. Surficial deposits associated with these 

lakes arose from reworked glacial debris including 

granitic rocks from the Canadian Shield and fossiliferous 

limestone and chert from the Hudson Bay basin (Huber 

1973b). Well- developed shoreline deposits indicate 

periods of relative stability. 

Isle Royale as a Natural Laboratory 

Preserved by its isolation and generally harsh climate, the 

environment at Isle Royale National Park allows 

researchers to all but remove human influence in their 

studies of the islands (Woodruff et al. 2003). Isle Royale 

is the single most studied area in the state of Michigan 

(Edstrom 1994). During the 1950s, an ecosystem model 

was developed allowing major parameters to be 

measured directly in the field. 

This model linked hydrology, topography, and geology 

with other ecosystem processes, thereby permitting 

quantification of biogeochemical cycles and other 

ecosystem processes sensitive to change. A network of 

sites for this research includes among other protected 

areas, Isle Royale National Park. The long- term goal of 

this research is to obtain increased understanding of 

ecosystem structure and function and the 

interrelationships among different facets of the 

environment. 
 ISRO Geologic Resource Evaluation Report 11

The primary focus of modern research is on the effects of 

global change, especially of atmospheric inputs and of 

climate. In order for this research to be quantifiable, the 

ecosystem must be studied as an integrated unit. This 

requires understanding both surficial and subsurface 

processes, linking these processes to geology and to 

hydrology. For instance, complex subsurface processes 

regulate the quality and quantity of nutrients and energy 

available to the entire ecosystem (Stottlemeyer 2002). 

Greenstones 

A greenstone is a common name for an assemblage of 

minerals in a metamorphic rock (typically 

metamorphosed basalt) that impart a green color to the 

rock unit. These minerals may include chlorite, 

actinolite, and/or epidote. Greenstones are prevalent on 

Isle Royale, forming the backbone of the entire island 

and many rock outcrops. They are the altered remnants 

of the Portage Lake Volcanics lava flows. 

These lava flows display a relatively limited number of 

textures based on the distribution, size, and percentage 

of minerals in the phenocrysts and in the groundmass (or 

matrix). Some features persisted throughout the change 

from lava to greenstone including the framework of 

plagioclase crystals in the groundmass that comprise 60-

70% of the total volume. They are randomly oriented, 

and interlocked creating a mesh- like effect (Rose 1995). 

Greenstone Ridge is the highest and longest ridge at Isle 

Royale. It is named for the distinct metabasalt flow that is 

up to 240 m (800 ft) thick and is one of several flows that 

underlie the island and reappear on the Keweenaw 

Peninsula. Over one billion years old, it is among the 

largest and thickest known lava flows on the earth. 

However, as discussed in the Chlorastrolite and other 

mineral specimens subsection above, the term 

greenstone has a double meaning unique to Isle Royale. 

The other greenstone is the Michigan state gemstone, 

chlorastrolite, found almost exclusively on the island as 

pebbles in beach and other shoreline deposits. 
 NPS Geologic Resources Division 12 

Figure 3. Regional distribution of rock types and geologic structures of the Lake Superior basin. Graphic is from Huber (1975).

Figure 4. Cross- sectional diagrammatic view of Isle Royale showing strata tilted towards a synclinal axis with ridges of the island 

defined by the tilted rock unit edges. Graphic adapted from Huber (1973a) by Trista L. Thornberry-Ehrlich (Colorado State University). 
 ISRO Geologic Resource Evaluation Report 13

Figure 5: Cross- sectional view of the Lake Superior basin showing basin bounding faults and strata tilted towards the Lake Superior 

synclinal axis. Diagram is not to scale. Graphic adapted from Huber (1973a) by Trista L. Thornberry-Ehrlich (Colorado State University). 

Figure 6. Diagrammatic view of a retreating glacier with scoured bedrock controlling in part the formation of a paleolake. Glacial 

deposits such as moraines, ice-margin deposits, and glacial till cover the landscape in front of the retreating glacier. Graphic is by 

Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 14 

Figure 7. Map showing maximum extent of glacial ice (dark purple line) during Wisconsin event and three later stages (lighter purplepink lines) during the overall glacial retreat in the Isle Royale area. Note dates on each line and purple arrows indicating direction of 

glacial movement. Graphic is by Trista L. Thornberry-Ehrlich (Colorado State University) from information by Huber (1973b) and John 

Anderton (written comm. 2007). 
 ISRO Geologic Resource Evaluation Report 15

Map Unit Properties 

This section identifies characteristics of map units that appear on the Geologic Resource 

Evaluation digital geologic map of Isle Royale National Park. The accompanying table is 

highly generalized and is provided for background purposes only. Ground- disturbing 

activities should not be permitted or denied on the basis of information in this table. 

More detailed map unit descriptions can be found in the help files that accompany the 

digital geologic map or by contacting the National Park Service Geologic Resources 

Division. 

The geologic bedrock units exposed at Isle Royale 

National Park fall into two groups: the Portage Lake 

Volcanics and the overlying Copper Harbor 

Conglomerate. Both of these groups are Precambrian in 

age in a period of time referred to in northern Michigan 

as the middle Keweenawan (Huber 1973a). The regional 

Keweenawan group of rocks includes the Precambrian 

and/or Cambrian Jacobsville Sandstone and Bayfield 

Group, the Precambrian Freda Sandstone and Nonesuch 

Shale, the Copper Harbor Conglomerate, Middle 

Keweenawan major intrusive rocks, a volcanic sequence 

(locally, the Portage Lake Volcanics), and a Lower 

Keweenawan sedimentary sequence. 

The Portage Lake Volcanic series is mostly comprised of 

numerous basaltic and andesitic lava flows. There are 

likely hundreds of individual flows containing 

porphyritic lavas, meaning large crystals in a finegrained matrix. Other flows contain ophitic textures 

characterized by crystals of pyroxene enclosing 

unoriented plagioclase laths, producing a mottled effect. 

During major intervolcanic episodes, interbeds of 

sandstone, conglomerate, and pyroclastic sediments 

were deposited (Huber 1973a). 

The Copper Harbor Conglomerate is a thick wedge of 

consolidated gravel and sand deposits that overly the 

Portage Lake Volcanics. These were originally deposited 

in alluvial fans and flood plains. The clastic materials in 

these conglomerates were largely derived from older 

Keweenawan volcanic rocks (pre- Portage Lake), though 

sandstone and siltstone fragments become more 

abundant on the eastern edge of the island near Siskiwit 

Bay (Huber 1973a). 

The transition between the two major formations at Isle 

Royale reflects a gradual cessation of volcanic activity 

(Wolff and Huber 1973). A major erosional unconformity 

separates the Copper Harbor Conglomerate from the 

overlying sediments. 

Paleozoic and all but the most recent Cenozoic rock 

units are missing from Isle Royale National Park. They 

eroded from the ancient tectonically stable craton (the 

Canadian Shield) in the vicinity of Isle Royale. The 

jumbled assortment of rock types, grain sizes, and lack of 

distinct bedding features characterizes the glacial till 

deposited on the landscape during the retreat of the last 

major ice sheets thousands of years ago. Some of these 

deposits were reworked as talus and beach deposits 

(Huber 1973a). Deposits of Quaternary alluvium line local 

river and stream valleys, bogs, wetlands, beaches, and 

lakes. These consist of unconsolidated clay, silt, sand, 

pebbles, organic material, and gravel. 

Geologic features and processes often occur in or can be 

restricted to a particular stratigraphic unit (group, 

formation, or member). This table ties together the 

geologic features and processes with the properties of 

the geologic units presented on the accompanying digital 

geologic map. 

Source data for the GRE digital geologic map are from: 

Huber, K. 1973a. Geologic map of Isle Royale National 

Park, Keweenaw County, Michigan. Scale 1:62,500.U.S. 

Geological Survey. Miscellaneous Geologic 

Investigations Report Map: I- 796. 

Huber, K. 1973b. Glacial and Postglacial Geologic History 

of Isle Royale National Park, Michigan. Scale 1:100,000. 

U.S. Department of the Interior and U.S. Geological 

Survey. Professional Paper 754- A. 

Huber, K. 1983. The Geologic History of Isle Royale 

National Park. U.S. Department of the Interior and 

U.S. Geological Survey. Formerly U.S Geological 

Survey Bulletin 1309. 

Wolff, R. and Huber K. 1973. The Copper Harbor 

Conglomerate (Middle Keweenawan) on Isle Royale, 

Michigan, and its Regional Implications. , Scale 1:62,500. 

U.S. Department of the Interior and U.S. Geological 

Survey. Professional Paper 754- B. 

The following pages present a tabular view of the 

stratigraphic column and an itemized list of features for 

each map unit. This sheet includes several properties 

specific to each unit present in the stratigraphic column 

including: age, map unit and symbol, description, 

resistance to erosion, suitability for development, 

hazards, cultural resources, mineral occurrence, habitat, 

recreational use potential, and geologic significance. 

 ISRO Geologic Resource Evaluation Report 17

Geologic History 

This section describes the rocks and unconsolidated deposits that appear on the digital 

geologic map of Isle Royale National Park, the environment in which those units were 

deposited, and the timing of geologic events that created the present landscape. 

Archean through Proterozoic 

The rock units at Isle Royale record two major sequences 

of events separated by over 850 million years of the 

earth’s history. The oldest rocks date back to the 

Proterozoic Era (fig. 8). These are the Keweenawan age 

sedimentary rocks interbedded with major tholeiitic 

basaltic lava flows with minor andesite and rhyolite flows 

within the Portage Lake Volcanics. These flows include 

the Scoville Point, Edwards Island, Middle Point, Long 

Island, Washington Island, Greenstone, Grace Island, 

Minong, Huginnin, Hill Point, and Amygdaloid Island 

flows (Huber 1975; Woodruff et al. 2003). Absolute ages 

are between 1,600 Ma and 900 Ma for the Middle 

Proterozoic lava flows. The youngest units at Isle Royale 

record the glacial activity during the Pleistocene Epoch. 

Glacial activity is largely responsible for the significant 

gap in the rock record, having removed much of the 

overlying units before depositing an unconsolidated 

mantle of material over portions of Isle Royale. 

Prior to the deposition of the Proterozoic rocks in the 

Lake Superior region, erosion had beveled the area into a 

peneplain. The underlying, basement rocks are among 

the oldest found on earth. These rocks do not crop out 

on Isle Royale, but appear both north and south of Lake 

Superior (Huber 1975). The region of northern 

Wisconsin, northeast Minnesota, and the Upper 

Peninsula of Michigan, forms part of the Canadian Shield 

and is the largest surface exposure of Archean and 

Proterozoic rocks (2.6 to 1.6 billion years old) in the 

United States (Vigil 2000). 

Deformation and metamorphism have obscured many of 

the original features as well as the complex history of the 

Archean basement rocks. Shallow seas covered part of 

the ancient peneplain in which a thick sequence of 

marine sedimentary rocks were deposited. These units 

included impure graywackes, sandstone, shale, dolomite, 

and iron- rich deposits (Huber 1975). Once deposited, 

they were deformed and metamorphosed into 

alternating belts of greenstone- granite and gneiss, and 

subsequently eroded (LaBerge 1994). During the Early 

Proterozoic, the Archean rocks, composing a large 

continental mass, fractured into smaller fragments 

(fig. 9a). 

The Middle Proterozoic rocks at Isle Royale, including 

the Portage Lake Volcanics and the overlying Copper 

Harbor Conglomerate, are part of the larger 

Keweenawan Supergroup. In fact, the period of time 

when these rocks were deposited is locally referred to as 

the Keweenawan (~1,100 to 1,000 Ma). 

The supergroup is divided into lower, middle, and upper 

parts. Below the Isle Royale units of the middle 

Keweenawan, are the Bessemer Formation (a basal 

sandstone and conglomerate), an unnamed volcanic 

sequence, and dikes and sills of gabbro including the 

Duluth Complex. Geologists interpret these sediments to 

have formed in a shallow- water, near- shore 

environment. At some point during the deposition of the 

unnamed volcanic sequence, the basin was likely filled 

and all later Keweenawan deposits are thought to be 

terrestrial (Huber 1975). 

The Portage Lake Volcanics are roughly the upper half of 

one of the largest deposits of lava ever recorded.These 

lavas erupted from fissures in a rift system that formed 

the long axis of an arc- shaped trough in what is now the 

center of Lake Superior (fig. 9b). This rift was extensive 

and probably extended southwest at least as far as 

Kansas, possibly to the Gulf of Mexico, and southeast 

into the Lower Michigan peninsula (Huber 1975). As the 

lava continued to erupt, followed intermittently by 

sedimentary deposition, the rift basin began to subside, 

both from the weight of the accumulation of deposits 

and from deflation of the magma chamber supporting 

the extrusion of the lava (LaBerge 1994). 

Basin- wide subsidence continued after the extrusion of 

lava ceased. Deposited over the basalt flows were the 

Copper Harbor Conglomerate and younger 

Keweenawan rocks (sandstones) forming a thick 

sedimentary rock cap (fig. 9c). The direction of sediment 

transport was generally northeast to southwest and the 

sediments were derived from predominantly felsic 

cobbles and pebbles of the earlier pre- Portage Lake 

volcanics (Wolff and Huber 1973). 

Near the end of the Keweenawan, hot fluids from lower 

volcanic units likely percolated upwards carrying 

dissolved minerals. These fluids under pressure altered 

the overlying rocks and precipitated some interesting 

secondary minerals in cavities, joints, and fissures at Isle 

Royale including native copper, chlorastrolite 

(pumpellyite), prehnite, and agate (Huber 1975 and 1983). 

Because the lava extruded primarily subaerially, the flows 

were highly vesicular, making them ideal for secondary 

mineralization (Woodruff et al. 2003). 

The downwarping of the basin altered stream patterns 

resulting in an inward flow of streams and rivers carrying 

mostly pebbles and cobbles. The basin thus formed a 

bowl- shaped syncline, called the Lake Superior syncline, 

with upturned beds exposed on the Keweenaw Peninsula 

to the southeast (bedding dips northwest) and on Isle 
 NPS Geologic Resources Division 18 

Royale to the northwest (bedding dipping southeast) 

(figs. 4- 5). The axis of the syncline trends between Isle 

Royale and the Keweenaw Peninsula along the length of 

Lake Superior. 

Late Proterozoic through Early Cambrian 

The syncline initially formed by subsidence rather than 

by tectonic compression. During the Late Proterozoic to 

Early Cambrian (~1,080 to 1,040 Ma) however, a local 

compressional event reactivated the reverse Isle Royale 

and Keweenaw faults (previously rift- bounding faults), 

and steepened the strata near the basin margins, 

accentuating the synclinal structure (fig. 10a) (Huber 

1975; Woodruff et al. 2003). 

The rocks on Isle Royale reflect the movement of the Isle 

Royale thrust fault just west of the island. Strata dip from 

between 10° and 50° to the southeast, with steeper dips 

on the north side. The Isle Royale fault pushed up the 

volcanics, sedimentary interbeds, and conglomerates. 

When the less resistant sedimentary rocks preferentially 

eroded into valleys, the volcanics (now metamorphosed 

to greenstones) formed the elongate ridges along the 

length of the island (Huber 1973). This fault movement 

also established the present geologic structure of the Isle 

Royale, thrusting the resistant volcanic rocks in an 

elongate ridge surrounded by less resistant sedimentary 

rocks that eroded away (Huber 1975). 

The Missing Record 

Erosion beveled most of the craton to a relatively flat 

surface prior to any Paleozoic, Mesozoic, and Cenozoic 

age deposition (Huber 1975). As a relatively stable 

landform, the craton has been exposed to millions of 

years of continuous erosion. The rocks of the Paleozoic, 

Mesozoic, and Cenozoic Eras, beginning with the 

sandstones of the upper part of the Keweenaw 

Supergroup in the Late Proterozoic to early Cambrian, 

were either never deposited or were eroded away from 

the landscape at Isle Royale. The next unit in the rock 

record is glacial till from the Pleistocene Epoch overlying 

the Late Proterozoic Copper Harbor Conglomerate 

leaving a gap (hiatus) of 570 million years as a mystery at 

Isle Royale. 

Surrounding areas give some clues as to the sequence of 

events for the 570 million years missing from the rock 

record at Isle Royale. The Lake Superior region, part of 

the Canadian Shield or craton, has been relatively stable 

geologically since the Precambrian (Huber 1975). In the 

Late Cambrian, deposition of a thick sequence of 

sediments extending to at least the Keweenaw Peninsula 

in the failed continental rift resulted in subsidence, 

which developed into the Michigan Basin, presently 

southeast of Isle Royale. 

The Michigan Basin is a typical interior cratonic basin 

bounded by a series of structural highs including, 

counterclockwise from the west, the Wisconsin Arch, the 

Kankakee Arch, the Findlay Arch of Ohio, Algonquin 

Arch of Ontario, and the Canadian Shield (Dolton 1995). 

The Michigan Basin has been a depositional center since 

the beginning of the Paleozoic Era. Sedimentary strata 

include various types of marine deposits such as 

dolomites, limestones and mixed calcareous shales 

interlayered with nearshore deposits such as sandstones, 

and reefal limestones as well as subaerial deposits 

including red beds and evaporites (Dolton 1995). Several 

periods of non- deposition and erosion punctuate the 

thick deposits of the basin, notably during the Lower 

Ordovician, Lower Silurian, Lower Devonian, late 

Mississippian to early Pennsylvanian, and the Permian to 

Triassic (Dolton 1995). The depositional sequence in the 

Michigan Basin attests to long- term tectonic stability of 

the area. 

Quaternary Period 

Prior to glaciation, broad river valleys occupied the failed 

continental rift valley and Michigan structural basin. 

Deformation associated with regional faults may have 

locally concentrated erosion by rendering the rocks 

weaker than surrounding areas. These valleys probably 

also served to exert a major influence on the direction of 

ice movement, focusing the successive lobes of ice 

through the topographic lows, and subsequently 

lowering them even further by glacial erosion 

In the Pleistocene Epoch, glacial advance and retreat 

significantly modified the topography at Isle Royale (fig. 

10b). Evidence indicates that glaciers overrode the island 

during all major Pleistocene advances and the final 

advance and retreat during the Wisconsin event, called 

the Marquette readvance (culminating at about 10,025 

years ago) obliterated nearly all evidence of the previous 

episodes on the island. The ice moved generally parallel 

with the prevailing bedding direction, so pre- existing 

landforms were accentuated. They scoured out the softer 

sedimentary rock layers leaving a valley- and- ridge 

topography, and also left many glacial features such as 

striations, ice margin deposits such as drumlins, 

moraines, glacial erratics, and layers of till (concentrated 

on the southwestern part of the island) (Huber 1973b). 

When the ice finally retreated from the axis of the basin, 

it left behind a large trough with Isle Royale remaining a 

ridge of more resistant rock (fig. 10c) (Huber 1975; 

Woodruff et al. 2003). 

Glacial advance and retreat controlled many of the 

paleolakes of the Lake Superior region. During warm 

periods, as the ice melted and the glaciers retreated, the 

corresponding lake levels rose from the addition of 

meltwater. Retreating glaciers also uncovered lower 

outlets for the meltwater that would have the effect of 

lowering lake levels. Upon the final glacial retreat, 

isostatic rebound of the Great Lakes region from 

removal of the enormous weight of the glacial ice 

resulted in uplift and a relative lowering of water level. 

The results of these fluctuations are several tilted 

paleoshorelines that correlate with certain paleolakes 

named from oldest to youngest: Lake Duluth, Lakes 

Washburn and Beaver Bay (~9,800 – 9,700 years before 

present (B.P.)), Lake Minong (~9,500 B.P), Lake 

Houghton (~8,000 B.P.), Lake Nipissing (~5,000 – 4,700 
 ISRO Geologic Resource Evaluation Report 19

B.P) and modern Lake Superior (Huber 1975; McRae et 

al. 1998). The first glacial lake, the highest at 331 m (1,085 

ft) above sea level, was Lake Duluth; this lake is inferred 

from features around the Lake Superior basin and is not 

well preserved on Isle Royale (Huber 1973b). Similarly, 

Lake Houghton paleoshorelines features are submerged 

in the Isle Royale area (John Anderton, written comm. 

2007). 

Several of these different paleolakes left shoreline 

features that are remarkably preserved on Isle Royale, 

especially Lake Minong and Lake Nipissing (Huber 

1975). Examples of these features include a 2 km (~1 mile) 

long barrier beach near Feldtmann Ridge, some 240 m 

(800 ft) above the present lake level, probably from Lake 

Minong. A wave- cut cliff south of Washington Harbor 

sits at 247 m (810 ft) elevation. Other paleoshoreline 

landforms include Suzy’s Cave, Monument Rock, and 

Amygdaloidal Arch (John Anderton, written comm. 

2007). 

The paleoshoreline features possibly associated with 

Lake Beaver Bay persist around 90 m (300 ft) above the 

present lake level including a cliff of conglomerate at the 

west end of Feldtmann Ridge (Huber 1973b). 

Paleoshorelines appear best developed along the 

southwest end of the island where unconsolidated glacial 

till and other reworked glacial moraine and outwash 

deposits are readily available for the development of 

beaches, terraces, and wave- cut benches (McRae et al. 

1998). 

Since the last glacial retreat, weathering and erosion have 

been active on Isle Royale. Soils developed on the island 

promoting vegetation including southern boreal forests 

and northern hardwoods (Edstrom 1994; Woodruff et al. 

2003). Sediment is filling glacial ponds and inland lakes 

(42 of which are named) are shrinking (Edstrom 1994). 
 NPS Geologic Resources Division 20 

Figure 8: Geologic time scale; adapted from the U.S. Geological Survey (http://pubs.usgs.gov/fs/2007/3015/). Red lines indicate major 

unconformities between eras. Included are major events in life history and tectonic events occurring on the North American continent. 

Absolute ages shown are in millions of years. 
 ISRO Geologic Resource Evaluation Report 21

Figure 9a-c. Evolution of the landscape in the Isle Royale area from the Precambrian rifting and volcanism through the early Paleozoic 

deposition of post-volcanic sediments. Graphic is by Trista L. Thornberry-Ehrlich (Colorado State University). 
 NPS Geologic Resources Division 22 

Figure 10a-c: Evolution of the landscape in the Isle Royale area continued from the Late Precambrian – Early Cambrian thrust faulting 

through the Pleistocene ice ages to the present day landscape. Graphic is by Trista L. Thornberry-Ehrlich (Colorado State University). 
 ISRO Geologic Resource Evaluation Report 23

Glossary 

This glossary contains brief definitions of technical geologic terms used in this report. Not all 

geologic terms used are referenced. For more detailed definitions or to find terms not listed 

here please visit: http://wrgis.wr.usgs.gov/docs/parks/misc/glossarya.html.

alluvial fan. A fan- shaped deposit of sediment that 

accumulates where a high gradient stream flows out of 

a mountain front into an area of lesser gradient such as 

a valley. 

alluvium. Stream- deposited sediment that is generally 

rounded, sorted, and stratified. 

alpine glacier. A glacier occurring in a mountainous 

region; also called a valley glacier. 

amygdule. A gas cavity or vesicle in an igneous rock 

which is filled with such secondary minerals as 

zeolites, calcite, quartz, or chalcedony. 

aquifer. Rock or sediment that are sufficiently porous, 

permeable, and saturated to be useful as a source of 

water. 

ash (volcanic). Fine pyroclastic material ejected from a 

volcano (also see tuff). 

axis (fold). A straight line approximation that when 

moved parallel to itself generates the shape of a fold 

(see and use hingeline) 

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie the rocks of 

interest. 

basin (structural). A doubly- plunging syncline in which 

rocks dip inward from all sides (also see dome). 

basin (sedimentary). Any depression, from continental 

to local scales, into which sediments are deposited. 

batholith. A massive, discordant pluton, greater than 100 

km2, (39.6 mi2) often formed from multiple intrusions. 

beach. A gently sloping shoreline covered with sediment, 

often formed by action of waves and tides. 

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above. 

bedrock geology. The geology of underlying solid rock 

as it would appear with the sediment, soil, and 

vegetative cover stripped away. 

block (fault). A crustal unit bounded by faults, either 

completely or in part. 

chemical weathering. The dissolution or chemical 

breakdown of minerals at Earth’s surface via reaction 

with water, air, or dissolved substances. 

clastic. Rock or sediment made of fragments or preexisting rocks. 

clay. Clay minerals or sedimentary fragments the size of 

clay minerals (<2 cm). 

conglomerate. A coarse- grained sedimentary rock with 

clasts larger than 2 mm in a fine- grained matrix. 

continental crust. The type of crustal rocks underlying 

the continents and continental shelves; having a 

thickness of 25- 60 km (16- 37 mi) and a density of 

approximately 2.7 grams per cubic centimeter. 

continental shield. A continental block of Earth’s crust 

that has remained relatively stable over a long period 

of time and has undergone only gentle warping 

compared to the intense deformation of bordering 

crust 

craton. The relatively old and geologically stable interior 

of a continent (also see continental shield). 

cross-bedding. Uniform to highly- varied sets of inclined 

sedimentary beds deposited by wind or water that 

indicate distinctive flow conditions. 

crust. The outermost compositional shell of Earth, 10- 40 

km (6- 25 mi) thick, consisting predominantly of 

relatively low- density silicate minerals (also see 

oceanic crust and continental crust). 

dike. A tabular igneous intrusion that cuts across or is at 

an angle to the orientation of adjacent rocks. 

dip. The angle between a structural surface and a 

horizontal reference plane measured normal to their 

line of intersection. 

erratic. A rock fragment carried by glacial ice to a 

location distant from its original outcrop. 

extrusive. Of or pertaining to the eruption of igneous 

material onto the surface of Earth. 

fault. A subplanar break in rock along which relative 

movement occurs between the two sides. 

formation. Fundamental rock- stratigraphic unit that is 

mappable and lithologically distinct from adjoining 

strata and has definable upper and lower contacts. 

graben. A down- dropped structural block bounded by 

steeply- dipping, normal faults (also see horst). 

horst. An uplifted structural block bounded by highangle normal faults. 

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes or rocks: 

igneous, metamorphic, and sedimentary. 

intrusion. A body of igneous rock that invades older 

rock. The invading rock may be a plastic solid or 

magma that pushes its way into the older rock. 

joint. A semi- planar break in rock without relative 

movement of rocks on either side of the fracture 

surface. 

lacustrine. Pertaining to, produced by, or inhabiting a 

lake or lakes. 

lava. Magma that has been extruded out onto Earth’s 

surface, both molten and solidified. 

magma. Molten rock generated within Earth that is the 

parent of igneous rocks. 

mechanical weathering. The physical breakup of rocks 

without change in composition (syn: physical 

weathering). 

metamorphism. Literally, “change in form”. 

Metamorphism occurs in rocks with mineral 
 NPS Geologic Resources Division 24 

alteration, genesis, and/or recrystallization from 

increased heat and pressure. 

normal fault. A dip- slip fault in which the hanging wall 

moves down relative to the footwall. 

ophitic. The texture of igneous rocks having plagioclase 

laths surrounded by pyroxene grains. 

orogeny. The tectonic process of mountain building 

usually accompanied by folding, faulting, thrusting, 

igneous activity, and metamorphism. 

outwash. Glacial sediment transported and deposited by 

meltwater streams. 

pebble. Generally, small, rounded, rock particles from 4 

to 64 mm in diameter. 

pegmatite. An exceptionally coarse- grained igneous 

rock with interlocking crystals, often containing rare 

minerals, derived from the most hydrous portion of a 

magma. 

peneplain. A geomorphic term for a broad area of low 

topographic relief resulting from long- term, extensive 

erosion. 

pillow lavas. Lavas displaying pillow structures 

(discontinuous bun- shaped masses) formed by the 

extrusion of basalt or andesite under water. 

pyroclastic. Particles of rock material formed by volcanic 

explosion or ejection from a volcanic vent; size ranges 

from ash to large boulders. 

rift. A long, continental depression formed by grabens 

along the crest of an oceanic spreading ridge or in a 

continental rift zone where extensional forces are 

pulling the crust apart.. 

sandstone. Clastic sedimentary rock of predominantly 

sand- sized grains. 

sediment. An eroded and deposited, unconsolidated 

accumulation of lithic and mineral fragments. 

sedimentary rock. A consolidated and lithified rock 

consisting of detrital and/or chemical sediment(s). 

shale. A clastic sedimentary rock made of clay- sized 

particles that exhibit parallel splitting properties. 

sill. A tabular, igneous intrusion that is concordant with 

the country rock. 

silt. Clastic sedimentary material intermediate in size 

between fine- grained sand and coarse clay (1/256- 1/16 

mm). 

siltstone. A variable- lithified sedimentary rock with siltsized grains. 

slump. A generally large, coherent mass movement with 

a concave- up failure surface and subsequent 

backward rotation relative to the slope. 

strata. Tabular or sheetlike masses or distinct layers (e.g., 

of rock). 

striations. Scratches or lines that are usually parallel 

formed by the scraping action of glaciers, faults or 

streams. 

strike. The compass direction of the line of intersection 

that an inclined surface makes with a horizontal plane. 

strike-slip fault. A fault with measurable offset where the 

relative movement is parallel to the strike of the fault. 

suture. The linear zone where two continental 

landmasses become joined due to obduction. 

tectonic. Relating to large- scale movement and 

deformation of Earth’s crust. 

tholeiite. A basalt characterized by the presence of 

orthopyroxene and/or pigeonite in addition to 

clinopyroxene and calcic plagioclase. Olivine may be 

present. 

thrust fault. A contractional, dip- slip fault with a 

shallowly dipping fault surface (<45o) where the 

hanging wall moves up and over relative to the 

footwall. 

trace (fault). The exposed intersection of a fault with 

Earth’s surface. 

transgression. Landward migration of the sea due to a 

relative rise in sea level. 

trend. The direction or azimuth of elongation or a linear 

geological feature. 

tuff. Generally fine- grained, igneous rock formed of 

consolidated volcanic ash. 

unconformity. A surface within sedimentary strata that 

marks a prolonged period of nondeposition or 

erosion. 

vent. An opening at the surface of Earth where volcanic 

materials emerge. 

weathering. The set of physical, chemical, and biological 

processes by which rock is broken down in place. 
 ISRO Geologic Resource Evaluation Report 25

References 

This section lists references cited in this report as well as a general bibliography that may 

be of use to resource managers. A more complete geologic bibliography is available from 

the National Park Service Geologic Resources Division. 

Breckenridge, Andy J. 2004. The Lake Superior varves; 

records of ice margin dynamics and eastern Lake 

Agassiz overflow, ca. 10,600 to 9,040 cal yBP (9.5- 8.1 

(super 14) C ka). Abstracts with Programs - Geological 

Society of America 36 (5): 67- 68. 

Cannon, W. F.; J. C. Behrendt, D. Hutchinson, M. Lee, 

W. F. Agena, A. Green, Bernd Milkereit, C. Spencer, 

Morel- a- l., Patrick Huissier. 1987. Mega half- grabens 

of the Mid- continent rift system. Eos, Transactions, 

American Geophysical Union 68 (44): 1356. 

Cannon, William F. and Laurel G. Woodruff. 2000. Some 

factors that control the distribution of mercury in the 

near- surface environment of Isle Royale National 

Park, Michigan; Earth, wind, and fire. Proceedings and 

Abstracts - Institute on Lake Superior Geology 46 (1): 8-

9. 

Cannon, William F.; L. G. Woodruff. 1999. Mercury 

distribution in bedrock, native copper ore, and soils; 

Isle Royale National Park, Michigan. Proceedings and 

Abstracts - Institute on Lake Superior Geology 45 (1): 9-

10. 

Coakley, B. J., D. J. Reynolds, H. F. Wang. 1992. 

Structural evolution of Lake Superior; I, Western 

basin. Abstracts with Programs - Geological Society of 

America 24 (7): 328. 

Dolton, G.L. 1995. Michigan Basin Province (063). 

http://certmapper.cr.usgs.gov/data/noga95/prov63/tex

t/prov63.pdf (accessed August 15, 2007). 

Dickas, Albert B. 1996. New structural and stratigraphic 

interpretation of Lake Superior basin from 

hydrocarbon exploration geophysics and geology. 

AAPG Bulletin 80 (9): 1522. 

Dyson, J.L. 1966. Glaciers and glaciation in Glacier 

National Park. Glacier Natural History Association in 

cooperation with the National Park Service. 

Edstrom, Nancy J. 1994. Island wilderness; Isle Royale 

National Park. In Exploration and mapping of the 

national parks, ed. Johnson, Jenny Marie, Occasional 

Paper - American Library Association, Map & 

Geography Round Table 4: 218- 250. 

Halls, H. C. and G. F. West. 1971. The Isle Royale fault. 

Proceedings and Abstracts - Institute on Lake Superior 

Geology 17: 25- 27. 

Huber, N. K. 1971. The Keweenawan geology of Isle 

Royale, Michigan. Open- File Report - U. S. Geological 

Survey, OF 71- 0154: 9 p. 

Huber, N. King. 1973a. Geologic map of Isle Royale 

National Park, Keweenaw County, Michigan: U.S. 

Geological Survey Miscellaneous Geologic Investigations

Map I- 796, scale 1:62,500. 

Huber, N. King. 1973b. Glacial and Postglacial Geologic 

History of Isle Royale National Park, Michigan: U.S. 

Department of the Interior and U.S. Geological Survey

Professional Paper 754- A, scale 1:100,000. 

Huber, N. King. 1973. Glacial and postglacial geologic 

history of Isle Royale National Park, Michigan. U. S. 

Geological Survey Professional Paper, P 0754- A: A1-

A15. 

Huber, N. K. 1975. The geologic story of Isle Royale 

National Park. U. S. Geological Survey Bulletin, B1309: 

66 p. 

Huber, N. King. 1983. Pink prehnite from Michigan, a 

mineral much confused. Rocks and Minerals 58 (3): 

132- 134. 

Isle Royale National Park. 2005. Wilderness and 

Backcountry Management Plan and Environmental 

Impact Statement. U.S. Department of the Interior, 

National Park Service: 287 p. 

LaBerge, G.L. 1994. Geology of the Lake Superior Region. 

Oshkosh, WI: Penokean Press. 

Lowell, T.V. 1999. Age Verification of the Lake Gribben 

Forest Bed and the Younger Dryas advance of the 

Laurentide ice sheet. Canadian Journal of Earth 

Science 36 (3): 383- 393. 

McRae, Michele E.; W. F. Cannon, L.G. Woodruff. 1998. 

Post- glacial shorelines of Isle Royale; where are they 

now? Proceedings and Abstracts - Institute on Lake 

Superior Geology 44 (1): 86- 87. 

Mineralogical Society of America. 2007. Virtual Field 

Trip to the Keweenaw Peninsula, Michigan – Structure. 

http://www.minsocam.org/MSA/collectors_corner/vft/m

i2a.htm (accessed August 14, 2007) 

NPS. 2005. Wilderness and Backcountry Management 

Plan and Environmental Impact Statement. Isle Royale 

National Park, National Park Service, Department of 

the Interior: 287p. 
 NPS Geologic Resources Division 26 

Nicholson, S. W.; James B. Paces, W. F. Cannon. 1988. 

Development of the Mid- continent Rift in the Lake 

Superior region; petrologic and geochemical 

constraints for a tectonic model. Proceedings and 

Abstracts - Institute on Lake Superior Geology 34 (1): 

76- 78. 

Rose, William I. [leader]. 1995. Field trip 1; Volcanic 

geology of Isle Royale, Michigan. In Field trip 

guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior region, ed. Miller, 

James D., Jr., Guidebook Series - Minnesota 

Geological Survey 20: 23- 27 

Sexton, John; and Harvey Henson. 1991. Lake Superior 

bedrock topography and rift structures. Abstracts with 

Programs - Geological Society of America 23 (5): 138. 

Stottlemyer, Robert. 2002. The effects of global change 

on watershed ecosystem biogeochemistry. Abstracts 

with Programs - Geological Society of America 34 (3): 5. 

Thurman, E. M. and Aaron E. Cromwell. 2000. 

Atmospheric transport, deposition, and fate of triazine 

herbicides and their metabolites in pristine areas at Isle 

Royale National Park. Environmental Science & 

Technology, ES & T 34 (15): 3079- 3085. 

Vigil, J.F., Pike, R.J., and Howell, D.G. 2000. A tapestry of 

time and terrain. U. S. Geological Survey Pamphlet to 

accompany Geologic Investigations Series: I- 2720. 

Wolff, Roger G., and King N. Huber. 1973. The Copper 

Harbor Conglomerate (Middle Keweenawan) on Isle 

Royale, Michigan, and its Regional Implications. U.S. 

Department of the Interior and U.S. Geological Survey 

Professional Paper 754- B, scale 1:62,500. 

Woodruff, Laurel G., William F. Cannon , Connie L. 

Dicken, James P. Bennett, Suzanne W. Nicolson. 2003. 

Bedrock soil and lichen geochemistry from Isle Royale 

National Park, Michigan. Open- File Report - U. S. 

Geological Survey, OF 03- 0276 : 17 p. 
 ISRO Geologic Resource Evaluation Report 27

Appendix A: Geologic Map Graphic 

The following page is a preview or snapshot of the geologic map for Isle Royale National 

Park. For a poster- size PDF of this map or for digital geologic map data, please see the 

included CD or visit the Geologic Resource Evaluation publications Web page 

(http://www.nature.nps.gov/geology/inventory/gre_publications). 

 ISRO Geologic Resource Evaluation Report 29

Appendix B: Scoping Summary 

The following excerpts are from the GRE scoping summary for Isle Royale National 

Park. The scoping meeting was on June 16, 2004; therefore, the contact information and 

Web addresses referred to in this appendix may be outdated. Please contact the Geologic 

Resources Division for current information. 

Executive Summary 

A Geologic Resource Evaluation scoping meeting for Isle 

Royale National Park was held at park headquarters on 

Mott Island June 16, 2004. The scoping meeting 

participants identified the following as the most 

significant geologic resources management issues. 

1. Park wants to know the submerged geomorphology 

using lidar and other appropriate techniques. 

2. Identify ancient shorelines in an effort to study early 

history. 

3. Identify old mining sites 

4. Park would like a new source of drinking water, 

preferably from a water well. 

Introduction 

The purpose of the meeting was to discuss the status of 

geologic mapping in the park, the associated 

bibliography, and the geologic issues in the park. The 

products to be derived from the scoping meeting are: (1) 

Digitized geologic maps covering ISRO; (2) An updated 

and verified bibliography; (3) A scoping summary (this 

report); and (4) A Geologic Resources Evaluation Report 

which brings together all of these products. 

There are 19 quads of interest covering Isle Royale 

National Park. From northeast to southwest they are: 

Passage Island OE (“Over Edge”) North, Passage Island, 

Rock Harbor Lodge, Belle Harbor, Mott Island, 

McCargoe Cove, Lake Richie, Lake Richie OE South, 

Todd Harbor, Malone Bay, Little Todd Harbor, Point 

Houghton, Point Houghton OE South, Sugar Mountain, 

Feldtmann Ridge, Windigo, Feldtmann Lake, Windigo 

OE West, and Feldtmann Lake OE West. Of these, 

Passage Island OE N, Passage Island, Windigo OE W and 

Feldtmann Lake OE W are entirely off- shore. 

Four known geologic maps cover ISRO, three in 1973 by 

N. King Huber: map I- 796, scale 1:62,500 (Huber 1973), 

Professional Paper 754- A, scale 1:120,000 (Huber 1973), 

and Professional Paper 754- B, scale 1:62,500 (Wolff and 

Huber 1973). Open- File Report OF- 03- 276, scale 

1:100,000, is a map of the bedrock, soil and lichen 

geochemistry (Woodruff, et al. 2003). The NPS Geologic 

Resources Division will digitize the Huber map of glacial 

deposits (Professional Paper 754- A) as well as the I- 796, 

for basic geologic coverage. By using the two maps 

together, the locations of the Copper Harbor 

Conglomerate can be plotted more accurately. A third 

order soil survey was done in the 1980s and published in 

June 1991. At present ISRO contracts with Michigan Tech 

in Houghton to produce all of their data layers for their 

park. There is also a need to look at available bathymetric 

data. 

Significant Geologic Resource Management Issues at 

Isle Royale National Park 

Identify and study submerged geomorphology 

About two- thirds of the park is below lake level. 

Therefore, it is important to understand the bathymetry 

and geology of the submerged areas. There are critical 

fish habitats that need study. USGS maps extend to only 

15 feet depth offshore. Many shoals and other fish 

habitats are not mapped. The method of choice is lidar, 

which is being used by the Natural Resources 

Conservation Service (NRCS) for terrestrial studies. lidar 

can measure through the leaf cover, but the best time for 

these studies would probably be after the leaves drop but 

before the first snow – probably September. 

Lidar for benthic studies use a different sensor than for 

soils. Two lasers are used, one infrared (IR) for the 

surface and one blue- green for the bottom. Cost is 

estimated to be about $44,000. The Army Corps of 

Engineers (ACE) in the ENG Shoals Program uses 

2mX2m, the highest resolution, down to 150 feet. The 

cost for this resolution is estimated to be $630,000. For 

4mX4m resolution, the cost is estimated at $225,000. The 

planning, mobilization, and demobilization alone would 

cost about $65,000. It may be more realistic to survey 

only the trout spawning areas rather than the whole 

coast. 

Identify ancient shorelines 

Identifying and mapping ancient shorelines can not only 

provide the history of the relative rise and fall of the lake 

level in response to glaciation, but also pinpoint the 

location of settlements and human artifacts. Some of this 

mapping was done by Huber, but much more remains to 

be done. Huber mapped the paleo- shorelines of glacial 

Lake Minong and Nipissing, published in USGS 

Professional Paper 754- A, but only in the southwest part 

of the park. These areas have a high potential for having 

archeological sites. Furthermore, Huber apparently was 

not aware at the time of the “Marquette advance.” Soils 

mapping by the NRCS could complement the surficial 

map by Huber. The NRCS is prepared to begin the 

process by late May 2005. John Anderton, Associate 

Professor of Geography at Northern Michigan 

University would like to accompany the soil survey either 

in Fall 2005 or the following year. 

The oldest known archeological sites are about 4,000 to 

5,000 years old. There could be older sites (8,000 to 
 NPS Geologic Resources Division 30 

9,000 years old) along some Minong shorelines. Some 

have been found at Thunder Bay, Ontario. These sites 

could be under water now. Stumps have been found at 

Grand Island at over 200 feet depth that are believed to 

be 8,000 years old. 

Identify old mining sites 

Historic mining, mainly for copper, occurred at three 

periods in time: 1840s- 1850s, 1870s, and 1890s. There is 

also evidence that there was some prehistoric mining by 

Native Americans. Unlike the Keweenaw Peninsula and 

other areas of the Great Lakes, Isle Royale did not have 

abundant deposits of copper. Native copper is widely 

distributed but there are no large lode deposits (Huber 

1983). A survey of old mining sites was done in the late 

1980s, mostly along trails and near campgrounds, but 

about 80% of the park is unsurveyed. There is also a need 

for bat surveys as well. 

Two mines of note are the Island Mine and the Minong 

Mine, both of which were financial failures. The Island 

Mine is near the west end of Siskiwit Bay. Native copper 

occurs in the matrix of a conglomerate. At the Minong 

Mine copper occurs as nodules and irregular masses in 

fractures in altered rock. About 250 tons of copper was 

produced from the Minong Mine (Huber 1983). 

Water quality and supply 

Currently, all drinking water comes from Lake Superior. 

It is expensive to pump, clean, and purify the water. 

There are some mineral springs that animals (e.g. moose) 

use, but most are too high in mineral content. The park 

would like to put down a water well, but the 

characteristics of the groundwater aquifer system are not 

well known. The water table is high and there may be 

some contamination from human wastes as well as from 

petroleum spills, especially at Rock Harbor, Mott Island, 

Amygdaloidal Island and Windigo. There may be 

residual PCBs, mercury, and other toxins in the surface 

and groundwater. 

Other issues 

• Species Isolation: When was the island isolated from 

the mainland? When Lake Superior freezes over, there 

is opportunity for some animals to migrate to the 

island. Once the ice thaws, the animals are isolated. 

Why are there plants native to the Pacific Northwest 

on the island? The island used to have caribou and 

lynx, but now there are more moose and wolves. 

Populations of some species periodically collapse. 

• State Gem: Isle Royale is known as the home of the 

state gem chlorastrolite, also known as “Isle Royale 

greenstone.” However, this is not greenstone in the 

petrologic since, but actually a variety of the mineral 

pumpellyite [Ca2

MgAl2(SiO4)(Si2O7)(OH)2·H2O]. The 

mineral was discovered on Isle Royale by C.T. Jackson 

and J.D. Whitney in 1847. It was later identified as 

pumpellyite from a similar mineral first described on 

the Keweenaw Peninsula in 1825 by Raphael Pumpelly 

(Huber 1983). The chlorastrolite variety on Isle Royale 

has a characteristic mosaic or mottled pattern know as 

turtleback. 

Since this variety is thought to occur only on Isle 

Royale, any specimens found elsewhere probably were 

removed illegally from the island. As visitation to the 

island increases, specimen theft may become a 

problem. 

• Surface Water Issues: There has been no watershed 

mapping in ISRO. There is a need for stream profile 

data as well. Human waste disposal in shallow privies 

has the potential to contaminate both surface and 

groundwater. There have been petroleum spills as well 

as lead contamination (on Passage Island).There is a 

need to identify and map the near surface fracture 

system to help understand surface and groundwater 

flow regimes. 

• Wetlands: There has been monitoring of aquatic 

vegetation in wetlands and no major changes in 

wetland extent have been observed. No coring of 

wetlands has been done, even though this could 

provide a wealth of information on the sediment, 

climate, cultural, and fire history of the island. How 

will drawing groundwater affect the springs and 

wetlands?
 ISRO Geologic Resource Evaluation Report 31

Scoping meeting participants 

• Sid Covington, Geologist, NPS, Geologic Resources 

Division 

• Anne Poole, Geologist, NPS, Geologic Resources 

Division 

• Phyllis Green, Superintendent, NPS, Isle Royale 

National Park 

• Jean Battle, Chief, Natural Resources Mgt., NPS, Isle 

Royale National Park 

• Mark Romanski, Biological Technician, NPS, Isle 

Royale National Park 

• John Anderton, Assoc. Professor of Geography, 

Northern Michigan University 

On ferry: 

• Liz Valencia, Chief, Cultural Resources Mgt., NPS, Isle 

Royale National Park 

Via conference call: 

• Pete Biggam, Soil Scientist, NPS, Natural Res. 

Information Div. 

• Ulf Galvert, GIS Specialist, NPS, Great Lakes Network 

• Larry Carey, Soil Scientist, USDA, NRCS 

References 

Huber, N. King, 1983, The Geologic Story of Isle Royale 

National Park, U.S. Geological Survey Bulletin 1309 

(Rev. by Isle Royale Natural History Association), 66p. 

Huber, N. King, 1973a, Glacial and postglacial geologic 

history of Isle Royale National Park, Michigan, U.S. 

Geological Survey Professional Paper 754- A, 15p. 

Huber, N. King, 1973b, The Portage Lake Volcanics 

(middle Keweenawan) on Isle Royale, Michigan, U.S. 

Geological Survey Professional Paper 754- C. 

Huber, N. King, 1973c, Geologic map of Isle Royale 

National Park, Keweenaw County, Michigan, 

U.S.Geological Survey Miscellaneous Geological 

Investigations Series Map I- 796, scale 1:62,500. 

LaBerge, Gene L., 1994, Geology of the Lake Superior 

Region, Penokean Press, Oshkosh, WI, 313p. 

Vigil, José F., Pike, Richard J., and Howell, David G., 

2000, A tapestry of time and terrain, U. S. Geological 

Survey Pamphlet to accompany Geologic 

Investigations Series I- 2720. 

Wolff, Roger G., and Huber, N. King, 1973, The Copper 

Harbor Conglomerate (Middle Keweenawan) on Isle 

Royale, Michigan, and its regional implications, U.S. 

Geological Survey Professional Paper 754- B, p.B1- B15. 

Woodruff, L.G., Cannon, W.F., Dicken, C.L., Bennett, 

J.P., and Nicholson, S.W., 2003, Bedrock, soil, and 

lichen geochemistry from Isle Royale National Park, 

Michigan, U.S. Geological Survey Open- File Report 

OF- 03- 276, scale 1:100,000.

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other 

information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated Island 

Communities. 

Isle Royale National Park 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/037 

NPS D-122, June 2008 

National Park Service 

Director • Mary A. Bomar 

Natural Resource Stewardship and Science 

Acting Associate Director • Mary Foley, Chief Scientist of the Northeast Region 

Natural Resource Program Center 

The Natural Resource Program Center (NRPC) is the core of the NPS Natural Resource Stewardship and 

Science Directorate. The Center Director is located in Fort Collins, with staff located principally in 

Lakewood and Fort Collins, Colorado and in Washington, D.C. The NRPC has five divisions: Air 

Resources Division, Biological Resource Management Division, Environmental Quality Division, Geologic 

Resources Division, and Water Resources Division. NRPC also includes three offices: The Office of 

Education and Outreach, the Office of Inventory, Monitoring and Evaluation, and the Office of Natural 

Resource Information Systems. In addition, Natural Resource Web Management and Partnership 

Coordination are cross- cutting disciplines under the Center Director. The multidisciplinary staff of NRPC 

is dedicated to resolving park resource management challenges originating in and outside units of the 

national park system. 

Geologic Resources Division 

Chief • Dave Steensen 

Planning Evaluation and Permits Branch Chief • Carol McCoy 

Credits 

Author • Trista Thornberry- Ehrlich

Review • John Anderton and Carol McCoy 

Editing • Sid Covington and Melanie Ransmeier 

Digital Map Production • Anne Poole and Heather Stanton 

Map Layout Design • Georgia Hybels
Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, CO 80225 

http://www.nature.nps.gov/geology/inventory/ 

(303) 969-2090 

EXPERIENCE YOUR AMERICATM 

National Park Service 

U.S. Department of the Interior