National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Apostle Islands National Lakeshore

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/972
ON THIS PAGE

An opening in an ice-fringed sea cave reveals ice flows on Lake Superior.

Photograph by Neil Howk (National Park Service) taken in winter 2008.

ON THE COVER 

Wind and associated wave activity created a window in Devils Island Sandstone at Devils Island.

Photograph by Trista L. Thornberry-Ehrlich (Colorado State University) taken in summer 2010.
Apostle Islands National Lakeshore

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/972

Trista L. Thornberry-Ehrlich

Colorado State University Research Associate

National Park Service Geologic Resources Division

Geologic Resources Inventory

PO Box 25287

Denver, CO 80225

May 2015

U.S. Department of the Interior

National Park Service

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, Colorado, publishes a 

range of reports that address natural resource topics. These reports are of interest and applicability to a broad audience 

in the National Park Service and others in natural resource management, including scientists, conservation and 

environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate comprehensive information and analysis about natural 

resources and related topics concerning lands managed by the National Park Service. The series supports the 

advancement of science, informed decision-making, and the achievement of the National Park Service mission. The 

series also provides a forum for presenting more lengthy results that may not be accepted by publications with page 

limitations.

All manuscripts in the series receive the appropriate level of peer review to ensure that the information is scientifically 

credible, technically accurate, appropriately written for the intended audience, and designed and published in a 

professional manner. This report received informal peer review by subject-matter experts who were not directly 

involved in the collection, analysis, or reporting of the data.

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and 

policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products 

does not constitute endorsement or recommendation for use by the U.S. Government. 

This report is available from the Geologic Resources Inventory website (http://www.nature.nps.gov/geology/inventory/

gre_publications.cfm), and the Natural Resource Publications Management website (http://www.nature.nps.gov/

publications/nrpm/). To receive this report in a format optimized for screen readers, please email irma@nps.gov.

Please cite this publication as:

Thornberry-Ehrlich, T. L. 2015. Apostle Islands National Lakeshore: Geologic resources inventory report. Natural 

Resource Report NPS/NRSS/GRD/NRR—2015/972. National Park Service, Fort Collins, Colorado.

NPS 633/128695, May 2015
iii

Contents

Page

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Products and Acknowledgments . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

GRI Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Geologic Setting and Significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Park Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Geologic Significance and Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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

Apostle Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Lake Superior Shoreline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Sea Caves and Other Shoreline Erosional Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Sandscapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Postglacial (Recent) Surficial Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Fluvial Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Glacial Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Slope Movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Sedimentary Rock Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Devils Island Sandstone Type Locality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Douglas Fault and Other Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Aeolian Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Paleontological Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Geologic Resource Management Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Shoreline Response to Lake-Level Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Slope Movement Hazards and Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Caves and Associated Landscape Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Fluvial and Other Surface Water Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Abandoned Mineral Lands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Disturbed Land Restoration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Renewable Energy Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

External Mineral Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Wind-Speed Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Seismic Activity Hazards and Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Paleontological Resource Inventory, Monitoring, and Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Geologic History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.5 Billion to 1.6 Billion Years Ago (Archean Eon and Paleoproterozoic Era)—North American Craton Forms . . . . . . . . . . . . . . . . . . 45

1.6 Billion to 1.0 Billion Years Ago (Mesoproterozic Era)—Continental Rifting, Volcanism, and Sandstone Deposition . . . . . . . . . . . . . . 45

541 Million to 66 Million Years Ago (Paleozoic and Mesozoic Eras)—Burial and a Missing Rock Record . . . . . . . . . . . . . . . . . . . . . 48

66 Million Years Ago to Present (Cenozoic Era)—Erosion, Ice Age Glaciation, and Rebound . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Geologic Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Source Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

GRI GIS Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Geologic Map Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

GRI Map Posters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Map Unit Properties Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

 Use Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
iv

Contents (continued)

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Additional References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Geology of National Park Service Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

NPS Resource Management Guidance and Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Climate Change Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Geological Surveys and Societies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

US Geological Survey Reference Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Appendix A: Scoping Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

2010 Scoping Meeting Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

2014 Conference Call Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Appendix B: Geologic Resource Laws, Regulations, and Policies . . . . . . . . . . . . . . . . . . . . . . . . . 75

Page

GRI Products CD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . attached

Bedrock and Surficial Geologic Map Posters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .in pocket

Bedrock and Surficial Geologic Map Unit Properties Tables . . . . . . . . . . . . . . . . . . . . . . . . .in pocket
v

Figures

Page

Figure 1. Map of Apostle Islands National Lakeshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . in pocket

Figure 2. Physiographic provinces of Wisconsin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 3. Geologic time scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 4. Map of the Midcontinent rift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 5. Cross section through Apostle Islands and the Bayfield Peninsula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 6. Cross section through Pleistocene and Holocene deposits in the Apostle Islands area . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 7. Apostle Islands and paleocurrent flow directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 8. Frozen Lake Superior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 9. Ice caves and ice formations along shorelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 10. Bedrock erosional features on Devils Island . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 11. Sea caves forming at water level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 12. Sandscape formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 13. Longshore drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 14. Model cuspate foreland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 15. Barrier island migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 16. Dune and beach features at Stockton Island of Apostle Islands National Lakeshore. . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 17. Aeolian sand transportation and dune movement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 18. Comparison of Ice-age and interglacial conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 19. Glacial features. Not every glacially-altered landscape contains all of these features or deposits . . . . . . . . . . . . . . . . . . . . 22

Figure 20. Bayfield Peninsula during glaciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 21. Drumlin formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 22. Schematic illustrations of slope movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 23. Eroding glacial-till bluffs at Raspberry Island . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 24. Eroding glacial-till bluffs into hoodoo-like forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 25. Bedrock features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 26. Schematic illustrations of fault types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 27. Aeromagnetic map showing the location of buried structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 28. Relative coastal-change potential for Apostle Islands National Lakeshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 29. Prograding and retrograding barrier features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 30. Coastal stabilization sites at Apostle Islands National Lakeshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 31. Disturbed lands at Apostle Islands National Lakeshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Figure 32. Probability of earthquakes with magnitude greater than 5.0. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Figure 33 A–C. Schematic graphics illustrating the evolution of the landscape and geologic foundation of Apostle Islands 

National Lakeshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Figure 33 D–E. Schematic graphics illustrating the evolution of the landscape and geologic foundation of Apostle Islands 

National Lakeshore (continued). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 34. Schematic graphics illustrating the evolution of the Midcontinent rift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Figure 35. Depositional settings of the Bayfield Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Figure 36. Paleogeographic map of North America 550 million years ago . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Figure 37. Glaciations in Wisconsin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Tables

Page

Table 1. Clastic sedimentary rock classification and characteristics. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Table 2. Pleistocene glacial advances recorded in the Apostle Islands region. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Table 3. Data layers in the Apostle Islands National Lakeshore bedrock geology GIS data (apis_geology.mxd; apis_geology.kmz). . . . . . . . . 54

Table 4. Data layers in the Apostle Islands National Lakeshore Pleistocene geology GIS data (aipg_geology.mxd; aipg_geology.kmz). . . . . . . 54

vii

This GRI report was written for resource managers 

to support science-informed decision making. It 

may also be useful for interpretation. The report was 

prepared using available geologic information, and the 

NPS Geologic Resources Division conducted no new 

fieldwork in association with its preparation. Sections 

of the report discuss distinctive geologic features and 

processes within Apostle Islands National Lakeshore, 

highlight geologic issues facing resource managers, 

describe the geologic history leading to the presentday landscape, and provide information about the 

GRI geologic map data. A poster (in pocket) illustrates 

these data. The Map Unit Properties Table (in pocket) 

summarizes report content for each geologic map unit. 

Established on 26 September 1970, Apostle Islands 

National Lakeshore (Wisconsin) preserves and 

protects the natural and cultural resources on 21 of 

the 22 islands in southwestern Lake Superior off the 

coast of the Bayfield Peninsula. The names of the 

islands are Basswood, Bear, Cat, Devils, Eagle, Gull, 

Hermit, Ironwood, Long, Manitou, Michigan, North 

Twin, South Twin, Oak, Otter, Outer, Raspberry, 

Rocky, Sand, Stockton, and York. The “22nd island,” 

Madeline Island, is privately owned. The lakeshore also 

includes 19 km (12 mi) of mainland shoreline along the 

northwestern edge of the peninsula. 

Among the myriad features of the Apostle Islands’ 

shorelines are sea cliffs and bluffs, sea caves, sandscapes, 

terraces, wave-cut benches, and arches. Sandscapes 

include sand spits, cuspate forelands, beaches, 

tombolos, barrier spits, and dunes. Inland features 

include glaciated landscapes where glaciers scoured 

bedrock, forming grooves and striations, while 

depositing thick mantles of glacial sediments elsewhere. 

The glacial-till bluffs are the most prolific source of 

sediment for the sandscapes within the lakeshore. 

The geologic units within the lakeshore reveal a long 

geologic history stretching back more than a billion 

years. Sandstone of the 1.1-billion-year-old Bayfield 

Group—Orienta, Devils Island, and Chequamegon 

sandstone (geologic map units PCo, PCdi, and PCch, 

respectively)—collected in a series of braided streams 

and shallow lakes within a valley that was the setting 

of a continental rift in the midst of the ancient North 

American continent (or “craton”). Volcanic rocks 

associated with the rift, analogous to the modern East 

African Rift, are buried beneath the sandstones of the 

Bayfield Group. These sandstones and volcanic rocks 

were deeply buried, but are now exposed at the surface 

following hundreds of millions of years of erosion and 

weathering. During the ice ages (Pleistocene Epoch), 

more than a billion years later, glaciers scoured the 

bedrock, following stream valleys between resistant 

rock knobs that would become the Apostle Islands. 

The glaciers left a series of glacial deposits across the 

local landscape. These deposits consists of the Miller 

Creek and Copper Falls formations (Qw, Qb, Qgh, Qgl, 

Qgw, Qou, Qsc, Qsg, Qsu, Qsuc, and Qgg). In a dramatic 

example of an unconformity (gap in the geologic 

record), the ancient sandstones are juxtaposed against 

the youngest unit in the lakeshore—postglacial deposits, 

shoreline sediment (Qc). This unconformity represents 

1,100,000,000 years of “missing time.”

Geologic features and processes include the following:

● Apostle Islands. Every island has its own landscape 

characteristics reflected in geologic features, 

habitats, histories, and modern stories. All except 

Long Island are underlain by sandstone bedrock. 

● Lake Superior Shoreline. Shorelines within the 

lakeshore are extremely dynamic. Approximately 

257 km (160 mi) of shoreline exist within the 

lakeshore boundaries and comprise a wide variety 

of wave energies, ranging from exposed, highExecutive Summary

The Geologic Resources Inventory (GRI) is one of 12 inventories funded by the National Park Service 

(NPS) Inventory and Monitoring Program. The Geologic Resources Division of the NPS Natural 

Resource Stewardship and Science Directorate administers the GRI. This report synthesizes discussions 

from a scoping meeting for Apostle Islands National Lakeshore (Wisconsin) on 20–21 July 2010 and a 

follow-up conference call on 19 June 2014, which were held by the NPS Geologic Resources Division to 

determine geologic resources, the status of geologic mapping, and geologic resource management issues 

and needs. It is a companion document to previously completed GRI digital geologic map data.
viii

energy locations to sheltered, low-energy sites.

● Sea Caves and Other Shoreline Erosional Features. 

Waves sculpt bedrock shores into sea caves, arches, 

stacks, and cliffs. Glacial-till bluffs form where 

waves reach portions of the islands underlain by 

thick glacial deposits.

● Sandscapes. In areas of lower energy, sand collects 

in a series of forms called sandscapes. These 

include cuspate forelands, tombolos, beaches, and 

barrier spits. 

● Fluvial Features. The primary fluvial feature within 

the lakeshore is the Sand River on the Bayfield 

Peninsula. On the islands, springs and seeps and 

ephemeral streams occur. Larger rivers throughout 

the region are important contributors of sediment 

to the regional lakeshore system, including to the 

local formation of the Long Island barrier spit. 

● Recent Surficial Deposits. Unconsolidated surficial 

deposits are constantly being deposited and 

reworked on the landscape. These deposits include 

stream sediment (Qsm), shoreline sediment (Qc), 

and organic sediment (Qp) such as peat collecting 

in quiet water lagoons and bogs. 

● Glacial Features. Glaciations affected the landscape 

in two major ways: (1) created or carved features 

into the landscape, and (2) deposited mantles of 

sediment over the landscape. Glaciers flowing 

between resistant knobs of bedrock defined the 

shapes of the Apostle Islands. Drumlins, ridges, 

and grooves provide information about which 

direction the glaciers flowed. Glacial deposits in 

the lakeshore are of two types: (1) those deposited 

directly by moving ice such as clay-rich glacial 

till, and (2) those deposited by meltwater in rivers 

(glaciofluvial) or lakes (glaciolacustrine). 

● Slope Movements. Slope movements (the 

downslope transfer of earth materials) occur as 

small-scale landslides, slumps, and slope creep on 

the steep glacial-till bluffs. The bedrock cliff areas 

are prone to blockfall, topple, and small-scale 

slides. 

● Sedimentary Rock Features. The rocks that 

form the foundation of Apostle Islands National 

Lakeshore contain many features indicative of their 

geologic history. The billion-year-old sedimentary 

rocks of the Midcontinent rift in Wisconsin are 

among the oldest in the Midwest. They collected in 

braided streams and shallow ponds of a rift valley. 

● Devils Island Sandstone Type Locality. The 

excellent exposures of ancient sandstone at Devils 

Island have made it the type locality for the Devils 

Island Sandstone (PCdi). A type locality showcases a 

particular unit’s mappable features, including rock 

type, color, texture, thickness, and distinguishing 

features such as depositional structures and the 

characteristic manner in which it weathers or 

erodes.

● Douglas Fault and Other Faults. Faults are fractures 

in rock along which rocks have moved. The 

Douglas Fault cuts across the base of the Bayfield 

Peninsula, south of Apostle Islands National 

Lakeshore. This fault is a reverse fault, thrusting 

older volcanic rocks over younger sedimentary 

rocks. It was likely a reactivated structure (i.e., 

normal fault) that first formed in association with 

the Midcontinent rift. 

● Aeolian Processes. Windblown erosion, 

transportation, and deposition of sediments have 

led to the formation of dunes, barriers, and sand 

spits at Apostle Islands National Lakeshore. Sand 

is blasted from the exposed faces of glacial-till 

bluffs by the local winds, serving as a vital source of 

sediment. 

● Paleontological Resources. Fossils are evidence of 

life preserved in a geologic context. The ancient 

bedrock of the lakeshore is unlikely to bear body 

fossils, but potential trace fossils, such as burrows, 

are in the Devils Island Sandstone (PCdi). The 

discovery of Pleistocene and Holocene fossils has 

potential at the lakeshore. These remains of past life 

could wash onto the shorelines or occur within peat 

(Qp). 

Geologic resource management issues identified during 

the GRI scoping meeting and follow-up conference call 

include the following:

● Shoreline Response to Lake-Level Change. The 

shorelines at the lakeshore are vulnerable to 

negative impacts from lake-level changes. Lake 

levels are controlled by factors including climate, 

local uplift or subsidence, and land use. The 

northeastern Lake Superior basin is uplifting faster 

than the Apostle Islands area, which could cause 

lake-level rise, flooding shorelines and inundating 

infrastructure. Contrary to a marine coastal setting 

where relative sea level is predicted to rise, climatic 

models predict lake levels will drop, exposing 

docks, sand bars, and submerged resources 
ix

throughout the area. Lake levels are intimately tied 

to climate changes. Lake-level change associated 

with climate change is a primary resource 

management concern at the lakeshore because 

much of the natural resources, fragile habitats, and 

visitor access areas are within a few meters of the 

shore.

● Slope Movement Hazards and Risks. Natural 

slope processes are to be expected on the 

shorelines under the onslaught of constant wave 

action. Geologic, morphological, physical, and 

anthropogenic factors contribute to slope instability 

and erosion. The mitigation of slope hazards, 

limited to areas where historic structures (e.g., light 

stations) are threatened, involves the construction 

of stabilizing structures to protect cultural 

resources and visitor access.

● Caves and Associated Landscape Management. 

The shallow sea caves at the lakeshore are nonrenewable resources formed in the sandstone cliffs 

at the shorelines. These features tend to weather 

relatively quickly and change rapidly. The lakeshore 

does not have a cave management plan, but 

climbing within the caves is restricted. 

● Fluvial and Other Surface Water Issues. As the Sand 

River meanders across its floodplain, it erodes into 

the unconsolidated deposits flanking its channel 

and causes undercutting and slope movements. 

Several open-water lagoons associated with 

sandscapes on the Apostle Islands are naturally 

acidic and lack the ability to buffer acid deposition 

from precipitation. 

● Abandoned Mineral Lands. Four abandoned 

mineral lands features occur at four sites on three 

islands at Apostle Islands National Lakeshore: 

Breckenridge Quarry and Bass Island Brownstone, 

Ashland Brownstone Company, and Excelsior 

Brownstone Quarry.

● Disturbed Land Restoration. Apostle Islands 

National Lakeshore includes a long history of 

human use. Most disturbed features are being 

allowed to reach a natural state of equilibrium. 

Some are interpreted for visitors.

● Renewable Energy Development. Generation and 

transmission of renewable energy includes utilityscale onshore and offshore wind, solar, geothermal, 

and hydroelectricity. Proposals exist to develop 

wind farms on Madeline Island and the mainland 

with potential impacts to the lakeshore’s viewshed 

and local traffic. 

● External Mineral Development. A large iron 

mine is proposed for an area in the Gogebic Iron 

Range, 50 km (30 mi) southeast of Apostle Islands 

National Lakeshore. Potential negative impacts to 

the lakeshore from mining include water quality 

degradation, air pollution, increased traffic, and 

noise. 

● Wind-Speed Change. Local wind speeds are 

increasing at rates approaching 5% per decade. 

The increase is probably driven by global climate 

change. Increased wind speeds will accelerate 

erosion of glacial-till bluffs, increase tree 

blowdowns, and change the formation of aeolian 

features in the sandscapes. 

● Seismic Activity Hazards and Risks. Earthquakes 

that are noticeable by humans are uncommon in 

Wisconsin. Buried faults and other structures could 

become active due to the isostatic rebound of the 

land surface as Earth’s crust continues to adjust to 

the retreat of glacial ice after the ice ages.

● Paleontological Resource Inventory, Monitoring, 

and Protection. The lakeshore lacks a field-based 

paleontological resource survey. Such a survey 

could provide detailed, site-specific descriptions 

and resource management recommendations for all 

paleontological resources at the lakeshore. 

xi

GRI Products

The objective of the Geologic Resources Inventory is 

to provide geologic map data and pertinent geologic 

information to support resource management and 

science-informed decision making in more than 

270 natural resource parks throughout the National 

Park System. To realize this objective, the GRI team 

undertakes three tasks for each natural resource park: 

(1) conduct a scoping meeting and provide a summary 

document, (2) provide digital geologic map data in a 

geographic information system (GIS) format, and (3) 

provide a GRI report (this document). These products 

are designed and written for nongeoscientists.

Scoping meetings bring together park staff and geologic 

experts to review and assess available geologic maps, 

develop a geologic mapping plan, and discuss geologic 

features, processes, and resource management issues 

that should be addressed in the GRI report. Following 

the scoping meeting, the GRI map team converts the 

geologic maps identified in the mapping plan to digital 

geologic map data in accordance with the GRI data 

model. After the map is completed, the GRI report team 

uses these data, as well as the scoping summary and 

additional research, to prepare the GRI report. 

The compilation and use of natural resource 

information by park managers is called for in the 1998 

National Parks Omnibus Management Act (section 

204), 2006 National Park Service Management Policies, 

and the Natural Resources Inventory and Monitoring 

Guideline (NPS-75). The “Additional References” 

section and Appendix B provide links to these and other 

resource management documents and information.

Additional information regarding the GRI, including 

contact information, is available at http://www.nature.

nps.gov/geology/inventory/ (accessed 3 April 2015). 

The current status and projected completion dates of 

products are available at http://www.nature.nps.gov/

geology/GRI_DB/Scoping/Quick_Status.aspx (accessed 

3 April 2015).

Acknowledgments

Additional thanks to: Ulf Gafvert (NPS Great Lakes 

Network), Peggy Burkman and Julie Van Stappen (NPS 

Apostle Islands National Lakeshore).

Author

Trista L. Thornberry-Ehrlich (Colorado

State University)

Review

Jason Kenworthy (NPS Geologic Resources Division)

Dale Pate (NPS Geologic Resources Division)

Courtney Schupp (NPS Geologic Resources Division)

Hal Pranger (NPS Geologic Resources Division)

Richard Ojakangas (University of Minnesota Duluth)

Julie Van Stappen (Apostle Islands National Lakeshore)

Lynda Bell (NPS Water Resources Division)

Laurel Woodruff (US Geological Survey)

Editing

Katie KellerLynn (Colorado State University)

Source Maps

US Geological Survey 

Wisconsin Geological and Natural History Survey

GRI Digital Geologic Data Production

Rachel Yoder (Colorado State University)

Jim Chappell (Colorado State University)

Derek Witt (Colorado State University)

Stephanie O’Meara (Colorado State University)

GRI Map Poster Design

Kari Lanphier (Colorado State University)

Georgia Hybels (Colorado State University)

GRI Map Poster Review

Georgia Hybels (Colorado State University)

Rebecca Port (NPS Geologic Resources Division)

Jason Kenworthy (NPS Geologic Resources Division)

Products and Acknowledgments

The NPS Geologic Resources Division partners with institutions such as Colorado State University, 

the US Geological Survey, state geological surveys, local museums, and universities to develop GRI 

products. This section describes those products and acknowledges contributors to this report.

1

Park Setting

Apostle Islands National Lakeshore, established 

on 26 September 1970, preserves and protects the 

natural and cultural resources on 21 of the 22 islands 

in southwestern Lake Superior off the coast of the 

Bayfield Peninsula in northern Wisconsin (fig. 1, in 

pocket). Jordahl (1994) detailed the story behind the 

establishment of the lakeshore. The islands’ name may 

stem from early French explorers, who referred to them 

collectively after the 12 biblical apostles, or perhaps 

the name is inspired by a band of “renegades” who 

used the narrow winding straits, caves, and hidden 

coves during their hijacking of the payloads of local 

fur traders (Stucker 1974). The 21 islands within the 

national lakeshore are Basswood, Bear, Cat, Devils, 

Eagle, Gull, Hermit, Ironwood, Long (currently a sand 

spit), Manitou, Michigan, North Twin, South Twin, 

Oak, Otter, Outer, Raspberry, Rocky, Sand, Stockton, 

and York. The largest Apostle Island, Madeline Island, 

is privately owned. The total area contained within 

the lakeshore’s boundaries is 28,074 ha (69,372 ac) of 

which 13,557 ha (33,500 ac) are designated wilderness 

and 11,020 ha (27,232 ac) are submerged; the lakeshore 

boundary extends 0.40 km (0.25 mi) offshore on the 

lake surface. The lakeshore also includes 19 km (12 mi) 

of mainland shoreline along the northwestern edge of 

the Bayfield Peninsula. The mainland unit, and Eagle 

Sand, York, and Raspberry islands are within Bayfield 

County; the remaining islands are in Ashland County. 

The shoreline of Lake Superior is 183 m (602 ft) above 

sea level. The highest point of the Bayfield Peninsula 

is 251 m (824 ft) above the lake’s surface. Most of the 

islands rise less than 60 m (200 ft) above the lake’s 

level. The highest island, Oak Island, is 329 m (1,081 ft) 

above sea level. Oak Island covers approximately 2,000 

ha (5,000 ac) and has about 146 m (479 ft) of relief. 

The shorelines of the Apostle Islands display sea cliffs 

and bluffs, sea caves, sandscapes, terraces, wave-cut 

benches, and arches. Sandscapes include sand spits, 

cuspate forelands, beaches, tombolos, barrier spits, and 

dunes. Inland features include glaciated landscapes 

where glaciers simultaneously scoured some areas and 

deposited thick mantles of glacial deposits in others. 

Geologic Setting

Wisconsin consists of five physiographic provinces: 

Lake Superior Lowland, Northern Highland, Central 

Plain, Western Upland, and Eastern Ridges and 

Lowlands (fig. 2; Martin 1916; Dott and Attig 2004). 

Physiography is determined largely by the variations of 

texture, composition, and structure of the underlying 

rocks and how earth surface processes have shaped 

their characteristic topography. Apostle Islands National 

Lakeshore is entirely within the northernmost province, 

the Lake Superior Lowland. The steeply rising shores 

of Lake Superior are testament to its setting as a lake set 

deeply within a highland area. The southern boundary 

of the province is the highest abandoned beach line of 

Lake Superior (Martin 1916). 

The geologic setting of Lake Superior and the Apostle 

Islands is reflective of its long geologic history. The 

geologic units within the lakeshore are from opposite 

ends of the geologic time scale (fig. 3): the billionyear-old bedrock from the Mesoproterozoic Era (or 

younger, possibly Neoproterozoic Era, as accurate age 

dates do not exist [Richard Ojakangas, University of 

Minnesota Duluth, geologist, written communication, 

12 January 2015]), and recent Pleistocene and Holocene 

surficial units, some of which are still forming. The 

bedrock exposed in the lakeshore (geologic map 

units PCch, PCdi, and PCo) is the youngest part of the 

Keweenawan Supergroup—rocks that formed as part 

of the Midcontinent rift approximately 1.1 billion years 

ago. The Middle Proterozoic Midcontinent rift system 

is one of Earth’s great continental rifts (Hinze et al. 

1997). The rift extends more than 2,500 km (1,500 mi) 

from Kansas in a northeasterly arc through the Lake 

Superior region and down into southern Michigan 

(fig. 4). More than 30 km (18 mi) of volcanic and 

sedimentary rocks fill the rift, but they are only exposed 

in the Lake Superior region as a result of long-term 

erosion and cover by younger Paleozoic rocks. Along 

the southern shore of Lake Superior, rocks of the 

Keweenawan Supergroup have been tilted to the north, 

forming the Montreal River monocline (Cannon et al. 

1999). Locally, the Keweenawan Supergroup is draped 

over an Archean (more than 2.5 billion years old) block 

Geologic Setting and Significance

This section describes the regional geologic setting of Apostle Islands National Lakeshore and 

summarizes connections among geologic resources, other park resources, and park stories.
2

North

0

0

35 Kilometers

35 Miles

MINNESOTA

IOWA

LAKE

MICHIGAN

MICHIGAN

WISCONSIN

GREEN BAY

Duluth

MILWAUKEE

MADISON

La Crosse

Ashland

LAKE

SUPERIOR

Northern Highland

Central Plain

Western Upland Eastern Ridges

and Lowlands

Lake Superior Lowland

Figure 2. Physiographic provinces of Wisconsin. Apostle Islands National Lakeshore (green box) is within the Lake 

Superior Lowland province, which contains some of the oldest geologic units in the state. Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University) using information from Martin (1916) and the Wisconsin Geological 

and Natural History Survey. Shaded-relief base map by Tom Patterson (National Park Service), available at http://www.

shadedrelief.com/physical/index.html (accessed 16 June 2014). 
3

Formation of the Earth

Eon Era Period Epoch MYA Geologic Map Units Apostle Islands NL Events Phanerozoic Proterozoic Archean Hadean Cenozoic (CZ) Mesozoic (MZ) Paleozoic (PZ)

Quaternary

 (Q)

Neogene

 (N)

Cretaceous (K)

Jurassic (J)

Triassic (TR)

Permian (P)

Pennsylvanian (PN)

Mississippian (M)

Devonian (D)

Silurian (S)

Ordovician (O)

Cambrian (C)P r e c a m b r i a n (PC)

Holocene (H)

Pleistocene 

 (PE)

Pliocene (PL)

Miocene (MI)

Oligocene (OL)

Eocene (E)

Paleocene (EP) 

2.6

5.3

23.0

33.9

56.0

145.0

201.3

298.9

323.2

358.9

419.2

443.7

485.4

2,500

4,600

66.0

252.2

541.0

Breakup of Pangaea begins; 

 Atlantic Ocean opens 

Supercontinent Pangaea intact

0.01

~4,000

~

Continued erosion, stream incision

Worldwide glaciation, periglacial

 conditions

Paleogene

 (PG)

 

Tertiary (T)

Erosion

Continued erosion and mass 

 wasting

Paleoproterozoic Mesoproterozoic (Y) Neoproterozoic (Z)

1,600

1,000

Human history, shoreline change

Granitic crust formed, 

 metamorphism

Uplfit, volcanism, widespread 

 erosion

Midcontinent rift began to open

 Lake Superior basin developed

Midcontinent rift volcanics

 (Yv) erupted

Widespread volcanism in rift

PCf deposited

PCo, PCdi, and PCch

 deposited Midcontinent rift becomes

 inactive; basins subside

Erosion of granitic crust and 

 deposition of Bayfield Group 

CPCrs

Qgg, Qsc, Qsg, Qsu, and

 Qsuc deposited

Qc and Qw deposited

Qsm, Qp, Qou, Qgw, Qgl, 

 Qgh, and Qb deposited Lake Superior forms

Burial and deformation

 of Bayfield Group

Erosion and weathering

 of overlying sediments

Supercontinent Rodinia rifted apart

Ync deposited

Figure 3. Geologic time scale. The divisions of the geologic time scale are organized stratigraphically, with the oldest 

divisions at the bottom and the youngest at the top. GRI map abbreviations for each time division are in parentheses. 

Boundary ages are millions of years ago (MYA). National Park Service graphic using dates from the International 

Commission on Stratigraphy (http://www.stratigraphy.org/index.php/ics-chart-timescale; accessed 5 April 2015).
4

called White’s Ridge (fig. 5). At Apostle Islands National 

Lakeshore the sandstones of the Bayfield Group, 

made up of the Orienta Sandstone (PCo), Devils Island 

Sandstone (PCdi), and Chequamegon Sandstone (PCch), 

accumulated after volcanism associated with active 

rifting had ceased. The rift was buried for hundreds of 

millions of years until glaciers—an agent of profound 

landscape change—scoured and sculpted the Lake 

Superior basin in the ice ages of the Pleistocene Epoch 

(2.6 million–11,700 years ago; fig. 3). 

Huge glaciers intermittently advanced and retreated 

through the Superior region. They scoured vast 

channels in the bedrock leaving “islands” between 

them. The glaciers and associated meltwater deposited 

vast amount of glacial drift or mixed glacial sediments 

across the region, including the Miller Creek Formation 

(Qw, Qb, Qgh, Qgl, Qgw, and Qou) and Copper Falls 

Formation (Qsc, Qsg, Qsu, Qsuc, and Qgg) (fig. 6; 

Clayton 1984, 1985). When they retreated, a series of 

glacial lakes, including Glacial Lake Duluth, formed 

North

0

0

100 Kilometers

100 Miles

CHICAGO

LAKE

MICHIGAN

LAKE

SUPERIOR

LAKE

HURON

LAKE

ERIE

DETROIT MILWAUKEE

MADISON

GREEN BAY

KANSAS CITY ST. LOUIS

MINNEAPOLIS

APOSTLE ISLANDS 

NATIONAL LAKESHORE

CANADA

MICHIGAN

NORTH

DAKOTA

SOUTH 

DAKOTA

NEBRASKA

KANSAS

MISSOURI

IOWA

MINNESOTA

WISCONSIN

ILLINOIS

INDIANA

OHIO

KENTUCKY

Figure 4. Map of the Midcontinent rift. Orange areas delineate the primary volcanic basins with associated 

sedimentary and plutonic rocks. The purple areas represent primarily sedimentary rocks deposited after volcanism had 

ceased. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) after Hinze et al. (1997, figure 3). Shadedrelief base map by Tom Patterson (National Park Service), available at http://www.shadedrelief.com/physical/index.html

(accessed 18 August 2014).
5

in the Superior basin. Remnants of these glacial-lake 

shorelines, in places 150 m (500 ft) above the modern 

shoreline, occur at Apostle Islands National Lakeshore 

(National Park Service 2005). Lake Superior and the 

other Great Lakes assumed their present configuration 

about 2,000 years ago when the lakes began to drain into 

the Atlantic Ocean via the St. Lawrence River (LaBerge 

1994; Busch 2008).

Geologic Significance and Connections

The natural beauty of Apostle Islands National 

Lakeshore belies a long history of human use. As 

described in detail in Jordahl (1994) and Busch (2008), 

the human history of the Apostle Islands incorporates 

American Indians, voyageurs (canoemen), fur 

traders, fishing and logging developers, and farmers. 

The geologic resources and features of the islands 

fundamentally influenced this history. 

American Indians took advantage of the natural 

richness of the landscape and established hunting and 

fishing camps, and harvested berries (Julie Van Stappen, 

written communication, 9 January 2015). Fluctuating 

lake levels may have obscured some of the earliest sites. 

A Paleo-Indian stone projectile from Manitou Island 

may indicate human presence as early as 11,000 years 

before present—a similar age to an archeological site 

on Chequamegon Bay (Busch 2008). Following the 

retreat of the glaciers, as vegetation became established, 

the woodland conditions allowed American Indians 

from the Archaic period (8,000 to 2,000 years before 

common era [BCE]) to find food, red clay for pottery, 

and stone tool material, including quartzite and native 

copper that was mined in the Lake Superior region, for 

example, at Isle Royale and the Keweenaw Peninsula 

(Jordahl 1994; Busch 2008). Copper was deposited in 

rift-related volcanic rocks by circulation of mineralladen geothermal fluids. Projectiles, knapping debris, 

fire-cracked rock, and a worked copper blank were all 

0

5,000

10,000

15,000

0

16,000

33,000

49,000

Late Archean crystalline

basement rocks, undivided (Wu)

Mesoproterozoic 

volcanic rocks, undivided (Yv) Yv

Ync

Freda Sandstone (PCf)

Orienta Sandstone (PCo)

Devils Island

Sandstone 

(PCdi)

Chequamegon

Sandstone 

(PCch) LAKE SUPERIOR

Sand 

Island

Bayfield

Peninsula

Madeline

Island

Nonesuch Formation and 

Copper Harbor Conglomerate, 

undivided (Ync)

Depth (meters) below sea levelDepth (feet) below sea level

White’s Ridge

Northwest Southeast

Mixed volcanic rocks and sediments Conglomerate Quartz-rich sandstone

Figure 5. Cross section through Apostle Islands and the Bayfield Peninsula. Note the buried structure of White’s Ridge. 

Cross section follows the northwestern part of the A–A ́ cross section line included in the GRI GIS data. Colors are 

standard colors approved by the US Geological Survey to indicate different time periods on geologic maps; they also 

correspond to the colors on the Map Unit Properties Table. See the Map Unit Properties Table (in pocket) for more 

detail. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) after Cannon et al. (1996, cross section A–A ́). 

Figure 6. Cross section through Pleistocene and 

Holocene deposits in the Apostle Islands area. Cross 

section is from south to north showing relationships 

between the two major Pleistocene into Holocene 

units (Copper Falls and Miller Creek formations). These 

units unconformably overlie rocks of the much older 

Precambrian Bayfield Group. Colors are standard US 

Geological Survey colors to indicate different time 

periods on geologic maps; they also correspond to the 

colors on the Map Unit Properties Table (in pocket). 

Graphic is not to scale and is vertically exaggerated. 

Graphic by Trista L. Thornberry-Ehrlich (Colorado State 

University) after Clayton (1984, figure 3). 
6

found at sites on Stockton Island, some of which dated 

to 5,300 to 4,880 years before present (Busch 2008). 

The earliest evidence of the next archeological period, 

the Woodland, in the Apostle Islands also appears on 

Stockton Island as hearths, a midden, a range of stone 

tools, pottery, and animal remains. Chipped quartz 

cobbles are also known remnants from this time—about 

1,200 years before present (Busch 2008), and were likely 

used as scrapers for fish processing (Julie Van Stappen, 

written communication, 9 January 2015).

American Indians speaking an Algonquian language, 

including the locally prominent Outchibouec or Ojibwe, 

were following the Late Woodland tradition when 

the first Europeans arrived in the early 17th century 

(Busch 2008). Chippewa legends describe the origin 

of the Apostle Islands as occurring when one of their 

ancestral figures and cultural heroes, Wanabozho, tried 

to trap a giant beaver behind a hastily built earth dam 

(Long Island) (Stucker 1974; Julie Van Stappen, written 

communication, 9 January 2015). The mythical beaver 

escaped, the dam failed, and the remnants of the dam 

are the Apostle Islands (Stucker 1974). 

Local geologic features affected the area’s early history. 

As described in the “Geologic History” section, 

drainage from glacial lakes following the last major ice 

age radically changed river courses. American Indians, 

early European explorers, and fur traders traversed a 

4-km (2-mi) portage from the St. Croix River into the 

Brule River across the modern divide north of Solon 

Springs (Ojakangas et al. 2011). Fur traders entered the 

area in the early 1600s using the natural harbors and 

bays for trading points. By the end of that century a 

trading post, La Pointe, was established on Madeline 

Island to figure prominently for nearly 150 years 

(Stucker 1974). Once the fur trade declined, the logging 

industry took over, forever changing the forests and 

landscapes of the islands (Stucker 1974). 

The Apostle Islands have a 30-year-history of 

brownstone (sandstone) quarrying. Beginning in 

1869, Chequamegon Sandstone (PCch) was extracted 

from Stockton, Hermit, and Basswood islands for 

urban Midwest buildings. More than 42,000 m3 (1.5 

million ft3) were removed by the Ashland Brownstone 

Company alone. The dark brown color of the sandstone 

comes from hematite (oxidized iron) that formed when 

the original sand deposits were exposed to oxygen 

in the atmosphere. Groundwater seeping through 

cracks in the sandstone left characteristic bleached 

fringes (Brown 2006); Thornberry-Ehrlich 2013). 

Lake Superior provided easy and economical boat 

transport of the quarried blocks (Brown 2006). The 

industry flourished until architectural styles changed, 

lighter colored stones (e.g., Indiana limestone) became 

fashionable, and extraction costs increased. Today, the 

dark, reddish brown stone is visible in the ca. 1883 Old 

Bayfield County Courthouse (now the Apostle Islands 

National Lakeshore visitor center) and other historic 

buildings throughout the Midwest (National Park 

Service 2005; Brown 2006). Apostle-Island quarries 

were abandoned by the late 1890s. Cut blocks are still 

visible on the shore of Hermit Island. 

The bedrock-cored Apostle Islands, jutting off the 

Bayfield Peninsula, were perfect perches for lighthouses 

to guide ships to mainland harbors and away from rocky 

shoals and islands. The eight lighthouses and six light 

stations on Devils, Long, Michigan, Outer, Raspberry, 

and Sand islands form the largest single-park collection 

in the National Park System (Thornberry-Ehrlich 2013). 

The lighthouse towers were constructed between 1856 

(Michigan Island) and 1929 (new Michigan light) (Julie 

Van Stappen, written communication, 9 January 2015). 

Apostle Islands Ecosystem

The Apostle Islands and Lake Superior region 

experiences distinct seasonal changes and supports a 

variety of ecosystems. Because of its great size, Lake 

Superior creates a maritime climate in an otherwise 

northern, continental setting. Inherent in this maritime 

climate are relatively cool summers and warm winters 

compared to adjacent areas.

The glacial history and setting of the islands and 

Bayfield Peninsula strongly influenced the types of soils 

that developed and in turn the types of plants. Soils 

on glacial tills sourced from the Bayfield Group are 

very sandy and acidic, whereas soils developed on the 

Pleistocene lacustrine sediments tend to be very clay 

rich and somewhat calcareous (Cary et al. 1979). Soils 

began to develop on the Apostle Islands during the late 

Pleistocene–early Holocene (fig. 3: Cary et al. 1979). Soil 

resources are not covered in this geologic report, but 

an NPS Soil Resources Inventory product for Apostle 

Island National Lakeshore is available at https://irma.

nps.gov/App/Reference/Profile/1049040 (accessed 3 

April 2015). 
7

Natural Resources Research Institute (2006) identified 

at least eight “important habitat sites” in the Apostle 

Islands National Lakeshore. Hardwood forests, 

dominated by maples and birches, are common 

throughout the Apostle Island region. Boreal forests 

of white spruce, balsam fir, tamarack, white cedar, 

birch, and aspen occupy a small portion of the islands’ 

forests (Natural Resources Research Institute 2001; 

Busch 2008). A pine savannah forest, peat bog, beach 

grass dune, and freshwater lagoon thrive on the double 

tombolo connecting Stockton and Presque Isle islands. 

A number of rare plants, including carnivorous pitcher 

plants, grow there. Throughout human history, the 

Apostle Islands were heavily logged for white pine, 

hemlock, yellow birch, and maple; however, some virgin 

stands of hemlock, pine, and sugar-maple oak were 

not logged and remain among the natural treasures of 

the Apostle Islands (Swain and Winkler 1983; Busch 

2008; Ojakangas et al. 2011). A stand of the original 

hemlock-hardwood forest exists on the northern end of 

Outer Island (Jordahl 1994). On islands where browsing 

deer have been restricted or absent, Canada yew is a 

dominant groundcover. Canada yew is all but gone on 

the adjacent mainland (Thornberry-Ehrlich 2013). 

Beyond the forests, the islands’ sandscapes, glacialtill bluffs, sandstone ledges and cliffs, and sphagnum 

bogs provide habitat for specialized plant communities 

(Busch 2008). These habitats support a variety of 

flora and fauna. More than 800 species of plants grow 

in the Apostle Islands, of which at least 37 are rare, 

endangered, or threatened (Jordahl 1994; National 

Park Service 2004). The geologic units of the lakeshore 

host 324 species of known lichen (Bennet and 

Wetmore 2005). Hundreds of species of nesting birds 

find habitat on the islands, including critical habitat 

for the endangered piping plover community that 

nests on Long Island (Thornberry-Ehrlich 2013). In 

addition, the islands provide an important stopover 

for migrating birds across the Great Lakes. More than 

20 species of salamanders, toads, frogs, and reptiles 

(e.g., turtles and snakes) find suitable habitat on the 

islands (National Park Service 2004). Splash pools, 

developed on bedrock platforms located at the shore 

host myriad species including lichens; the arctic 

disjunct Birdseye Primrose (Primula farinose) at Devils 

Island; stone flies; predacious diving beetles; spring 

peeper and salamander; and invasive spiny water fleas 

(Bythotrephes longimanus) (Lafrançois et al. 2010). 

Apostle-Island shores are important fish spawning 

areas. At least 22 whitefish–important habitats and 

five lake trout–important habitats exist among the 

islands (Natural Resources Research Institute 2006). 

Mammal species such as black bears, beaver, deer, red 

squirrels, snowshoe hares, deer mouse, voles, foxes, 

coyotes, and otters occur on the islands (National 

Park Service 2004). The lakeshore’s lighthouses, as 

well as the honeycombed sandstone cliffs, caves, and 

other shoreline exposures provide some bat habitat 

(Thornberry-Ehrlich 2013).

9

Geologic Features and Processes

This section describes noteworthy geologic features and processes in Apostle Islands National 

Lakeshore.

During the 2010 scoping meeting (Thornberry-Ehrlich 

2013) and 2014 conference call, participants (Appendix 

A) identified the following geologic features and 

processes:

● Apostle Islands

● Lake Superior Shoreline

● Sea Caves and Other Shoreline Erosional Features

● Sandscapes

● Recent Surficial Deposits

● Fluvial Features

● Glacial Features

● Slope Movements

● Sedimentary Rock Features

● Devils Island Sandstone Type Locality

● Douglas Fault and Other Faults

● Aeolian Processes

● Paleontological Resources

Nuhfer and Dalles (2004) provide a comprehensive 

guidebook to the geology of each of the Apostle Islands 

at the lakeshore. The Apostle Islands serve as a natural 

classroom providing visitors the opportunity to learn 

about geological features and processes, including 

braided stream deposits, glacial scouring, shoreline 

erosion, sandscapes, sea caves, wave-cut platforms, and 

sedimentary structures (Nuhfer and Dalles 1994). 

Apostle Islands

Each Apostle Island has its own unique character 

and landscape expressed through geologic features, 

habitats, histories, and modern uses as described in 

detail by Nuhfer and Dalles (2004). With the exception 

of Long Island—a barrier sand spit—the Apostle Islands 

are cored by sandstone bedrock extending outwards 

in a northeasterly trend from the tip of Bayfield 

Peninsula into Lake Superior (see poster, in pocket). 

Prior to the scouring of glaciers during the ice ages 

of the Pleistocene Epoch, the islands were part of the 

mainland. Their northeasterly trend parallels ancient 

streamflows, which deposited some of the sandstones of 

the Bayfield Group (geologic map units PCo, PCdi, and 

PCch), as well as more recently flowing glaciers (fig. 7; 

Cannon et al. 1999; Nuhfer and Dalles 2004). 

The largest island in the lakeshore is Stockton Island at 

4,069 ha (10,054 ac) and 36.5 km (22.7 mi) of shoreline. 

The smallest island is Gull Island at 1.2 ha (3 ac) and 

0.6 km (0.4 mi) of shoreline. Most islands are two to 

three times longer than they are wide. Elevations above 

the Lake Superior shoreline range from 146 m (479 ft) 

at Oak Island to 3 m (10 ft) for Long and Gull islands. 

Visitors can access all of the islands by beach except 

Basswood, Devils, Eagle, Gull, and North Twin. With 

the exception of Gull Island, rock landings provide 

access to these islands. 

The Apostle Islands experience constant landform 

change. When the Apostle Islands were surveyed by 

the General Land Office in 1856, Gull Island was 11 

m (35 ft) above the lake level, vegetated, and inhabited 

(Jordahl 1994). By the 1920s after devastating storms, it 

was 0.6–1 m (2–3 ft) above the lake. Other islands such 

as Steamboat, which existed in 1898, have disappeared 

altogether (Stucker 1974; Ulf Gafvert, Great Lakes 

Network, GIS specialist; and Julie Van Stappen and 

Peggy Burkman, Apostle Islands National Lakeshore, 

chief of Resource Management and biologist, 

conference call, 19 June 2014). 

Park resource managers are interested in how the 

shorelines and sandscapes (see “Sandscapes” section) 

have changed with time. In preparation of the book 

A guidebook to the geology of Lake Superior’s Apostle 

Islands National Lakeshore (Nuhfer and Dalles 2004), 

Ed Nuhfer took a number of photographs that extend 

back at least 20 years. This photographic record has 

management applications. Digitizing shorelines of the 

sand spits, repeat photography, and aerial photographs 

would all contribute to a quantitative understanding of 

shoreline change at Apostle Islands National Lakeshore. 

The Geoscientist-in-the-Parks program (http://www.

nature.nps.gov/geology/gip/index.cfm; accessed 3 April 

2015) may provide a means of facilitating such a project.
10

Lake Superior Shoreline

Coastal (shoreline) resources are in the transition 

between terrestrial (on land) and open-water 

environments. Coastal environments—shaped by 

waves, tides, and wind—include tidal flats, estuaries, 

dunes, beaches, and barrier islands (Bush and 

Young 2009). As of November 2011 (the most recent 

servicewide statistics), the National Park Service 

managed 84 ocean, coastal, and Great Lakes parks with 

more than 18,000 km (11,200 mi) of shoreline. The 

seven Great Lakes parks—Apostle Islands National 

Lakeshore, Grand Portage National Monument, 

Isle Royale National Park, Pictured Rocks National 

Lakeshore, Sleeping Bear Dunes National Lakeshore, 

Indiana Dunes National Lakeshore, and Perry’s Victory 

and International Peace Memorial—have more than 998 

13

Sand River

Sioux River

Raspberry River

Rocky

Island

North Twin

Island

Cat

Island

Manitou

Island

Stockton Island

Hermit

Island

Devils

Island

Eagle Island

MADELINE

ISLAND

Gull

Island

Michigan

Island

Outer 

Island

Long Island

Ironwood Island

Bear

Island

York

Island

Raspberry

Island

Otter

Island

South

Twin

Island

Oak

Island

Basswood

Island

Sand 

Island

Presque

Isle Bay

Cornucopia

Bayfield

Red Cliff

Point

La Pointe 

South Channel

N o r t h C h a n n e l

We st Ch a n ne l

Oak Point

Big Bay

LAKE

SUPERIOR

Chequamegon

Bay

Houghton

Point Chequamegon Point

Lighthouse

Bay

Point

Sand

Point

Pikes

Bay

Vandeventer

Bay

Washburn

Raspberry

Bay

Frog

Bay Quarry

Bay

Julian Bay

Chebomnicon

Bay

Detour

Sea

Caves

Meyers Beach

Kayak launch

BAYFIELD

PENINSULA

LAKE

SUPERIOR

Presque Isle

Point

13

North

0

0

8 Kilometers

8 Miles

Geologic Units

Chequamegon Sandstone 

(Mesoproterozoic) PCch 

Devil's Island Sandstone 

(Mesoproterozoic) PCdi 

Orienta Sandstone 

(Mesoproterozoic) PCo 

NPS Boundary

Bedrock Outcrop

Paleocurrent 

flow directions

Figure 7. Apostle Islands and paleocurrent flow directions. Measurements on sedimentary structures such as cross-beds 

and ripple marks provide paleocurrent flow directions of rivers and shorelines that deposited the Mesoproterozoic 

bedrock of the Apostle Islands a billion years ago. Devils Island Sandstone (PCdi) has a wider range of flow-direction 

orientations due to its deposition in a shoreline environment compared to the stream currents that deposited the 

Orienta (PCo) and Chequamegon (PCch) sandstones. Because of the predominant stream flow directions to the 

northeast, bedrock exposures on the northwest or southeast sides of the islands will show layers deposited along 

the length of the ancient stream channels. Therefore, bedrock exposures on the northeast and southwest sides will 

typically show a “cross-section” of the ancient stream channels. Graphic by Trista L. Thornberry-Ehrlich (Colorado State 

University) and Georgia Hybels (Colorado State University) with data from Nuhfer and Dalles (2004, figure 5). 
11

km (620 mi) of shoreline (Curdts 2011). Approximately 

257 km (160 mi) of shoreline exist within Apostle 

Islands National Lakeshore boundaries (Curdts 2011). 

The NPS Geologic Resources Division Coastal Geology 

website, http://nature.nps.gov/geology/coastal/

index.cfm (accessed 3 April 2015), provides additional 

information.

The Apostle Islands are on the southwestern edge of 

Lake Superior, the largest freshwater lake (by surface 

area) in the world; it holds 10% of the liquid freshwater 

on Earth. Even at its great size of 82,170 km2 (31,700 

mi2), Lake Superior is much smaller than an ocean 

and thus has smaller waves and currents (Engstrom 

1985). Like all lakes, sediment is slowly accumulating in 

Lake Superior and streams incise the edges. For these 

reasons, lake waves and currents have insufficient time 

to create large landforms, such as would occur at an 

oceanic coast (Wycoff 1999). 

Lake Superior experiences only minimal astronomical 

tides, but wind sometimes pushes water up against one 

shore and it flows back the opposite direction in an 

event called a seiche. Seiche (“slosh”) waves can reach 

several meters high, have a period of 7.9 hours, and are 

almost always present in Lake Superior (Minnesota 

Sea Grant 2014). Seiches can cause “freak” or “rogue” 

waves that surprise kayakers, as well as stir up the water 

column, releasing nutrients or potentially contaminants, 

from different water levels and/or bottom sediments 

(Minnesota Sea Grant 2014). They may also form as a 

result of other events, including earthquakes, changes 

in atmospheric pressure, heavy rains, and variations in 

water density (Wycoff 1999). Lake Superior’s maritime 

lake effects and the jet stream can cause severe winter 

storms with high winds and waves (Eichenlaub 1979; 

Busch 2008). 

Within Apostle Islands National Lakeshore, a wide 

variety of wave energy settings exist, from exposed, 

high-energy locations to sheltered, low-energy 

locations (Engstrom 1974b). Waves wear away bedrock 

that fringes the lakeshore, undercutting the exposures 

(Wycoff 1999; National Park Service 2005). Bedrock 

outcrops tend to form cliff exposures on the north- and 

northeast-island shores in the face of the highest wave 

energy. Some of the larger bedrock exposures (e.g., 

CPCrs and “Outcrops” layer) are included in the GRI 

GIS data. Erosive waves sculpt the bedrock into myriad 

forms, including caves, arches, stacks, and cliffs. In 

shorelines underlain by glacial deposits, mostly on the 

western shores of the islands, waves have eroded steep 

till bluffs (see “Sea Caves and Other Shoreline Erosional 

Features” section). Waves and currents also transport 

sediments and deposit them in areas of lower energy. 

Sandscapes, including beaches with scarps, berms, and 

ridges, as well as sand spits, barrier spits and islands, 

tombolos, cuspate forelands, and dunes are formed 

and reworked by waves and wind (see “Sandscapes” 

section). 

Shorelines within the lakeshore are dynamic areas 

capable of rapid change, which is commonly a function 

of lake level. Mean annual lake levels can fluctuate 

nearly 2 m (6 ft) from one year to the next, so the 

conditions are highly variable (National Oceanic and 

Atmospheric Administration 2014). Entire islands have 

disappeared or diminished dramatically (Julie Van 

Stappen and Peggy Burkman, conference call, 19 June 

2014). Humans have witnessed entire sand beaches 

stripped away to bedrock by storms in a few hours. 

Then, the beaches were rebuilt as longshore currents 

moved sand back to the area over a period of a few 

weeks. Fallen trees and landslide scars mark areas of 

rapid erosion in the glacial-till bluffs—collectively, the 

single largest source of sediment to Lake Superior (Huff 

and Swenson 2002; Nuhfer and Dalles 2004). Along 

bedrock shores, arches collapse (e.g., Lookout Point 

off of Hermit Island in 1975); stacks and balancing 

rocks form and then crumble due to the relentless 

wave energy (e.g., at Stockton Island) (Nuhfer and 

Dalles 2004). New sea caves are rapidly developing at 

the southern end of Sand Island. The sea caves near 

Mawikwe Bay along the mainland unit shoreline are 

also changing rapidly (Nuhfer and Dalles 2004). See 

“Sea Caves and Other Shoreline Erosional Features” 

section and “Slope Movements” section).

Waves are not the only agent of shoreline change 

in Lake Superior. As water, seeps along cracks and 

fractures then freezes and expands, it wedges rock 

apart in a process called frost weathering. Furthermore, 

expanding lake ice pushes sand and gravel up the beach 

areas, creating ridges (Wycoff 1999; National Park 

Service 2005). Ice also pushes against low-lying shelves 

in the bedrock, and frozen spray icicles add tremendous 

weight to bedrock outcrops (Nuhfer and Dalles 2004). 

Because of its large volume, Lake Superior seldom 

freezes over completely. The winter of 2013–2014 had 
12

prolonged cold spells and the lake was as much as 96% 

covered with ice (fig. 8; Rodman 2014). The lake was 

covered with a similar percentage of ice during the 

2014–2015 winter. This ice cover prevented extensive 

winter-time evaporation and was responsible in part 

for higher than usual lake levels in spring 2014 after a 

trend and predictions of dropping levels (see “Shoreline 

Response to Lake-Level Change” section). The ice also 

formed ice caves and contributed to breaks and erosion 

of the sandstone cliffs. Ephemeral ice caves formed 

along the Lake Superior shorelines (fig. 9). The caves 

are formed by wind and wave action pushing slushy ice 

along the shoreline and large formations and columns 

are formed from seepage between the rock layers (Julie 

Van Stappen, written communication, 9 January 2015). 

The ice piles up in layers pocked with caves. The caves 

may feature ice stalactites.

Sea Caves and Other Shoreline Erosional 

Features

Caves are naturally occurring underground voids such 

as solutional cavities, lava tubes, sea caves, talus caves (a 

void among collapsed boulders), regolith caves (formed 

by soil piping), and glacier caves (ice-walled caves) 

(Toomey 2009). As of May 2015, cave or karst resources 

are documented in 153 parks, including Apostle Islands 

National Lakeshore, which contains sea caves. The NPS 

Geologic Resources Division Cave and Karst Resources 

website, http://www.nature.nps.gov/geology/caves/

index.cfm (accessed 10 April 2015), provides more 

information. 

Sea caves are recent features carved into the ancient 

bedrock at Apostle Islands National Lakeshore. They 

formed in exposures of the Bayfield Group (PCch, 

PCdi, and PCo) and are somewhat limited in scope. 

Figure 8. Frozen Lake Superior. This NASA satellite image of the Great Lakes was taken on 16 March 2014. Lake 

Superior was roughly 95% frozen on 19 March. Wave action pushed ice flows around the lakes’ surface and had 

marked impacts on the shorelines at Apostle Islands National Lakeshore (red box). NASA image from the Moderate 

Resolution Imaging Spectroradiometer (MODIS) available at http://earthobservatory.nasa.gov/NaturalHazards/view.

php?id=85535 (accessed 7 May 2015).
13

Most are fairly shallow and close to the water’s surface. 

The lakeshore restricts climbing in caves and bluffs 

(see “Caves and Associated Landscape Management” 

section). Sea caves are erosional features grouped with 

cliffs, embayments, rocky headlands, arches, stacks, 

windows, and wave-cut platforms, common along the 

rocky shores of the islands and Bayfield Peninsula (fig. 

10). The type of feature created depends on the nature 

of the bedrock and the orientation of its exposure to the 

waves of Lake Superior. Sea caves typically form where 

less erosion-resistant layers, exposed at water level (fig. 

11), are overlain by solidly cemented, thickly bedded, 

more resistant layers. Waves erode the less durable 

layers, incising shoreward. The more resistant layers 

remain as overhangs to incised areas called “reentrants.” 

As the reentrant deepens and sometimes coalesces with 

other reentrants, a sea cave forms until the overhang 

eventually collapses. Lateral spreading of reentrants 

is particularly common in the thinly laminated, flatbedded, flaggy sandstone of the Devils Island Sandstone 

(PCdi). Exposures can become honeycombed with 

cavities. Where Devils Island Sandstone is exposed 

along the shorelines at Sand Island, Devils Island, and at 

Mawike Bay cliffs, intricate systems of sea caves occur 

(Wright 1997; Nuhfer and Dalles 2004). 

Joints (fractures in rock, commonly parallel) are nearly 

vertical in the Bayfield Group (PCo, PCdi, and PCch). 

These joints create localized zones of weakness, which 

are exploited by wave erosion and cause the rock faces 

Figure 9. Ice caves and ice formations along shorelines. Significant ice accumulation leads to frost weathering of the 

bedrock outcrops and likely accelerates blockfall and erosion. These features are particularly popular with visitors and 

accessible when the lake surface freezes adequately. National Park Service photographs courtesy of Julie Van Stappen 

(Apostle Islands National Lakeshore), taken in January 2014.
14

Figure 10. Bedrock erosional features on Devils Island. Waves from Lake Superior carve the rocky shorelines of 

Apostle Islands National Lakeshore into features such as reentrants, sea caves, arches, windows, platforms, and stacks. 

Photographs show some of these features throughout the lakeshore. Photographs by Trista L. Thornberry-Ehrlich 

(Colorado State University), taken in July 2010. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) after 

Nuhfer and Dalles (2004, figure 9).
15

to retreat unevenly, producing a jagged shoreline with 

protruding spurs and undercut reentrants. Such a 

landscape is visible at the northeast corner of Stockton 

Island (Nuhfer and Dalles 2004). Additionally, joint 

orientations influence the formation of arches and 

stacks (see fig. 10). As a protruding spur of bedrock 

between joints is exposed to wave energy, an arch 

may be excavated. When enough of the spur material 

is removed, pillars or stacks of rock are isolated from 

the island and remain until falling or eroding away. 

Stack development is particularly evident at Eagle 

Island. Other stacks include “Honeymoon Rock” of the 

northern end of Basswood Island, and some pedestals 

on the northern end of Cat Island (Nuhfer and Dalles 

2004). 

Wave-cut platforms are also erosional shoreline features 

present at the lakeshore. They represent periods of 

relatively constant lake water elevations. Waves and 

cobbles rolled by the waves erode these platforms. 

Wave-cut platforms or benches are most common 

where less resistant shaly sandstone lies over more 

resistant, cemented sandstone near the lake’s surface. 

These frequently record older, higher lake levels and are 

visible in exposures of the Chequamegon and Orienta 

sandstones (PCch and PCo) at Outer Island, Stockton, 

and North Twin islands (Nuhfer and Dalles 2004). 

The erosional, western shoreline of Rocky Island has 

wave-cut benches and a gradual, curving shore trace 

(Thornberry-Ehrlich 2013). 

At the lake-to-land interface, most islands have 

some isolated splash pools or pools on cliff shelves. 

Dense systems of rock pools occur on the bedrock 

platform shores of Bear, Devils, and Stockton 

islands. As discussed in the “Geologic Significance 

and Connections” chapter, these pools host diverse 

chemical, biological, and ecological features 

(Lafrançois et al. 2010). Some of the pools are 

dominated by wave action and experience regular 

scouring. Interconnected splash pool zones are 

typically ephemeral with dry periods between rain 

or wave incursions. Some pools, particularly those 

closer to the islands’ trees, experience less frequent 

wave impacts, are likely influenced and/or nourished 

by groundwater, and can support lichen growth. 

They are called lichen zone pools (Lafrançois et al. 

2010). 

In shoreline areas not underlain directly by bedrock, 

other shoreline erosion features form. Thick glacial 

tills underlie most of the Apostle Islands. Glacial-till 

bluffs form where waves are eroding into the thick, 

unconsolidated material deposited by the Pleistocene 

glaciers atop many of the Apostle Islands, including 

Stockton, Raspberry, Outer, and Sand islands. Erosion 

of these bluffs produces slumping and landslide scars 

(see “Slope Movements” section). This erosion process 

also provides vast amounts of sediment to the shoreline 

system of sandscapes. Erosion is most prolific in the 

springtime when the glacial till is saturated with water 

and the wave energy is high (Nuhfer and Dalles 2004).

Sandscapes

Among the most dynamic geologic features 

within Apostle Islands National Lakeshore are the 

sandscapes—collections of landforms (however 

temporary) composed of loose particles that are 

transported and reworked by wind and water (fig. 12). 

Development of sand landforms requires (1) a source 

of sand, (2) wave energy to erode sand from its source 

and transport it elsewhere, and (3) a sheltered place 

to accumulate (Nuhfer and Dalles 2004). In general, 

sand is transported in a southwesterly direction 

along the eastern and western sides of the islands and 

accumulates on the sheltered, southern or lee sides of 

Figure 11. Sea caves forming at water level. Waves from Lake 

Superior carve sea caves into the rocky shorelines of Apostle 

Islands National Lakeshore. Note the smooth indentations 

at other levels on the cliffs. These may be the outcome of 

differential erosion among rock layers or indicate former lake 

levels. Photograph by Hal Pranger (NPS Geologic Resources 

Division) taken in August 2005 along the mainland unit’s 

shoreline on the northwestern side of the Bayfield Peninsula.
16

the islands (Engstrom 1985; Thornberry-Ehrlich 2013). 

Sandscapes at the Apostle Islands consist of beaches, 

sand spits, tombolos, cuspate forelands, barrier spits 

and islands, and dunes. They are mostly composed of 

postglacial deposits, shoreline sediment (Qc), which is 

weathered from glacial-till deposits such as the Miller 

Creek Formation (Qgl and Qgw) (Clayton 1984, 1985; 

Nuhfer and Dalles 2004). This sand supply affects the 

mean grain size of the sand (Engstrom 1974b).

Beaches

Beach configuration depends on wave conditions. 

Beaches form where waves have stacked sand onto the 

shoreline in protected coves where the sand circulates 

(fig. 12). Storm waves move sand from the beach to 

offshore bars leaving cobble-covered surfaces. By 

contrast, gentler waves move sand from the offshore 

bars onto the beach to form gently sloping beaches. 

Beach height is a function of wave energy and the 

direction of dominant wave approach; vigorous swash 

moves the sand further inland (Engstrom 1974a, 

1974b). Lower wave energy may promote wider 

beaches (Engstrom 1974a, 1974b). The forebeach is 

the lakeward-sloping zone of the beach that is between 

the low-water mark and beneath the berm of sediment 

that marks the highest waves. It is frequently washed 

by waves (see fig. 16). Berms are low, impermanent 

benches or shelves formed landward of the forebeach 

composed of material transported and deposited by 

larger storm waves (see fig. 16).

Beaches that are particularly well developed occur at 

Figure 12. Sandscape formation. Sand deposits are weathered from glacial-till bluffs of the Apostle Islands. Waves 

and longshore currents transport the weathered material, depositing it in sheltered coves and leeward sides of the 

islands. Photographs show examples from the shorelines of Apostle Islands National Lakeshore. National Park Service 

photographs courtesy of Julie Van Stappen (Apostle Islands National Lakeshore). Graphic by Trista L. Thornberry-Ehrlich 

(Colorado State University) after Nuhfer and Dalles (2004, figure 8).
17

Julian Bay and Presque Isle Bay at Stockton Island 

and on the northern side of York Island. Glacial-till 

bluffs provide nourishment for beaches at Sand Bay 

on the Bayfield Peninsula; Quarry Bay on Stockton 

Island; Lighthouse, Justice, and West bays on Sand 

Island; and a beach on the eastern side of Raspberry 

Island (Engstrom 1985; Nuhfer and Dalles 2004). 

Sand Spits, Tombolos, and Cuspate Forelands

Sand spits, tombolos, and cuspate forelands (fig. 12) 

can be transitional (e.g., sandscapes change from one 

feature to another over time) or physically connected 

to each other. Sand spits (elongate “fingers” of sand) 

typically develop on the sides of islands leeward to 

the dominant wave direction. Sand spits are well 

developed on Outer, Michigan, Rocky, South Twin, 

and Cat islands (Nuhfer and Dalles 2004). If a sand 

spit grows and extends all the way to another island, 

connecting the two, it forms a tombolo. A double 

tombolo, where sand spits extended from two 

headlands toward each other, connects Stockton 

Island and Presque Isle Point, enclosing a lagoon. 

This tombolo formed where wind and wave action 

accumulated sand between the formerly separated 

islands. The reddish sand is probably from the Miller 

Creek Formation, (Qgh, Qgl, and Qgw). The tombolo 

itself has beaches, dunes, and washover fans.

The Holocene sand (Qc), juxtaposed against the 

Mesoproterozoic Chequamegon Sandstone (PCch) 

represents more than a billion years of geologic time 

at a tangible unconformity. Cuspate forelands are 

broad, crescent-shaped landforms often projecting 

off the leeward side of an island to the dominant 

wave direction. Examples of cuspate forelands 

appear on Bear, Raspberry, Ironwood, Oak, Otter, 

South Twin, and Stockton islands (Nuhfer and Dalles 

2004; Thornberry-Ehrlich 2013). 

Barrier Spits and Islands

Barrier spits and islands (fig. 12) require abundant 

sand and strong longshore currents (fig. 13). A large, 

6.7-ha (16.5-ac) sand spit, containing a freshwater 

lagoon, occurs off the southeastern end of Outer 

Island. Rocky and Michigan islands have 2-ha (5-

ac) and a 3-ha (7.5-ac), lagoon-trapping cuspate 

forelands (fig. 14; Thornberry-Ehrlich 2013). Long 

Island is the most prominent example of a barrier 

spit and island at Apostle Islands National Lakeshore. 

It is a long, narrow barrier trending northwestward 

into the mouth of Chequamegon Bay. It features a 

complex of parallel beach rides and adjacent bogfilled swales (Bona 1990; Bona and Kiesel 1990). It is 

Net movement

of sand grains

Longshore current

Waves break

oblique to the shoreface

Sand 

washes

up

Sand 

washes 

down

Path of sand

particles

Figure 13. Longshore drift. Sand grains are transported by 

waves and longshore currents to be deposited in sheltered 

coves and leeward sides of the islands. Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University) after Capital 

Regional District graphic at https://www.crd.bc.ca/education/inyour-community/geology-processes/coastal-sediment (accessed 

9 September 2014). 

Bedrock

Washovers

Lagoon

Retreating side

Prograding side

North

Apparent migration direction

Old cliff line

Active beach

Beach ridge

Berm crest

Swamp

Foredune

Marsh trees

Immature trees

Mature trees

Juniper

Toppled mature trees

Figure 14. Model cuspate foreland. Plan view of a cuspate 

foreland off the southern shore of an Apostle Island. As sand 

is transported from the waveward to the leeward side of 

the island, the cuspate foreland retreats. Note the changes 

in vegetation from one area to another. Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University) after Engstrom 

(1985, figure 2). 
18

the only island in the lakeshore composed entirely of 

unconsolidated deposits (Qc). It lacks a bedrock core, 

which on other islands stabilizes sand deposits. The 

sand of Long Island is susceptible to barrier movement 

as a response to storms and variable longshore currents 

(Bona 1990; Bona and Kiesel 1990). Long Island 

formed as northeastward-flowing streams, such as 

the Kakagon and Bad rivers, transported sand onto 

the southern shore of Lake Superior. The sand was 

then carried northwestward by longshore currents of 

the lake (Nuhfer and Dalles 2004). The last dramatic 

change for Long Island occurred in November 1975 

Overwash fans Storm 

surge

Former 

overwash fans Peat exposed

on beach

Migrating barrier

Peat

Mainland Bay Marsh

Dunes, ridge and 

swale topography Beach LAKE SUPERIOR

A

B

C

D Migrating barrier

Rising lake levels

Falling lake levels

Figure 15. Barrier island migration. At Apostle Islands National Lakeshore, barriers composed of unconsolidated 

sediment display a characteristic array of features spanning from the open lake, up a beach front, through dunes and 

ridges, across a marsh, and to the bay toward the mainland (A). Overwash transports sand onto and across the island 

(B). Over time, with rising lake levels, the island migrates landward, building on top of the old marsh (C). If lake levels 

fall, the opposite occurs as the barrier migrates lakeward (D), which is currently the case of Long Island. However, no 

peat is exposed on the lakeshore of Long Island (Julie Van Stappen, written communication, 9 January 2015). Graphic 

by Trista L. Thornberry-Ehrlich (Colorado State University).
19

as a large storm—the same storm that sank the SS 

Edmond Fitzgerald, the largest ship to have sunk on the 

Great Lakes—connected a series of sand bars into the 

large spit that persists today (Ojakangas et al. 2011). 

The barrier spit is currently migrating lakeward, and 

the southern side is somewhat stabilized by vegetation 

(Bona 1990; Bona and Kiesel 1990). Geologists expect 

the barrier to be breached in a storm forming a true 

barrier island once again (fig. 15; Thornberry-Ehrlich 

2013). 

Dunes

Dunes are less common and much smaller than other 

sandscapes at Apostle Islands National Lakeshore (figs. 

16 and 17; see “Aeolian Processes” section). Rocky 

Island’s sandscapes include foredunes and ridge and 

swale complexes on the eastern shore (ThornberryEhrlich 2013). Foredunes are sand ridges oriented 

parallel to the shoreline that occur at the landward 

margin of a beach area (fig. 16). Excellent examples of 

foredunes occur at Stockton Island, where many are 

stabilized by vegetation. The double tombolo between 

Stockton Island and Presque Isle has the second largest 

dune area in the lakeshore. Long Island has 2.4 km (1.5 

mi) of active dunes (Thornberry-Ehrlich 2013). 

Postglacial (Recent) Surficial Deposits

Surficial deposits—including shoreline sediments, 

organic sediment, and stream (fluvial) sediment (Qc, Qp, 

and Qsm, respectively; see Map Unit Properties Table, in 

Figure 16. Dune and beach features at Stockton Island of Apostle Islands National Lakeshore. Clockwise from the 

upper left: wide beach area with foredunes in the background, sea grass stabilizing dune areas, a sand blowout 

resulting from a windstorm threatens the integrity of a boardwalk trail, and beach berm fronting a narrow forebeach. 

Photographs by Trista L. Thornberry-Ehrlich (Colorado State University; upper and bottom right; taken in July 2010) 

and Hal Pranger (NPS Geologic Resources Division; bottom left; taken in August 2005).
20

pocket)—are constantly being deposited and reworked 

on the Apostle Islands’ landscape. As described in the 

“Sandscapes” section, shoreline sediment (Qc) fringes 

much of the dynamic shoreline at the lakeshore. Long 

Island is composed of this unit. Organic sediments (Qp), 

including peat, are collecting in lagoons and marshes 

at the lakeshore. These are useful for dating purposes 

because they contain organic carbon and fossil pollen 

records (see “Paleontological Resources” section). 

As described in the “Fluvial Features” section, stream 

sediments (Qsm) are collecting along local streams and 

rivers within the lakeshore. They are mapped at the 

mouth of the Sand River (Clayton 1985). 

Fluvial Features

Many rivers that flow into Lake Superior near the 

Apostle Islands contribute to the lakeshore ecosystem. 

Sand, Siskiwit, Raspberry, Sioux, Kakagon, Bad, 

Montreal, and White rivers, and North Fish, South Fish, 

Red Cliff, Pikes, Saxine, Squaw, and Beartrap creeks are 

among the larger waterways. Bayfield Peninsula streams 

can transport upwards of 40 million kg (44,000 tons) 

of suspended sediment to Apostle Islands National 

Lakeshore area per year (Rose 1987).

Rose (1987) inventoried the water resources of Apostle 

Islands National Lakeshore and noted that few island 

streams flow perennially; most are ephemeral. Oak and 

Stockton islands have some streams with year-long base 

flows dominated by groundwater discharge and seepage 

from wetlands and beaver ponds, respectively. Smaller 

islands such as York, Raspberry, Eagle, Devils, South 

Twin, North Twin, Gull, and Ironwood have virtually no 

distinguishable surface drainage channels. 

The Sand River, which flows through the mainland 

unit, is the primary fluvial feature within the lakeshore 

(Rose 1987; Thornberry-Ehrlich 2013). Several of the 

larger rivers reach the lake forming a wetland, estuary, 

or slough. The delta at the mouth of the Sand River is an 

estuary-like area with a sand spit, bogs, and some lowlevel terraces flanking the river channel (ThornberryEhrlich 2013). These features are composed of stream 

sediment (Qsm). The Sand River valley was carved 

before the deposition of the Pleistocene Miller Creek 

Formation (Qw, Qb, Qgh, Qgl, Qgw, and Qou). During 

glaciations, it was filled with sediment then became 

re-excavated to create the modern river valley. A similar 

situation exists for Fish Creek and the Sioux River 

gorge (Ojakangas et al. 2011). Besides the Sand River, 

the smaller streams within the lakeshore, for example, 

Raspberry River and Red Cliff Creek, are generally 

short (less than 2 km [1 mi]), with steep gradients, and 

small drainage areas (Rose 1987). 

The evolution of local drainages has impacted the 

development of the sediment supply system for the 

Apostle Islands. Approximately 800 years ago, the White 

River captured the flow of the Bad River increasing the 

sediment supply to the Long Island barrier spit system 

(Ojakangas et al. 2011). The Bad and Kakagon rivers 

were once joined, and the sediments they transported 

to Lake Superior supplied the estuary system on the 

western side. As described in “Sandscapes” section, 

these sediments contributed to the formation of Long 

Island. Today, the rivers are separated and Bad River 

enters the lake on the east side of the Kakogan-Bad 

River Slough—the largest undeveloped wetland system 

on the Great Lakes (Tynan 2012). Local predominant 

longshore movement is to the east, now supplying 

Figure 17. Aeolian sand transportation and dune 

movement. Prevailing winds transport sand grains up 

the dunes, depositing them in cascades down the steep 

side. Graphic by Trista L. Thornberry-Ehrlich (Colorado 

State University).
21

sediment to Waverly Beach (Ulf Gafvert, Julie Van 

Stappen, and Peggy Burkman, conference call, 19 June 

2014).

Glacial Features

During the Pleistocene Epoch (2.6 million to 11,700 

years ago; fig. 3), climate alternated between glacials 

(ice ages) and interglacials (warm periods similar to 

modern climate). Black (1974) described the glacial 

deposits and history of Wisconsin—the namesake of 

the Wisconsinan glaciation, the most recent of the four 

major recognized ice-age events of the Pleistocene 

Epoch. Repeated glaciations scoured and reshaped 

the landscape of the northern United States, including 

Apostle Islands National Lakeshore, when massive ice 

sheets advanced from the Arctic (fig. 18). As the glacial 

lobes descended into the Great Lakes area and beyond, 

they became progressively thinner with distance from 

their source. Locally, the Superior Lobe was some 400 

m (1,300 ft) thick, a massive quantity of ice but only half 

of its maximum thickness farther north (ThornberryEhrlich 2013). 

The two major categories of glacial features at Apostle 

Islands National Lakeshore are (1) those created 

or carved by glacial ice, and (2) those deposited by 

meltwater, either “glaciofluvial” (deposited by rivers 

flowing beneath or out of glaciers) or “glaciolacustrine” 

(deposited in lakes near glaciers). Figure 19 provides 

schematic illustrations of these features. Features 

associated with glacial ice include till, moraines, 

drumlins, kettles, grooves, striations, roches 

moutonnées, and glacial erratics. Glaciofluvial and 

glaciolacustrine features include kames; eskers; braided 

streams; and outwash fans, deltas, or plains.

Investigators mapped two prominent glacial units on 

the Bayfield Peninsula and Apostle Islands—the older 

glacial till and outwash of the Copper Falls Formation 

(Qsc, Qsg, Qsu, Qsuc, and Qgg) dating back to 11,500 

years before present, and the younger lacustrine and 

glacially reworked sediments of the finer-grained 

Miller Creek Formation (Qw, Qb, Qgh, Qgl, Qgw, and 

Qou), deposited between 11,500 and 9,500 years before 

present (Clayton 1984; Ojakangas et al. 2011).

Glacial Landforms

Perhaps the greatest examples of local, glacially 

influenced landforms are the Apostle Islands 

themselves. As the glaciers descended southward, 

they flowed preferentially through topographic 

lows such as stream valleys. In the Apostle Islands 

National Lakeshore area, the Bayfield Peninsula was a 

CHICAGO NEW 

YORK

DENVER

Laurentide ice sheet

Cordilleran ice sheet

Pleistocene ice age extent

ATLANTIC

OCEAN

PACIFIC

OCEAN

CHICAGO NEW 

YORK

DENVER

Present interglacial conditions

ATLANTIC

OCEAN

PACIFIC

OCEAN

Figure 18. Comparison of Ice-age and interglacial 

conditions. Nearly half of North America was covered 

by ice during Pleistocene glaciations. Relative sea level 

dropped during glaciations (note the expanded width 

of Florida [white shading]). The location of Apostle 

Islands National Lakeshore is denoted by a green star. 

Graphic by Trista L. Thornberry-Ehrlich (Colorado State 

University), produced using base paleogeographic maps 

created by Ron Blakey (Colorado Plateau Geosystems, 

Inc.), available at http://cpgeosystems.com/index.html

(accessed 2 July 2014).
22

topographic high composed of resistant Precambrian 

bedrock (PCo, PCf, PCdi, and PCch). The peninsula 

posed an impediment to glacial flow and split the glacier 

into two lobes, the Superior to the south and west, and 

the Chippewa to the east (fig. 20). Thick, coarse-grained 

sands and glacial deposits collected between the lobes 

down the axis of the Bayfield Peninsula (Ojakangas et 

al. 2011). When the ice retreated to the north and Lake 

Superior filled with water, the resistant remnants of 

sandstone at the flanks of the ancient Midcontinent rift 

were left as islands sheltered by the Bayfield Peninsula. 

Mapped features associated with moving glaciers, part 

of the GRI GIS data, include drumlins, glacial icemovement directions, and glacial ridges. Drumlins are 

elongated linear hills that formed when a glacier flowed 

over a mass of sediment (fig. 21). They are mapped 

(Glacial Feature Lines in GRI GIS data; see table 3) on 

Stockton, Outer, Cat, Michigan, and Devils islands, as 

well as on the Bayfield Peninsula and Madeline Island. 

Primarily oriented northeast–southwest, drumlins 

indicate the direction of glacial movement in the area 

(Clayton 1984, 1985). Glacial grooves and striations also 

record the direction of ice movement as rocks entrained 

at the base of a glacier are scraped over the underlying 

bedrock. Stockton and Bear islands have excellent 

examples of glacial grooves (Nuhfer and Dalles 2004). 

Glacial Deposits

Glaciers deposit all sorts of sediment and create a 

variety of landforms (see fig. 19). As described in detail 

in the Map Unit Properties Table and the “Geologic 

History” section, glacial deposits in the area of presentday Apostle Islands include those deposited directly by 

moving ice and those deposited by meltwater in rivers 

or lakes. Such features and deposits are significant in the 

mapped area. Clayton (1984, 1985) provided detailed 

glacial histories and feature descriptions for the Apostle 

Islands area. 

Figure 19. Glacial features. Not every glacially-altered landscape contains all of these features or deposits. Prominent 

features and deposits within Apostle Islands National Lakeshore are labeled in green and include lacustrine fans, 

drumlins, glacial erratics, glaciofluvial deposits, outwash fans, outwash deltas, glaciolacustrine deposits, kames, and 

glacial grooves and striations (see fig. 19), and. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University).
23

Figure 20. Bayfield Peninsula during glaciation. The Bayfield Peninsula split the descending glacier into two distinct 

lobes—the Superior Lobe to the south and west, and the Chippewa Lobe to the east. Glaciers flowed in valleys 

between prominent knobs of bedrock that became the Apostle Islands. Location of glacial ice is approximated; the 

actual Pleistocene ice extent was much farther south (see fig. 36) to near St. Louis, Missouri (Richard Ojakangas, 

University of Minnesota Duluth, geologist, written communication 12 January 2015). Graphic by Trista L. ThornberryEhrlich (Colorado State University) with assistance from Georgia Hybels (Colorado State University).
24

Deposits associated with moving ice, and captured 

in the GRI GIS data, include reddish, loamy till from 

both the Miller Creek (Qgh, Qgl, and Qgw) and older 

Copper Falls (Qgg) formations. Glacial till (a mixed 

assortment of sediments deposited directly from glacial 

ice) mantles much of the bedrock in and around the 

Bayfield Peninsula, Apostle Islands, and Lake Superior 

itself. Within the lakeshore, only the Miller Creek 

Formation (Qgl and Qgw) is mapped. The Copper Falls 

Formation appears farther inland (Clayton 1984, 1985). 

The clay layers within the Miller Creek Formation act 

as aquicludes (rock bodies that confine groundwater) 

among more permeable layers. The buried Copper Falls 

Formation acts as a confined aquifer in the lakeshore. Its 

recharge area is at the height of the Bayfield Peninsula 

axis. Numerous artesian wells and springs occur 

throughout the Chequamegon Bay area (Ojakangas et 

al. 2011). 

The 1- to 46-m- (2- to 150-ft-) high, reddish bluffs, 

characteristic of the western sides of several Apostle 

Islands, such as Michigan, York, Bear, Oak, Raspberry, 

and Outer, are mostly glacial till (Nuhfer and Dalles 

2004). Some till forms moraines (ridges of material that 

mark the edges of a glacier). Terminal moraines, also 

called “end moraines,” mark the farthest advance of a 

glacier and help determine the extent of various glacial 

advances (see “Geologic History” section). The most 

conspicuous terminal moraines are located south of 

the Bayfield Peninsula (Peterson 1986). Glacial erratics 

(rocks transported some distance by a glacier that now 

rest upon bedrock of a different type) occur on many 

Apostle Islands. A notable example is a greenstone 

boulder, likely plucked from Canadian bedrock, 

discovered on the edge of the tombolo at Stockton 

Island (Thornberry-Ehrlich 2013).

Glaciolacustrine deposits are common in the map area 

and are associated primarily with previous stages of 

Glacial Lake Duluth and later, Lake Superior. Unlike 

glacial till, these sediments tend to have more rounded 

clasts and be well sorted with respect to grain size 

(Nuhfer and Dalles 2004). The glaciolacustrine layers 

tend to be clay rich, varved, and interbedded with other 

glacial deposits and some wave-worked glacial till. Clayrich, glaciolacustrine deposits occur on the highlands of 

some islands, including Raspberry Island (ThornberryEhrlich 2013). Repeated glacial advances over the area 

“smeared” the deposits together like peanut butter 

on bread (Ulf Gafvert, Julie Van Stappen, and Peggy 

Figure 21. Drumlin formation. Drumlins are low, 

smoothly rounded, elongate oval hills that are formed 

by the movement of a glacier atop previously deposited 

glacial till. The long axis of the drumlin is parallel to the 

direction of glacial ice movement. Drumlin locations in 

Apostle Islands National Lakeshore are indicated in the 

GRI GIS data and displayed here for Outer Island (red 

lines). Graphic by Trista L. Thornberry-Ehrlich (Colorado 

State University) with US Geological Survey topographic 

map coverage available from ESRI online.
25

Burkman, conference call, 19 June 2014). The GRI 

GIS data differentiate a variety of shoreline sediment, 

offshore sediment, lake-modified topography, waveplaned topography, and stream sediment units (see Map 

Unit Properties Table, in pocket, and “Geologic Map 

Data” section). 

Glacial Lake Stages and Holocene Lake-Level 

History

When glacial ice retreated from the Lake Superior 

basin for the last time, approximately 9,900–10,000 

years ago, lake levels fluctuated as a function of several 

factors: climate, isostatic rebound, outlet incision, and 

sedimentation (Blewett 2006). Lake levels fluctuated 

dramatically as various outlets were first exposed by 

deglaciation and then abandoned as they isostatically 

rose above other outlets (Pranger 2005). As described 

in the “Geologic History” section, meltwater ponded 

and many water-level stages of Lake Superior left 

features on the Apostle Islands landscape as a result 

of successive glacial retreats. Wave-cut terraces and 

benches mark these former highstands in a record of 

now-stranded shoreline features such as beach deposits 

(visible at Outer, South Twin, Oak, and Rocky islands), 

and wave-cut benches (visible at Oak and Bear islands) 

(Nuhfer and Dalles 2004).

Understanding the nature and timing of these major 

fluctuations is complicated due to the relatively small 

scale of the features involved (Blewett 2006) and the 

post-glacial erosion of features. The natural resource 

staff at Apostle Islands National Lakeshore would 

like additional information regarding the lake level 

history. High-resolution topographic and bathymetric 

data will reveal subtle wave-cut terraces, benches, and 

submerged shorelines (Ulf Gafvert, Julie Van Stappen, 

and Peggy Burkman, conference call, 19 June 2014). 

Analysis of the data may be a project for a GeoscientistsIn-the-Parks Program participant. Such a map sheet 

was prepared for Crater Lake by Robinson et al. (2012), 

which may be a useful resource.

Slope Movements

Slope movements are the downslope transfer of soil, 

regolith, and/or rock under the influence of gravity. 

Slopes become unstable when downward forces exceed 

the strength of the material composing the slope 

(Anderson 2003). Soil creep, rockfalls, debris flows, 

and avalanches are common types of slope movements. 

These processes and the resultant deposits are also 

known as “mass wasting” and commonly grouped 

as “landslides.” Slope movements occur on time 

scales ranging from seconds to years. Figure 22 shows 

schematic illustrations of slope movements. Slope 

movements create geologic hazards and associated 

risk throughout the National Park System. Hazards 

and risks associated with slope movements in Apostle 

Islands National Lakeshore are described in the “Slope 

Movement Hazards and Risks” section. 

Slope processes at Apostle Islands include sheetwash 

and runoff, slumping, liquefaction and mudflow, 

undercutting by seeps and springs, and undercutting 

by waves causing collapse, topple, and rockfall (Nuhfer 

and Dalles 2004). The type of slope movement in a 

particular area depends on the type of geologic material, 

the nature and steepness of the slope, and other factors 

such as vegetation, the presence of seeps and springs, 

and/or exposure to erosion by moving water (fig. 22). 

The bedrock shorelines are most prone to topple, 

blockfall, slides, or collapse of sea arches and stacks. 

The glacial-till bluffs at Apostle Islands Lakeshore 

are particularly prone to slope movements such as 

translational and rotational slides, debris flows, and 

creep. Anderson (2003) recognized that the stability 

of the glacial till bluffs along the Wisconsin Lake 

Superior shoreline is controlled by the average slope 

angle, bluff composition (particularly cohesiveness), 

amount of water in the bluff, and amount and type of 

vegetation cover. The Miller Creek Formation (Qw, 

Qb, Qgh, Qgl, Qgw, and Qou) is notorious for its 

instability, particularly in areas where oversteepened 

and/or undercut by erosion or excavation (Ojakangas 

et al. 2011). Notable slumps occur at Michigan Island 

Light and Raspberry Island (fig. 23). The glacial-till 

banks tend to erode vertically where a more resistant 

glaciolacustrine clay layer caps the deposit (see “Glacial 

Features” section). The resistant layer supports a 

columnar-shaped pillar or “hoodoo” (fig. 24) of eroding 

glacial till (Qgl and Qgw) at places such as Outer and 

Raspberry islands (Ulf Gafvert, Julie Van Stappen, and 

Peggy Burkman, conference call, 19 June 2014). 

Sedimentary Rock Features

Sedimentary rock features may contain clues as to 

the conditions at the time of deposition. For example, 

texture (size, shape, and orientation of individual 

grains) of sedimentary rocks reflects the nature 

of transport and depositional processes. Higher-
26

BEDROCK UNCONSOLIDATED MATERIALS

(including rock fragments, sheared bedrock, weathered regolith, soils)

NO BEDROCK

FLOW EQUIVALENT

EXISTS

FALLS TOPPLES SLIDES LATERAL SPREADS FLOWS

Translational Rotational

Water

content Wet Dry

Rockfall Soil fall

Debris topple

Rockslide Debris slide

Slump

Rock block slide

Rockfall avalanche Sand flow Loess flow

MAINLY LARGE

ROCK FRAGMENTS

NON-PLASTIC

SORTED SAND OR SILT

MIXED ROCKS

AND SOIL MOSTLY PLASTIC

Rapid earthflow

Sand or silt flow

Debris flow

Debris avalanche

Slow 

earthflow

Extremely 

rapid

Very 

rapid Rapid Moderate Slow Very 

slow

Extremely 

slow

3 m (10 ft)

per second

0.3 m (1 ft)

per minute

1.5 m (5 ft)

per day

1.5 m (5 ft)

per month

1.5 m (5 ft)

per year

0.3 m (1 ft)

per 5 years

Extremely 

rapid

Joint opened (e.g., by 

hydrostatic pressure,

frost or root 

wedging

Removal of support

(e.g., by river erosion,

quarrying, wave action)

B B A

C D E

Ruptured

surface

Extremely slow

to moderate

Undercut by

river

Very 

rapid

Mixed 

sediments

Clayey gravel

Clean sand

Slump

Earth flow

Loess

Glacial clay

Slow

Planar

Failure

along faults

Moderate

Dip slope controlled by 

Scarp face bedding planes

controlled 

by joints

Very slow to

extremely rapid

Very slow 

to rapid

Bedrock

Firm clay

Soft clay with

water-bearing silt

Firm clayey gravel

General lateral

movement of 

soft clays Very 

rapid

Sand

Rapid to

Very rapid

Very 

rapid

Extremely 

rapid

Dry 

sand Firm 

silt

Sand

Rapid to

Very rapid

Extremely 

rapid

Loess

Shale

Weathered

shale

Very 

rapid

Very rapid to

extremely rapid

Bedrock

Regolith

APPROXIMATE RATE OF MOVEMENT

Figure 22. Schematic illustrations of slope movements. Different categories of slope movement are defined by material 

type, nature of the movement, rate of movement, and moisture content. Gray-shaded areas depict slope movements 

that are not likely to occur in Apostle Islands National Lakeshore. Graphic by Trista Thornberry-Ehrlich (Colorado State 

University) after Varnes (1978, figure 4.33 and information therein).
27

Figure 23. Eroding glacial-till bluffs at Raspberry Island. Note the slump scarps and vegetation sliding downslope. At 

the base of the slope, the finer material has been removed by the longshore current and wave activity, leaving gravel 

and boulders. The finer material provides sediment to the sandscape system. Photograph by Trista Thornberry-Ehrlich 

(Colorado State University) taken in July 2010.

Erosion-resistant cl ay layer

Vertical erosion causes

stacks or hoodoos to 

form at the glacial till

banks

Slumping causes 

trees to fall from 

bluff edge

Clay-layer cap

Once cap is removed 

the hoodoo erodes rapidly away

Eroding glacial till

bluffs supply sediment

to the sandscape

system

Glacial

till

Figure 24. Eroding glacial-till bluffs into hoodoo-like forms. The formation of hoodoo-like vertical stacks along the 

glacial-till bluffs in places such as Raspberry Island requires a resistant cap of fine-grained clay. The cap temporarily 

protects the underlying column, but once the cap is breached, the unconsolidated material erodes quickly away into 

the waves of Lake Superior. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University).
28

energy depositional environments, such as fastmoving streams, deposit larger (heavier) clasts while 

transporting smaller (lighter) clasts. Where water 

moves slowly or is stagnant, such as in lakes, the water 

cannot transport even the smallest clasts and they are 

deposited. Wind also transports and deposits sandsized or smaller clasts (table 1). Characterization of 

texture can aid in interpreting ancient environmental 

settings. Because the ancient bedrock (PCo, PCdi, and 

PCch) formed in similar (moving water) environments 

as the modern shoreline system (Qc), the saying “the 

present is the key to the past” (an adage summarizing 

uniformitarianism) is truly applicable at Apostle Islands 

National Lakeshore. 

Sedimentary structures such as ripple marks are 

common in the Devils Island Sandstone (PCdi) (fig. 25). 

Providing an analogy, ripple marks can be seen forming 

today in the soft sands of bays such as Presque Isle or 

Julian Bay. Modern cross-bedding is revealed by digging 

into river sandbars and viewing a cross-sectional profile, 

while ancient cross-beds occur in the Chequamegon 

Sandstone (PCch). The mud cracks of the Chequamegon 

Sandstone look like mud cracks on any exposed mudflat 

today. Mud cracks are common in the quieter water 

areas such as lagoons and marshes (Cannon et al. 1999; 

Nuhfer and Dalles 2004). 

As detailed in the Map Unit Properties Table (in pocket) 

and “Geologic History” section, the sandstones of 

Figure 25. Bedrock features. Ripple marks are common in the layers of Devils Island Sandstone (geologic map unit 

PCdi) at Devils Island landing. The thin laminations typical of this unit are also visible. This thin layering lends itself to 

the formation of the spectacular sea caves, arches, windows, and stacks along some shorelines. Seeps flow along layers 

to emerge at cliff faces, causing staining of some bedrock outcrops. When the water freezes in winter, large icicles 

form on cliffs. Photographs by Trista L. Thornberry-Ehrlich (Colorado State University) taken in July 2010.
29

the Bayfield Group contain sedimentary features that 

indicate they were deposited in sediment-choked 

braided streams (PCo and PCch) and along the 

shorelines of shallow ponds and lakes (PCdi) more than 

a billion years ago.

Devils Island Sandstone Type Locality

Formal geologic formations take their names from 

geographic features such as towns, islands, and 

mountains that are near its type locality; that is, a 

place with exposures extensive enough to show all 

the mappable features of the formation, including the 

rock type, color, texture, thickness, and distinguishing 

features such as depositional structures (e.g., crossbeds) and the characteristic manner in which the 

rock weathers (e.g., forming cliffs or dissolving). Type 

localities are searchable at the US Geological Survey’s 

“Geolex” database: http://ngmdb.usgs.gov/Geolex 

(accessed 6 April 2015). 

Devils Island is the type locality for Devils Island 

Sandstone (PCdi). The type locality was first recorded 

by Thwaites (1912) and studied by Cannon et al. (1999) 

and Nuhfer and Dalles (2004) (see http://ngmdb.usgs.

gov/Geolex/Units/DevilsIsland_1390.html; accessed 6 

April 2015). Devils Island Sandstone and the adjacent 

geologic units of the Chequamegon Sandstone (PCch) 

and the Orienta Sandstone (PCo) compose the Bayfield 

Group—a formal group (composed of two or more rock 

formations) whose name is derived from the Bayfield 

Peninsula where these rocks are so well exposed (http://

ngmdb.usgs.gov/Geolex/Units/Bayfield_6720.html; 

accessed 6 April 2015). 

Douglas Fault and Other Faults

A fault is a fracture in rock along which rocks have 

moved. Faults are classified based on motion of rocks 

on either side of the fault plane (fig. 26). The three 

primary types of faults are normal faults, reverse faults, 

and strike-slip faults (fig. 26). Thrust faults are reverse 

faults with a low angle (less than 45°) fault plane. Thrust 

faults and faults of unknown offset or displacement are 

mapped within GRI GIS data. 

The Douglas Fault is the only major, named fault 

included in the GRI GIS data. It cuts across the base 

of the Bayfield Peninsula, south of Apostle Islands 

National Lakeshore, and runs from the Twin Cities into 

northwestern Wisconsin. Along portions of its length, 

the fault separates the Chequamegon Sandstone (PCch) 

of the Bayfield Group from the older Freda Sandstone 

(PCf) of the Oronto Group (Cannon et al. 1999). The 

Douglas Fault is a reverse fault, thrusting older volcanic 

rocks over younger sedimentary rocks. It forms the 

northern margin of the St. Croix horst (fig. 27; Cannon 

et al. 1999; Nicholson et al. 2006). The Douglas Fault 

is likely a structure that originally formed as part of 

the Midcontinent rift and was later “reactivated” as a 

reverse fault. Displacement estimates approach 3,000 m 

(10,000 ft). Following its formation, the Douglas Fault 

was then offset by smaller strike-slip faults.

Aeolian Processes

Aeolian processes refer to windblown erosion, 

transportation, and deposition of sediments (Lancaster 

2009). Features created by aeolian processes include 

dunes, loess, sand sheets, desert pavement, yardangs, 

and alcoves. The NPS Geologic Resources Division 

Aeolian Resource Monitoring website, http://

www.nature.nps.gov/geology/monitoring/aeolian.

cfm (accessed 6 April 2015), provides additional 

information.

Aeolian features such as dunes, barriers, and sand spits 

(fig. 16) are described in the “Sandscapes” section. 

Rock Name and Relative Energy of 

Depositional Environment

Clast Size Examples at Apostle Islands

Conglomerate (rounded clasts) or

Breccia (angular clasts)

[higher energy]

>2 mm (0.08 in)

[larger] 

Isolated layers in Orienta Sandstone (PCo) and Chequamegon 

Sandstone (PCch)

Sandstone 1/16–2 mm (0.0025–0.08 in) Devils Island Sandstone (PCdi)

Siltstone 1/256–1/16 mm (0.00015–0.0025 in) None documented

Claystone

[lower energy]

<1/256 mm (0.00015 in) 

[smaller]

Isolated shale layers in Orienta Sandstone (PCo) and 

Chequamegon Sandstone (PCch)

Table 1. Clastic sedimentary rock classification and characteristics.

Note: Claystones and siltstones can also be called “mudstone,” or if they break into thin layers, “shale.”
30

Winds scouring the surface of the glacial till banks 

along island shorelines contribute to their formation. 

Wind dries the banks causing erosion (“granular 

disintegration”) at locations such as Michigan Island 

lighthouse (Julie Van Stappen and Peggy Burkman, 

conference call, 19 June 2014). High winds also 

contribute to blowdowns (areas of downed trees) that 

occur in broad swaths through the forest. Blowdowns 

cause more disturbance than fires to lakeshore forests 

(Thornberry-Ehrlich 2013). Where trees are uprooted 

by strong winds, mounds of soil and sediment are 

exposed, creating depressions and mounds. This 

churns the local soils and is an important factor in soil 

development on the islands (Cary et al. 1979). Wind 

erosion causes the formation of blowouts which are 

small, saucer-shaped or trough-shaped depressions 

in the sand, particularly in areas where stabilizing 

vegetation is lacking or has been disturbed.

Loess (windblown silt) associated with glacial retreat 

at Apostle Islands National Lakeshore may cover some 

isolated areas of glacial outwash. These deposits are 

typically easily eroded and/or incorporated into the soil 

profile (Thornberry-Ehrlich 2013). They do not appear 

in the GRI GIS data.

Paleontological Resources

Paleontological resources (fossils) are any evidence 

of life preserved in a geologic context (Santucci et 

al. 2009). Body fossils are any remains of the actual 

organism such as bones, teeth, shells, or leaves. Trace 

fossils are evidence of biological activity; examples 

include burrows, tracks, or coprolites (fossil dung). 

Fossils in NPS areas occur in rocks or unconsolidated 

deposits, museum collections, and cultural contexts 

such as building stones or archeological resources. As of 

May 2015, 258 parks, including Apostle Islands National 

Lakeshore, had documented paleontological resources 

in at least one of these contexts. The NPS Geologic 

Resources Division Paleontology website, http://

www.nature.nps.gov/geology/paleontology/index.cfm

(accessed 10 April 2015), provides more information. 

Also see the “Paleontological Resources Inventory, 

Monitoring, and Protection” section of this report.

At more than 1 billion years old, the bedrock of the 

lakeshore is unlikely to retain body fossils. Havholm 

(2007) described potential trace fossils from the Devils 

Island Sandstone (PCdi). These structures appear to be 

bedding-parallel, sinuous burrows (Galston 2008). As 

of summer 2014, the status of these fossils is still unclear 

and a subject of ongoing research (Julie Van Stappen 

and Peggy Burkman, conference call, 19 June 2014). 

Potential microbial fossils are known from rocks of 

similar age in the surrounding region (Hunt et al. 2008). 

As described in Hunt et al. (2008), the Apostle Islands 

museum collection contains six fossil specimens, 

Figure 26. Schematic illustrations of fault types. Movement occurs along a fault plane. Footwalls are below the fault 

plane and hanging walls are above. In a normal fault, crustal extension (pulling apart) moves the hanging wall down 

relative to the footwall. In a reverse fault, crustal compression moves the hanging wall up relative to the footwall. A 

thrust fault is similar to a reverse fault, but has a dip angle of less than 45°. In a strike-slip fault, the relative direction 

of movement of the opposing plate is lateral. When movement across the fault is to the right, it is a right-lateral 

(dextral) fault, as illustrated above. When movement is to the left, it is a left-lateral (sinistral) fault. A strike-slip 

fault between two plate boundaries is called a transform fault. Graphic by Trista Thornberry-Ehrlich (Colorado State 

University).
31

including corals and brachiopods in chert and from Stockton and Brander bogs, Hemlock Lake, and 

limestone cobbles. These were likely not in situ within Lake Mary contain peat, plant and animal remains, 

but park but rather transported to the area from the and a pollen record as old as 9,500 years; they record 

north by advancing glaciers during the ice ages. Fossils vegetation and climate changes since the last major ice 

also may wash onto the Lake Superior shoreline. advance (Wilson 1935; Swain and Winkler 1983; Davis 

Pleistocene or Holocene aged fossils may be discovered et al. 1998; Hunt et al. 2008).

within the lakeshore (Hunt et al. 2008). Sediment cores 

Figure 27. Aeromagnetic map showing the location of buried structures. Aeromagnetic maps display differences in 

Earth’s magnetic field as measured from a moving aircraft along a regional grid. The maps show subsurface crustal 

structures because different rocks have different magnetic intensities. These structures are not visible at the surface 

nor do they necessarily reflect topography at the surface. Note the location of the Douglas Fault and St. Croix horst, 

which is a “block” of curst that moved upward along faults on either side. Adjacent blocks moved downward (relative 

to the St. Croix horst) along those faults. Green star is the location of Apostle Islands National Lakeshore. Red box is 

the area of the geologic map presented in Nicholson et al. (2006). Graphic from Nicholson et al. (2006) modified by 

Trista Thornberry-Ehrlich (Colorado State University).

33

Geologic Resource Management Issues

This section describes geologic features, processes, or human activities that may require management 

for visitor safety, protection of infrastructure, and preservation of natural and cultural resources in 

Apostle Islands National Lakeshore. The NPS Geologic Resources Division provides technical and 

policy assistance for these issues.

Apostle Islands National Lakeshore has abundant and and includes detailed recommendations for resource 

varied shorelines and is, therefore, considered “coastal” managers and suggested methods of monitoring. In 

and managed accordingly. The coastal environment addition, Hart and Gafvert (2006) outlined a data 

is defined broadly as the area lying at the interface management strategy to provide scientifically and 

between land and sea (or other large body of water). statistically sound data to support management 

The coast includes a zone of shallow water, where decisions for the protection of park resources for 

waves are able to transport sediment onto a beach. The the Great Lakes Network including Apostle Islands 

coastal environment also incorporates areas landward National Lakeshore. 

of this shallow water zone, including beaches, cliffs, 

bluffs, and marshes, that are affected to some degree by Shoreline Response to Lake-Level Change

the direct or indirect effects of waves and currents. The Pendleton et al. (2007) assessed the susceptibility of 

coastal environment—characterized by factors such as the shoreline to lake-level change, creating a changewave energy, sediment supply, sediment type, slope and potential index (CPI) for Apostle Islands National 

width, and past geologic history—may extend inland for Lakeshore (fig. 28). This CPI used geomorphology, 

many kilometers/miles (Wyckoff 1999). shoreline change potential (m/year), coastal slope (%), 

historical rate of relative lake-level change (mm/year), 

During the 2010 scoping meeting (see Thornberry- annual ice cover (days), and wave height (m) to create a 

Ehrlich 2013) and 2014 conference call, participants relative measure of the coastal system’s vulnerability to 

(see Appendix A) identified the following geologic the effects of lake-level change. Areas where the physical

resource management issues: effects of coastal and lake-level change might be greatest

are highlighted. The CPI provides data for resource ● Shoreline Response to Lake-Level Change management and lakeshores facilities plans. For more ● Slope Movement Hazards and Risks information about CPIs and the coastal-vulnerability 

● Caves and Associated Landscape Management index (CVI), refer to the US Geological Survey CVI 

website: http://woodshole.er.usgs.gov/project-pages/ ● Fluvial and Other Surface Water Issues

nps-cvi/. ● Abandoned Mineral Lands 

● Disturbed Land Restoration The nearly 257 km (160 mi) of shorelines in Apostle 

Islands National Lakeshore consist of sand and gravel ● Renewable Energy Development beaches (Qc), rock outcrops (PCdi, PCch, and PCo), and 

● External Mineral Development dune and glacial bluffs (Qgl and Qgw). The shoreline 

● Wind-Speed Change geomorphology is a function of the type of geologic 

foundation (i.e., rock type) present as well as its position ● Seismic Activity Hazards and Risks relative to the wave energy of Lake Superior (i.e., on the ● Paleontological Resource Inventory, Monitoring, lee side or facing direct wave energy). Rocky, cliff-lined and Protection coasts have the lowest coastal change potential whereas 

Resource managers may find Geological Monitoring sandy beaches and mudflat areas have the highest 

(Young and Norby 2009; http://go.nps.gov/ (Pendleton et al. 2007). Overall, 21% of the mapped 

geomonitoring) useful for addressing these geologic shoreline was classified as having a very high change 

resource management issues. The manual provides potential, 30% as high, 25% as moderate, and 24% as 

guidance for monitoring vital signs—measurable low change potential due to future lake-level change 

parameters of the overall condition of natural resources. (Pendleton et al. 2007).

Each chapter covers a different geologic resource 
34

Engstrom (1985) noted that if lake levels rose, beaches 

on the inner islands and spits (Qc) would decrease 

due to a limited sand supply. Declining lake levels are 

locally associated with progradation of sand features, 

building outward into the lake (fig. 29). Glacial till (Qgl

and Qgw) is a local source of sand (and other sediment) 

nourishment. Islands with thick glacial till cover, such 

as Outer, Michigan, Rocky, Cat, and South Twin, have a 

local supply of sand and would be less affected. Islands 

lacking a thick glacial till cover, such as Devils, North 

Twin, Eagle, and Gull would experience more loss of 

depositional features (Engstrom 1985; Nuhfer and 

Dalles 2004).

In the recent past, lake levels have changed profoundly 

and will continue to change (Nuhfer and Dalles 2004). 

Throughout the 1900s, the Great Lakes showed a 

trend toward increasing lake levels (Engstrom 1985), 

but since 1998, the levels have dropped to near 1930s 

Dust Bowl conditions. In 2007, the lake was 56 cm (22 

in) below normal (Climate Change Response Program 

2012). As described in the “Lake Superior Shoreline” 

section, the winter of 2013–2014 caused a lake-level 

Figure 28. Relative coastal-change potential for Apostle Islands National Lakeshore. Investigators determined the 

relative change potential index (CPI) of shorelines at Apostle Islands National Lakeshore based on six variables: 

geomorphology, regional coastal slope, rate and direction (i.e., rise and fall) of relative lake-level change, historical 

shoreline change rates, annual ice cover and mean significant wave height. The colored shorelines on the graphic 

represent these rankings from low (green) to very high (red). The highest change-potential shorelines are along 

sandy stretches with significant wave heights. The lowest change-potential shorelines are bluffs and cliffs where wave 

heights are lower. Graphic from Pendleton et al. (2007, figure 13A) available at http://pubs.usgs.gov/of/2005/1249/html/

fig13a.html (accessed 19 June 2014).
35

spike and in turn, accelerated shoreline erosion and 

change. Due to higher lake levels, beaches throughout 

the lakeshore were greatly reduced and a considerable 

amount of erosion occurred, as well as changes to sand 

spits. Near the Outer Island sand spit campsite, as much 

as 3 to 5 m (10 to 15 ft) eroded, and the tip of the sand 

spit was breached (Julie Van Stappen, chief of Resource 

Management, Apostle Islands National Lakeshore, 

written communication, 9 January 2015). The riprap 

stabilizing the northern end of Outer Island (below 

the lighthouse) used to be fronted by a large beach. In 

spring 2014, the beach disappeared due to high lake 

levels and wave action (Julie Van Stappen and Peggy 

Burkman, Apostle Islands National Lakeshore, chief of 

Resource Management and biologist, conference call, 

19 June 2014). 

Lake levels and shoreline changes are controlled by 

myriad factors such as isostatic rebound (commonly 

in response to removal of continental glaciers present 

during ice ages), sedimentation patterns, anthropogenic 

influences, and climate (Engstrom 1985; Nuhfer and 

Dalles 2004). As a result of uneven isostatic rebound, 

the Lake Superior basin is tilting, causing the northern 

and eastern portions of the lake’s bottom to rise relative 

to the southern portion (Engstrom 1985). At Outer 

Island, this rise was measured at 3 mm/year (0.1 in/

year) (Gable and Hatton 1983; Nuhfer and Dalles 2004). 

Potentially this tilt will cause southern shorelines to 

recede and lake level to rise; however, other factors, 

such as climate change, will have impacts. 

Global climate models predict the Great Lakes region 

to experience warmer, stormier, and drier climate 

conditions into the next century; summer temperatures 

in the region are projected to rise by at least 3°C (5.4°F) 

by 2100 (Pendleton et al. 2007; Great Lakes Network 

2007, 2009; Melillo et al. 2014). A warmer, drier climate 

will result in more evaporation, less recharge, and 

ultimately lower lake levels and warmer water. Since 

Lagoon

Older deposits

Lagoonal marsh

Overwash ramp

Offshore

Lower shoreface

Upper shoreface

 Dropping lake level

Beach

Progradation

Beach dune ridges

Shoreface

Retrogradation

Rising lake level Island narrowing

Dune ridges 

migrating inland Marsh deposits 

exposed on 

shoreface

Figure 29. Prograding and retrograding barrier features. The location and morphology of larger sand features such 

as Long Island and the double tombolo connecting Stockton Island and Presque Isle will be affected by progradation 

(building outward into the lake) and retrogradation (landward stepping of the shoreline) as a result of changing lake 

levels. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) based on an unpublished figure provided by 

Kathleen Farrell (North Carolina Geological Survey) in 2010. 
36

2002, warmer temperatures and lower precipitation 

have led to drought-like conditions (Great Lakes 

Network 2013). Despite recent exceptions to the trend 

(e.g., the cold winter of 2013–2014), the winter ice 

cover will also decrease in area and duration. Winter 

ice reduces surface-water evaporation and protects the 

shorelines from winter storms; decreased ice cover will 

allow the impacts of erosive wave energy to increase. 

Data from climate models suggest Lake Superior 

levels could fall at a rate of 8 mm/year (0.3 in/year) 

by 2090 (US Global Change Research Program 2000; 

Pendleton et al. 2007). Modeling by Caffrey (2007) 

suggested decreases in lake levels would eventually join 

Sand Island with the Bayfield Peninsula. Much of the 

southern Chequamegon Bay would also be exposed. 

In contrast to global oceans, the water level in the Great 

Lakes is not directly connected to the melting of polar 

ice. Thus contrary to marine coastal settings where 

relative sea level is predicted to rise, climatic models 

predict lake levels will drop. Lower lake levels will 

impact groundwater recharge, cause nearby streams and 

wetlands to shrink or disappear, and necessitate harbor 

and channel dredging to maintain shipping facilities. 

Dredging could expose contaminated sediments 

(Pendleton et al. 2007). Lower lake levels could also 

expose submerged archeological resources such as 

shipwrecks or prehistoric sites (National Park Service 

2011). Impacts to visitor use from lower lake levels 

and climate change include a shorter winter recreation 

season, infrastructure problems such as high-and-dry 

docks, and new navigational hazards such as sand bars 

(National Park Service 2011). 

Monitoring and Research of Lake-Level Change

Resource management specialists at the National Park 

Service are examining and considering the National 

Oceanographic and Atmospheric Administration’s 

(NOAA) National Ocean Service’s Lake Level Viewer 

(http://coast.noaa.gov/llv/) as a monitoring tool for 

lake level changes in Great Lakes national parks and 

lakeshores (Lynda Bell, NPS Water Resources Division, 

sea level specialist, written communication, 21 January 

2015). The US Geological Survey/National Park Service 

Water Quality Partnership Program at Sleeping Bear 

Dunes National Lakeshore has recently completed an 

investigation on the role of beaches and shallow waters 

in identifying areas prone to botulism toxin production 

and how it is related to changes in lake levels and 

surface water temperatures (Lafrançois et al. 2011). 

This work in the Great Lakes Region is being applied 

and used to enhance the understanding of lake-level 

and shore-level change in all of the Great Lakes parks, 

including Apostle Islands National Lakeshore (Lynda 

Bell, written communication, 21 January 2015).

Management of Shoreline Change

In order to protect natural and cultural resources 

located on the shorelines of Apostle Islands National 

Lakeshore, at least eight coastal engineering projects 

were undertaken. Bank and slope stabilization occurred 

at Raspberry Island to protect the lighthouse there. 

Stone revetments and slope stabilization occurred on 

Outer Island. Revetments were also constructed at Little 

Sand Bay Harbor on the Bayfield Peninsula (Coburn 

et al. 2010). No beach nourishment occurs with the 

lakeshore. Some dredging for local road work occurs at 

the Little Sand Bay marina and at Stockton Presque Isle 

Harbor (Thornberry-Ehrlich 2013).

The sandscapes within the lakeshore are very dynamic 

and heavily used. Understanding the details of how 

sand is moving around and between the islands and 

the mainland is a resource management need (Julie 

Van Stappen and Peggy Burkman, conference call, 19 

June 2014). Repeat aerial photography, GIS analysis, 

and detailed surficial geologic mapping would help in 

coastal-processes and shoreline-change models, and 

would refine the lake level-change history. This need 

was identified in 2010; discussions with the Wisconsin 

Geological and Natural History Survey to complete 

mapping are ongoing (Thornberry-Ehrlich 2013). The 

Great Lakes Network uses satellite imagery to monitor 

landscape dynamics in the Apostle Islands; analysis of 

disturbances occurs every six years as part of the Land 

Cover and Land Use program (Gafvert 2009; Great 

Lakes Network 2012b). 

Geospatial data layers available for the lakeshore 

include national land cover data, coarse-scale Landsat 

from 1984–present, color infrared (CIR) imagery, 

recent aerial photographs, high-resolution satellite 

imagery, early historical imagery, and light detection 

and ranging (LiDAR) from the US Army Corps of 

Engineers (Gafvert 2009). Such data could also be 

used in sandscape monitoring. As discussed in the 

“Sandscapes” section, Engstrom (1974a, 1974b) used 

quantitative measurements of wave statistics, wave 

characteristics, and shore morphology to create a model 

of foreshore sedimentology and morphology. 
37

Because shorelines are so dynamic, any inventory of 

these features provides merely a snapshot in time. 

Thus an inventory followed by long-term monitoring 

is needed to establish trends, understand change 

through time, and predict future conditions. In the 

Geological Monitoring chapter about coastal features 

and processes, Bush and Young (2009) described 

the following methods and vital signs for monitoring 

coastal features and processes: (1) shoreline change, 

(2) coastal dune geomorphology, (3) coastal vegetation 

cover, (4) topography/elevation, (5) composition of 

beach material, (6) wetland position/acreage, and (7) 

coastal wetland accretion. In the Geological Monitoring

chapter about marine features and processes, Bush 

(2009) described five methods and vital signs for 

monitoring marine features and processes, which also 

may be applicable to lake settings: (1) general setting of 

the environment, of which water depth is the primary 

indicator; (2) energy of the environment, waves, and 

currents; (3) barriers, including reefs and other offshore 

barriers, which block energy; (4) seafloor composition 

or substrate; and (5) water column turbidity.

Recognizing that climate is a primary driver of most 

biological and physical processes in natural ecosystems, 

the Great Lakes Network monitors land-cover and 

land-use disturbances, water quality, vegetation, 

persistent contaminants, phenology, and weather 

and climate to understand how climate change is 

affecting the entire ecosystem (Great Lakes Network 

2009, 2012a). In 2007, low lake levels required shortand long-term responses to ensure visitor access to 

facilities such as docks. The strategy for adaptation and 

mitigation responses to climate change at the lakeshore 

is evolving and ongoing (Climate Change Response 

Program 2012). Climate change management strategies 

are part of general-management planning and include 

monitoring and assessing predicted and actual impacts 

of climate change (National Park Service 2011). 

Multidisciplinary and multi-resource assessments are 

ongoing research needs at the lakeshore, although 

limited resources exist. Michigan Technological 

University and other groups are currently assessing 

climate change vulnerability of park resources. Melillo 

et al. (2014) provided a comprehensive summary of 

climate-change observations, impacts, and predictions 

for the United States. They also presented response 

strategies and guidance for mitigating climate-change 

impacts.

Slope Movement Hazards and Risks

Slope movements are a common type of geologic 

hazard—a natural or human-caused condition that may 

impact park resources, infrastructure, or visitor safety. 

Risk is the probability of occurrence combined with 

the expected degree of damage or loss that may result 

from exposure to a hazard, or the likelihood of a hazard 

causing losses (see also Holmes et al. 2013).

As described in the “Slope Movements” section, slope 

processes occur on the landscape at Apostle Islands 

National Lakeshore. These processes are natural 

elements of shoreline evolution. For example, along 

the Wisconsin Lake Superior shoreline, 85% of glacial 

till bluffs (Qgh, Qgl, Qgw, and Qou) were identified as 

unstable (Clayton 1985; Anderson 2003). This situation 

is likely comparable to that at the Apostle Islands. Some 

slope processes could threaten cultural resources and/

or visitor safety at the lakeshore. 

At Raspberry and Outer islands, shoreline stabilization 

techniques were implemented to protect historic 

lighthouses. At Raspberry Island, riprap (gabbro 

boulders), slope reduction, vegetation, log cribs, facines, 

brush layering, and drainage systems were installed to 

slow shoreline erosion (see “Management of Shoreline 

Change” section). The importation of soil unfortunately 

introduced exotic species, which required further 

resource management action. On Outer Island, riprap, 

drainage trenches, and bioengineering combined to 

maintain a more stable, natural looking slope (fig. 

30; Thornberry-Ehrlich 2013). As described in the 

“Shoreline Response to Lake-Level Change” section, 

higher lake levels have since removed a large beach that 

was lakeward of the riprap installed on the northern 

end of Outer Island (Julie Van Stappen and Peggy 

Burkman, conference call, 19 June 2014).

Rockfall occurs in the lakeshore on the islands rimmed 

by sandstone cliffs (PCdi, PCch, and PCo), although 

the parkwide frequency and magnitude of rockfall 

is not known quantitatively. Falling material ranges 

in size from pebbles and rocks to larger blocks and, 

in the case of Oak Island in 2010, an entire sea arch. 

The intersections of joints and fractures create weak 

areas susceptible to rockfall. Wave erosion and frost 

weathering—whereby water expands upon freezing, 

and the resulting ice wedges rocks apart along cracks 

and fractures—contribute to the instability of the cliffs. 

These natural processes become hazards when kayakers 
38

or visitors hiking on winter ice travel near the base of 

the cliffs or in sea caves (see “Caves and Associated 

Landscape Management” section).

Particularly hazardous areas are those with visible 

cracks, loose material, or overhangs. Alerting visitors 

to the hazards associated with rockfall near the base 

of cliffs or in sea caves is a first step toward reducing 

the risk. Such information could be presented via 

the park website, brochures, signage, and/or verbal 

communication from park staff.

If funding permits, resource managers could consider 

obtaining quantitative information to assess the 

frequency and magnitude of rockfall (and other slope 

movements) in high visitation areas. A photomonitoring 

program is one possibility. The Geoscientist-in-theParks program (http://www.nature.nps.gov/geology/

gip/index.cfm; accessed 3 April 2015) is an option to 

support such a project. The NPS Geologic Resources 

Division Photogrammetry website (http://www.

nature.nps.gov/geology/monitoring/photogrammetry/

index.cfm) provides examples of how photographic 

techniques support structural analysis of rockfall areas.

The following references provide additional 

background information, suggested vital signs, and 

resources for assessing and documenting slope 

movements: Wieczorek and Snyder (2009), Highland 

and Bobrowsky (2008), the US Geological Survey 

landslides website (http://landslides.usgs.gov/), the 

NPS Geologic Resources Division Geohazards (http://

www.nature.nps.gov/geology/hazards/index.cfm) and 

Slope Movement Monitoring (http://www.nature.nps.

gov/geology/monitoring/slopes.cfm) websites. In the 

Geological Monitoring chapter about slope movements, 

Wieczorek and Snyder (2009) provided guidance 

and described five vital signs for understanding and 

monitoring slope movements: (1) types of landslide, 

(2) landslide causes and triggers, (3) geologic materials 

in landslides, (4) measurement of landslide movement, 

and (5) assessment of landslide hazards and risks.

Caves and Associated Landscape 

Management

Cave features are non-renewable resources. The Federal 

Cave Resources Protection Act of 1988 requires the 

identification of “significant caves” in NPS areas, the 

regulation or restriction of use as needed to protect 

cave resources, and inclusion of significant caves in land 

management planning. The act also imposes penalties 

for harming a cave or cave resources and exempts park 

managers from releasing specific location information 

for significant caves in response to a FOIA request (see 

also Appendix B). 

A park-specific cave management plan has not yet been 

Figure 30. Coastal stabilization sites at Apostle Islands 

National Lakeshore. (A) shows Raspberry Island 

shoreline before stabilization, and (B) shows the 

shoreline below the lighthouse after stabilization. (C) 

shows riprap installed at the base of the slope below 

the lighthouse at Outer Island. Photographs from 

Coburn et al. (2010; figures 3, 6, and 7, respectively). 
39

completed for Apostle Islands National Lakeshore. Such 

plans include a comprehensive evaluation of current 

and potential visitor use and activities, such as climbing, 

which is currently restricted at caves in the lakeshore. 

Rockfall within and near caves is a potential visitor 

safety concern particularly during the winter when 

visitation increases dramatically (see “Slope Movement 

Hazards and Risks” section). A cave management plan 

would also address issues associated with ephemeral 

but very popular ice caves. Cave management plans 

also propose ways to survey and study known caves 

and discover new ones. The NPS Geologic Resources 

Division can facilitate the development of a cave 

management plan.

The sea caves at the lakeshore provide some limited 

bat habitat. Scientists from the Department of Natural 

Resources surveyed for bat habitat and found several 

areas. The extent is limited by the exposed nature of 

the sea caves. Resource managers are interested in a 

summer survey (Ulf Gafvert, Julie Van Stappen, and 

Peggy Burkman, conference call, 19 June 2014).

Toomey (2009)—the Geological Monitoring chapter 

about caves and associated landscapes—described 

methods for inventorying and monitoring the following 

cave-related vital signs: (1) cave meteorology, such 

as microclimate and air composition; (2) airborne 

sedimentation, including dust and lint; (3) direct visitor 

impacts, such as breakage of cave formations, trail use in 

caves, graffiti, and artificial cave lighting; (4) permanent 

or seasonal ice; (5) cave drip and pool water, including 

drip locations, rate, volume, and water chemistry, pool 

microbiology, and temperature; (6) cave microbiology; 

(7) stability issues associated with breakdown, rockfall, 

and partings; (8) mineral growth of speleothems, such 

as stalagmites and stalactites; (9) surface expressions 

and processes that link the surface and the cave 

environment, including springs, sinkholes, and cracks; 

(10) regional groundwater levels and quantity; and 

(11) fluvial processes, including underground streams 

and rivers. This information would augment cave 

management planning.

Fluvial and Other Surface Water Issues

The fluvial features at Apostle Islands National 

Lakeshore are largely ephemeral in nature. As described 

in the “Fluvial Features” section, the Sand River 

is the primary fluvial resource within the park. As 

this river meanders across its floodplain within the 

unconsolidated glacial till deposits (Qgw and Qgl) and 

its own postglacial stream sediment (Qsm), slumping 

and undercutting are occurring. 

In the Geological Monitoring chapter about fluvial 

geomorphology, Lord et al. (2009) described methods 

for inventorying and monitoring geomorphologyrelated vital signs: (1) watershed landscape (vegetation, 

land use, surficial geology, slopes, and hydrology), 

(2) hydrology (frequency, magnitude, and duration 

of stream flow rates), (3) sediment transport (rates, 

modes, sources, and types of sediment), (4) channel 

cross section, (5) channel planform, and (6) channel 

longitudinal profile.

Several open-water lagoons are associated with 

sandscapes on the Apostle Islands. Some of these are 

acidic and lack the ability to buffer acid deposition 

from precipitation. High levels of methyl mercury 

have been recorded in the Outer and Stockton islands’ 

lagoons (Rolfhus et al. 2005). The Great Lakes Network 

currently monitors water quality in the coastal wetlands 

and open-water lagoons on Outer, Stockton, and 

Michigan islands, and at the mainland’s Little Sandy 

Bay (Great Lakes Network 2011b). Detailed water 

quality discussions are beyond the scope of this report. 

The NPS Water Resources Division can provide more 

information and assistance (see http://www.nature.nps.

gov/water/; accessed 6 April 2015). 

Abandoned Mineral Lands 

According to the NPS Abandoned Mineral Lands 

(AML) database (accessed 3 June 2014) and Burghardt 

et al. (2014), Apostle Islands National Lakeshore 

contains four AML features at four sites: Breckenridge 

Quarry and Bass Island Brownstone (Basswood Island), 

Ashland Brownstone Company (Stockton Island), 

and Excelsior Brownstone Quarry (Hermit Island). 

Features include waste rock piles and a well into the 

Chequamegon Sandstone (PCch). Servicewide, AML 

features present a variety of resource management 

issues for visitor and staff safety; air, water, and soil 

quality; as well as providing habitat for animals. 

Resource management of AML features requires an 

accurate inventory and reporting. All AML features 

should be recorded in the AML database (the NPS 

Geologic Resources Division may be able to provide 

assistance). An accurate inventory can identify 

human safety hazards, and facilitate reclamation and 

restoration of features. When appropriate for resource 
40

management and visitor safety, features can also present 

opportunities for interpretation as cultural resources. 

Access to two AML features within the lakeshore 

has already been restricted with wooden fences and 

railings. The two other listed features are not ranked 

for mitigation priority (Burghardt et al. 2014). The NPS 

AML Program website, http://nature.nps.gov/geology/

aml/index.cfm, provides further information.

Disturbed Land Restoration

Apostle Islands National Lakeshore has a long history 

of human use. As detailed in the “Geologic Significance 

and Connections” section, land use ranged from 

homesteads, sandstone quarries, and fishing villages 

to lighthouses and landings. Many of the “disturbed” 

lands within the lakeshore are now considered cultural 

resources and are interpreted with signs and exhibits 

(Julie Van Stappen and Peggy Burkman, conference 

call, 19 June 2014; Burghardt et al. 2014). For example, 

bedrock quarries are now interpretive targets and 

vegetation is cleared to maintain the cultural resource 

(Thornberry-Ehrlich 2013). Structures without cultural 

significance mostly are being allowed to reach a state 

of equilibrium with the natural environment (fig. 31; 

Thornberry-Ehrlich 2013). Extensive logging took 

place between the mid-1800s and 1940 with some 

sporadic logging occurring as recently as 1974. Forests 

are recovering, but with targeted species in lower 

abundance than historically. 

Figure 31. Disturbed lands at Apostle Islands National Lakeshore. Bedrock outcrops are occasionally vandalized with 

graffiti or removal of material. The quarry on Hermit Island still contains cut Chequamegon Sandstone (geologic 

map unit PCch) blocks. These blocks were ready for transport but left in place, and are now interpretive targets. Old 

homesteads are being naturally reclaimed in the forests on some islands such as Ironwood Island. Photographs by 

Trista L. Thornberry-Ehrlich (Colorado State University; left and upper right) taken in July 2010; and Hal Pranger (NPS 

Geologic Resources Division; bottom right) taken in August 2005. 
41

Coastal beaches and dune areas are particularly 

vulnerable to degradation from disturbances caused by 

past logging, heavy visitor use, and exotic vegetation. 

Restoration strategies include revegetation, removing 

exotic species, limiting access, and recontouring the 

land surface (Pranger 2005). Restoration, primarily 

consisting of planting vegetation, is done where impacts 

have been caused by current and/or past human 

activities. Restoration work has occurred on Ironwood, 

Long, Oak, Raspberry, South Twin, and Stockton 

islands, and on a number of sandscapes (Julie Van 

Stappen, written communication, 9 January 2015).

Park resource managers are uncertain how much 

upland soil erosion resulted from logging activities 

(Thornberry-Ehrlich 2013). Comparison of the 

composition of diatoms (siliceous algae) in lakesediment cores reveals a significant shift in water quality 

as a result of farming and logging around the time of 

Euro-American settlement. Changes could be due to 

increased sedimentation. Monitoring modern diatom 

community composition and comparing it to historical 

variations are providing a means of understanding the 

nature of water quality changes in Lake Superior and 

whether changes are due to climate change, land use, or 

natural factors (Great Lakes Network 2011a). 

Renewable Energy Development

Generation and transmission of renewable energy 

includes utility-scale solar, wind, geothermal, off-shore 

wind technologies, and hydroelectricity. The National 

Park Service uses a combined technical and policy 

approach to manage and protect park resources and 

values as renewable energy resources are identified and 

developed near NPS areas. Park resources and values 

that may be impacted by renewable energy development 

include water quantity and quality, air quality, wildlife, 

dark night skies, natural soundscapes, cultural 

resources, scenic views, soils, geologic and hydrologic 

processes, and visitor experience. The NPS Geologic 

Resources Division Renewable Energy website, http://

nature.nps.gov/geology/energy/index.cfm, provides 

more information.

Proposals were being circulated in 2010 to develop 

wind farms on Madeline Island and the mainland. 

Development would have significant impacts to the 

lakeshore’s viewshed and temporarily increase local 

traffic during the construction period (ThornberryEhrlich 2013). Since that time, no new proposals have 

been made and no renewable energy development 

has occurred (Julie Van Stappen and Peggy Burkman, 

conference call, 19 June 2014).

External Mineral Development

The National Park Service works with adjacent land 

managers and other permitting entities to help ensure 

that National Park System resources and values are not 

adversely impacted by external mineral exploration and 

development. Potential impacts include groundwater 

and surface water contamination, erosion and siltation, 

introduction of exotic plant species, reduction of 

wildlife habitat, impairment of viewsheds and night 

skies, excessive noise, and diminished air quality. Visitor 

safety and overall degradation of the visitor experience 

are particular concerns. 

As of June 2014, a large iron mine is being proposed 

for an area 50 km (30 mi) south of Apostle Islands 

National Lakeshore (Laurel Woodruff, US Geological 

Survey, geologist, conference call, 19 June 2014). 

Preliminary drilling for an economic assessment of 

the Gogebic Iron Range of Wisconsin is underway. 

The Gogebic Iron Range includes Proterozoic strata of 

the Keweenawan Supergroup and, prior to the 1960s, 

was one of the Lake Superior region’s major ironproducing districts (Cannon et al. 1999). A possible 

outcome is a large, open-pit mine. An operation of 

this scale would introduce significant impacts to the 

lakeshore area including increased traffic, air pollution, 

and noise. The most important potential impact is 

related to water quality from streams such as Bad River 

flowing from the mine area to the lakeshore. In order to 

anticipate potential impacts, factors such as water flow, 

sedimentation, and currents should be incorporated 

into modeling (Laurel Woodruff, Ulf Gafvert, Julie Van 

Stappen, and Peggy Burkman, conference call, 19 June 

2014). 

Miller (date unknown) detailed the mineral 

occurrences within and adjacent to the lakeshore, 

including early efforts to search for oil and gas. The 

NPS Geologic Resources Division is available to 

provide lakeshore managers with policy and technical 

assistance regarding minerals and energy issues. 

Recommendations include remaining aware of public 

and private mineral ownership and speculation, 

exploration, or drilling activity on lands in the vicinity 

of the lakeshore. Regulations and permit procedures 

vary among states. The NPS Geologic Resources 
42

Division Energy and Minerals website, http://nature.

nps.gov/geology/minerals/index.cfm, provides 

additional information. 

Wind-Speed Change

Blowing wind is one of the agents of landform change 

at Apostle Islands National Lakeshore (see “Aeolian 

Processes” section). Local wind speeds are increasing 

at rates approaching 5% per decade (Desai et al. 2009). 

This is likely related to climate change (Desai et al. 2009; 

Julie Van Stappen and Peggy Burkman, conference call, 

19 June 2014; see “Shoreline Response to Lake-Level 

Change” section). Increasing temperatures of air and 

surface water, decreases in winter ice, and a reduction 

in the temperature gradient between air and water are 

destabilizing the atmospheric surface layer above the 

water surface causing an increase in wind speeds (Desai 

et al. 2009). Increased wind speeds may accelerate 

erosion of glacial till bluffs, increase tree blowdowns, 

and promote the formation of aeolian features in 

sandscapes. 

In the Geological Monitoring chapter about aeolian 

features and processes, Lancaster (2009) described 

the following methods and vital signs for monitoring 

aeolian resources: (1) frequency and magnitude of dust 

storms, (2) rate of dust deposition, (3) rate 

of sand transport, (4) wind erosion rate, (5) 

changes in total area occupied by sand dunes, 

(6) areas of stabilized and active dunes, (7) 

dune morphology and morphometry, (8) dune 

field sediment state (supply, availability, and 

mobility), (9) rates of dune migration, and (10) 

erosion and deposition patterns on dunes. 

Monitoring aeolian features and processes may 

aid in predicting the impacts of changing wind 

speeds on the landforms at the lakeshore.

Seismic Activity Hazards and Risks

Seismic activity (earth shaking) may occur 

when rocks suddenly move along a fault, 

releasing accumulated energy and causing 

earthquakes (see Braile 2009). Earthquake 

intensity or magnitude ranges from 

imperceptible by humans to total destruction 

of developed areas and alteration of the 

landscape. The “Richter magnitude” is 

a measure of the energy released by an 

earthquake. Earthquakes can damage site 

infrastructure directly, or trigger other hazards 

such as slope movements on the bluffs above the river 

that may impact site resources, infrastructure, or visitor 

safety. 

Apostle Islands National Lakeshore is not located near 

an active seismic zone. The US Geological Survey’s 

earthquake probability maps (http://geohazards.usgs.

gov/eqprob/2009/, accessed 1 August April 2014) 

indicate an almost 0% probability of a magnitude-5.0 

(moderate) or greater earthquake occurring in the 

next 100 years at Apostle Islands National Lakeshore 

(fig. 32; Peterson et al. 2008). Since melting of the last 

ice age continental glaciers, isostatic rebound of the 

land surface may cause small earthquakes to occur 

occasionally at the lakeshore as the crust adjusts to the 

lack of weight of the ice; however, most of these events 

range in magnitude between 2 and 3 and are too minor 

to be felt by humans (Thornberry-Ehrlich 2013). 

If resource management staff members are interested 

in seismic potential or activity in the area, the 

multidisciplinary Earthscope program is serving seismic 

data to measure motions of Earth’s surface (see http://

www.earthscope.org; accessed 6 April 2015). This 

program has magnetotelluric stations and seismometers 

in the Bayfield area. The NPS Geologic Resources 

Figure 32. Probability of earthquakes with magnitude greater than 

5.0. This probability assumes a 100-year timespan and a 50-km (30-

mi) radius around Apostle Islands National Lakeshore (green star). 

The very low probability does not rule out smaller earthquakes or 

the chance that a larger earthquake may occur. Graphic generated 

using the US Geological Survey earthquake probability mapping 

program (https://geohazards.usgs.gov/eqprob/2009/index.php; 

accessed 1 August 2014).
43

Division Seismic Monitoring website (http://nature.

nps.gov/geology/monitoring/seismic.cfm; accessed 6 

April 2015) and the US Geological Survey Earthquakes 

Hazards website (http://earthquake.usgs.gov/; accessed 

6 April 2015) provide additional information.

Paleontological Resource Inventory, 

Monitoring, and Protection

As discussed in the “Paleontological Resources” section, 

Apostle Islands National Lakeshore contains possible 

ancient trace fossils and fossils in glacial deposits, and 

has the potential for younger fossils. All paleontological 

resources are non-renewable and subject to scienceinformed inventory, monitoring, protection, and 

interpretation as outlined by the 2009 Paleontological 

Resources Preservation Act (see Appendix B). As 

of May 2015, Department of Interior regulations 

associated with the act were still under development.

A field-based paleontological resource survey has not 

been completed for Apostle Islands National Lakeshore 

but could provide detailed, site-specific descriptions 

and resource management recommendations. Hunt et 

al. (2008) suggested development of a paleontological 

resource management plan including in situ fossils 

and those that may potentially wash onto the shoreline 

at the national lakeshore and become subjected to 

“beachcombing.” In the Geological Monitoring chapter 

about paleontological resources, Santucci et al. (2009) 

described five methods and vital signs for monitoring 

in situ paleontological resources: (1) erosion (geologic 

factors), (2) erosion (climatic factors), (3) catastrophic 

geohazards, (4) hydrology/bathymetry, and (5) human 

access/public use. Brunner et al. (2009) discussed policy 

and resource management considerations associated 

with NPS fossils along coastlines.

45

Geologic History

This section describes the chronology of geologic events that formed the present landscape of Apostle 

Islands National Lakeshore.

2.5 Billion to 1.6 Billion Years Ago (Archean and Michigan. Later basalts of the Bergland Group 

Eon and Paleoproterozoic Era)—North include the Chengwatana Volcanics in Wisconsin and 

American Craton Forms the Portage Lake Volcanics in Michigan (figs. 34B and 

Wisconsin contains some of the oldest rocks in North 34C; Hinze et al. 1997; Cannon et al. 1999). The Portage 

America. Before 2.5 billion years ago, granitic crust Lake Volcanics, famous for their native copper deposits, 

formed and was metamorphosed in a series of events were flood basalts erupted rapidly between 1.096 

that related to the formation of the North American billion and 1.094 billion years ago (fig. 34D; Davis and 

craton—the old, stable, core of the continent (fig. 33A). Paces 1990; Cannon et al. 1999). Those copper deposits 

Periodic mountain-building orogenies, uplift, erosion, would eventually drive centuries of copper mining 

and volcanism occurred from about 1.8 billion to 1.6 and associated industry as interpreted at Keweenaw 

billion years ago (Nuhfer and Dalles 2004). These rocks National Historical Park in Michigan. During periods 

(“W” geologic map units shown only in cross sections) of volcanic quiescence, conglomerate and sandstone 

are now buried deep beneath the Apostle Islands (see were deposited. Collectively, the 30-km (19-mi) thick 

fig. 5), but their presence impacted the deposition of the accumulated volcanic, plutonic, and sedimentary rocks 

rocks exposed in the lakeshore. Mesoproterozoic (1.6 compose the Keweenawan Supergroup (Behrendt et 

billion–1.0 billion years ago) sedimentary units thicken al. 1988; Cannon et al. 1999; Thornberry-Ehrlich 2013; 

away from White’s Ridge, one such structure comprised Richard Ojakangas, University of Minnesota Duluth, 

of “W” rocks. More than 2.5 billion years ago White’s geologist, written communication, 12 January 2015). 

Ridge was a regional highpoint. The Freda Sandstone 

(PCf)—which is on the geologic map for Apostle Downward movement along normal faults associated 

Islands National Lakeshore but not mapped within the with the rift created a major graben (basin) in the Lake 

boundaries—is the oldest geologic unit draped over the Superior region. Following the eruption of the Lake 

ancient ridge (Cannon et al. 1999). Shore Traps approximately 1.086 billion years ago, 

volcanism ceased during the next phase of the rift 

1.6 Billion to 1.0 Billion Years Ago development (Cannon and Nicholson 1992; Ojakangas 

(Mesoproterozic Era)—Continental Rifting, et al. 1997; Cannon et al. 1999). Volcanic rocks on the 

Volcanism, and Sandstone Deposition flanks of the rift were eroded, supplying sediments that 

Approximately 1.1 billion years ago, the craton of the would be deposited as conglomerate and sandstone of 

North American continent began to split in response the Oronto Group, including the Freda Sandstone (PCf), 

to a rising mantle plume or body of molten rock. In which is the youngest formation of the group (fig. 34E; 

a setting analogous to today’s East African rift, the Cannon et al. 1999; Nuhfer and Dalles 2004; Ojakangas 

Midcontinent rift formed in several stages as the crust et al. 2011). 

thinned over tens of millions of years from about 1.1 

billion (figs. 34A and 33B) to 1.05 billion years ago Older, granitic rock on the rift flanks supplied sediment 

(Hinze et al. 1997; Ojakangas et al. 1997). At this time, to the basin as it continued to subside. Streams carried 

the proto–North American landmass called “Laurentia” sediments northward and eastward from the high relief 

was part of the supercontinent Rodinia, which included flanks (fig. 35). These younger sediments of the Bayfield 

most continental crust in existence at the time. Group buried the older rocks of the Oronto Group. 

The Bayfield Group, which is exposed at Apostle 

Initial rifting involved fracturing of Earth’s crust, Islands National Lakeshore, comprises the Orienta 

which led to eruptions of lava and intrusions of molten Sandstone (PCo), the Devils Island Sandstone (PCdi) and 

material (plutons) along the rift. Lava flowed from vents the Chequamegon Sandstone (PCch); it was deposited 

along the center of the rift, which spread laterally. The approximately 1.0 billion years ago (Cannon et al. 1999; 

earliest lavas to erupt (about 1.109 billion years old) are Nuhfer and Dalles 2004). 

basalts of the Powder Mill Volcanic Group in Wisconsin 
46

The Orienta Sandstone (PCo) is an arkose (feldsparrich sandstone) derived, in this case, from weathered 

granites (Culler and Berendsen 1988; Nuhfer and Dalles 

2004). The mineral feldspar tends to break down faster 

than quartz, so as sediments containing these minerals 

are transported, weathered, and worked by wind and 

water, the feldspar component gradually diminishes 

with respect to the quartz component. Feldspar-rich 

sediment typically accumulated relatively close to its 

source rock. Sedimentary features such as ripples and 

Figure 33 A–C. Schematic graphics illustrating the evolution of the landscape and geologic foundation of Apostle 

Islands National Lakeshore. Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) using information from 

Ojakangas et al. (2011), Nuhfer and Dalles (2004), Clayton (1984), and Cannon et al. (1999).
47

cross-beds indicate the Orienta Sandstone collected 

in channels of northeastward-flowing streams. As 

the accumulating sands began to fill the local basin, 

shallow lakes surrounded by sandy beaches formed 

in the Apostle Islands area. Waves, wind, and streams 

reworked the sediments to a fine-grain size and nearly 

pure-quartz composition of the Devils Island Sandstone 

(PCdi) (Cannon et al. 1999; Nuhfer and Dalles 2004; 

Galston 2008). 

After the deposition of the Devils Island Sandstone, 

the area subsided again, allowing further accumulation 

of sediment (fig. 33C). Streams resumed their 

northeasterly flows across the landscape and deposited 

the feldspar-rich sediments of the Chequamegon 

Sandstone (PCch). These streams were likely 

sediment-choked, shallow, and braided across several 

simultaneous or successive channels (Nuhfer and Dalles 

2004). As sediment from one channel built up high 

enough, streamflow was diverted to a lower level, thus 

accumulating a nearly continuous blanket of thick sand, 

conglomerate, and clay-poor deposits throughout the 

area (Nuhfer and Dalles 2004). 

At the end of Bayfield Group deposition, fluvial 

and lacustrine sediment totaled more than 800 m 

(2,600 ft) in thickness. Encroaching Paleozoic seas 

deposited thick layers of sediments atop the Bayfield 

Group (Thornberry-Ehrlich 2013; Nuhfer and Dalles 

2004; Richard Ojakangas, written communication, 

12 January 2015), then the entire stack of rocks was 

buried and slightly deformed (Nuhfer and Dalles 

2004). Deformation may have occurred as a result of 

compression during the Grenville Orogeny east of the 

rift (fig. 34F). 

Figure 33 D–E. Schematic graphics illustrating the evolution of the landscape and geologic foundation of Apostle 

Islands National Lakeshore (continued). Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) using 

information from Ojakangas et al. (2011), Nuhfer and Dalles (2004), Clayton (1984), and Cannon et al. (1999).
48

The third stage in rift history, 

which began about 1.08 billion 

years ago and culminated at 

about 1.06 billion years ago, was 

a compressional event coincident 

with major mountain building 

during the Grenville orogeny that 

ultimately resulted in the assembly 

of the supercontinent Rodinia 

(Cannon 1994). The normal 

faults along the Midcontinent rift 

were reversed, and the central 

graben was uplifted relative 

to the rift flanks between 1.04 

billion and 950 million years 

ago. This was after and perhaps 

concurrent with Bayfield Group 

deposition (Ojakangas et al. 2011; 

Thornberry-Ehrlich 2013). 

During the Neoproterozoic, 

approximately 760 million years 

ago, in a setting similar to the 

Midcontinent rift, tectonic forces 

(extension) began to pull the 

supercontinent Rodinia apart. 

Breaking up of the supercontinent 

formed a basin that eventually 

became the Iapetus Ocean (a 

precursor to the Atlantic Ocean) 

far to the east of the Lake Superior 

region, which was in the center of 

the North American craton (fig. 

36). Rifting was completed by 

about 541 million years ago (start 

of the Paleozoic Era), and the 

Iapetus Ocean separated ancient 

North America (Laurentia) from 

the other continents (Volkert and 

Drake 1999).

541 Million to 66 Million 

Years Ago (Paleozoic and 

Mesozoic Eras)—Burial and 

a Missing Rock Record

After millions of years of tectonic 

stability at the beginning of 

the Paleozoic, tectonic unrest 

commenced once again along 

the eastern margin of North 

Figure 34. Schematic graphics illustrating the evolution of the Midcontinent rift. 

The rift developed in a series of events: A–F. Large arrows indicate the tectonic 

stress direction (e.g., extension or compression). The graphics are not to scale 

and do not represent any particular cross section through the rift. Graphic by 

Trista L. Thornberry-Ehrlich (Colorado State University) after Hinze et al. (1997, 

figure 19).
49

America. A series of landmass collisions and mountainbuilding orogenies throughout the Paleozoic Era built 

the Appalachian Mountains and culminated in the 

formation of another supercontinent—Pangaea. At this 

time, the Bayfield Group remained buried under thick 

piles of sediment in northern Wisconsin. 

During the Triassic Period, approximately 200 million 

years ago, the supercontinent Pangaea began to 

rift apart forming fault-bounded rift basins along 

the eastern coast of North America. Unlike the 

Proterozoic Midcontinent rift, this rifting resulted in the 

development of an ocean (Atlantic Ocean). The rifting 

led to the configuration of continents that persists 

today and the Atlantic Ocean continues to widen. The 

effects of these events were focused east of northern 

Wisconsin. 

66 Million Years Ago to Present (Cenozoic 

Era)—Erosion, Ice Age Glaciation, and 

Rebound

Rocks from much of the Paleozoic, all of the Mesozoic, 

and early Cenozoic eras does not exist in this part of 

Wisconsin. Whatever rocks were deposited atop the 

Bayfield Group were stripped away by erosion and 

weathering over millennia. Approximately 2.5 million 

years ago, after hundreds of millions of years of erosion 

and weathering, northeastward-flowing, ancestral 

streams had eroded valleys down into the sandstones 

of the Mesoproterozoic Bayfield Group in the Apostle 

Islands area. The rock types, textures, and grain sizes 

Crossbeds with

current

orientations

Channel cross

sections with 

cut-and-fill 

structures

Older 

source

rocks

Channel sand

and gravel

Grainsize and

feldspar component

decrease downstream

Shallow lakes 

and ponds Ripple marks

and planar 

bedding

Well-sorted, quartz

sand, beach sediments

Depositional setting of Orienta (PCo)

and Chequamegon (PCch) sandstones

Depositional setting of Devils Island Sandstone (PCdi)

Figure 35. Depositional settings of the Bayfield Group. 

Sediment-choked, braided streams deposited the 

Orienta and Chequamegon sandstones (PCo and PCch).

The Devils Island Sandstone (PCdi) was deposited as 

beach, dune, and shallow-lake sands. Sedimentary 

structures resulting parallel and perpendicular to flow 

are show in the cross sections. Colors in cross section 

are standard colors approved by the US Geological 

Survey to indicate different time periods on geologic 

maps; they also correspond to the colors on the Map 

Unit Properties Table (in pocket). Graphic by Trista L. 

Thornberry-Ehrlich (Colorado State University) after 

Nuhfer and Dalles (2004, figure 4). 

Figure 36. Paleogeographic map of North America 550 

million years ago. At this time, northern Wisconsin 

was in the center of the craton and the sediments 

of the Bayfield Group were deeply buried. Red star 

indicates approximate present-day location of Apostle 

Islands National Lakeshore. Graphic compiled by Trista 

L. Thornberry-Ehrlich (Colorado State University). Base 

paleogeographic map by Ron Blakey (Colorado Plateau 

Geosystems), available at http://cpgeosystems.com/nam.

html (accessed 8 May 2015).
50

of the sandstones (directly related to their depositional 

setting) probably influenced stream activity and the 

location of these early valleys. During deposition, major 

channels of the Mesoproterozoic streams would have 

carried coarser sediments, creating more resistant 

sandstone layers while adjacent smaller streams would 

have deposited finer, more easily eroded materials. 

Those streams preferentially incised the softer, 

more easily eroded layers. As ridges divising ancient 

watershed eroded away, isolated hills (underlain by the 

coarser sandstones) remained. Those hills are likely the 

precursors to the Apostle Islands (Nuhfer and Dalles 

2004). 

As described in the “Glacial Features” section, during 

Figure 37. Glaciations in Wisconsin. Glacial ice advanced and retreated many times over the landscape. Each advance 

obscured or obliterated evidence of previous advances, thus the record is best for the most recent major glaciation—

the Wisconsinan (purple on graphic). A large area in the southwestern part of the state, called the “driftless area,” 

was not covered by glacial ice. “Drift” is rock material transported and deposited by either a glacier or meltwater. 

Graphic by Trista L. Thornberry-Ehrlich (Colorado State University) after Syverson and Colgan (2004) and Ojakangas et 

al. (2011, figure 9) and. Shaded-relief base map by Tom Patterson (National Park Service), available at:

http://www.shadedrelief.com/physical/index.html (accessed 18 August 2014).
51

the Pleistocene Epoch, global climate shifts brought 

alternating cold periods—ice ages—and relatively warm 

periods (similar to modern climate). Continental ice 

sheets descended south from the Arctic, reshaping the 

landscape. The impacts to the Apostle Islands and Lake 

Superior were extensive. Successive glaciations tend to 

obliterate or obscure evidence of former glaciations. 

The record of glacial effects at the Bayfield Peninsula 

and Apostle Islands only extends back to about 

30,000 years ago, but adjacent areas provide evidence 

that glaciers advanced and retreated across the Lake 

Superior basin many times before then (fig. 37; Nuhfer 

and Dalles 2004). 

Twenty-thousand years ago, at the time of the St. Croix 

advance (table 2), glacial ice flowed southwestward 

into an area of isolated hills that were the precursors to 

the Apostle Island. The glaciers flowed into the valleys 

first and were divided into lobes (Superior Lobe to the 

west and Chippewa sublobe to the east) by the hills 

and the highlands that would become the Bayfield 

Peninsula. Subsequent flows of thicker ice likely 

covered everything (fig. 33D), scouring the hills to bare 

rock. Stones plucked from other areas were scraped 

over the landscapes along the base of the flowing 

glaciers forming glacial grooves and striations. A series 

of other local advances—named for areas where they 

were described; Tiger Cat, Hayward, Swiss, Airport, 

Lake Ruth, Porcupine, and Lake View—followed the 

St. Croix advance (Clayton 1984). The Airport advance 

was the last glacial advance to cover the interior of the 

Bayfield Peninsula at about 12,300 years ago (Clayton 

1984; Ojakangas et al. 2011). 

As climate warmed in the late Pleistocene and early 

Holocene epochs, the glaciers melted and retreated 

from south to north. Incredible volumes of meltwater 

were released. The southern edge of the retreating ice 

sheet barricaded local streams, which formerly flowed 

north, and water ponded up until it reached a level 

that flowed through a southerly outlet in the ice. The 

modern basins in which these lakes stood lent their 

names to the glacial lakes (e.g., Lake Nemadji, Lake 

Brule, Lake Ashland, and Lake Ontonagon). The glacial 

lakes along the southern edge of the retreating ice 

eventually coalesced to form Lake Superior (Clayton 

1984). Lacustrine sediments from Lake Superior 

and earlier lakes were deposited on top of glacial tills 

throughout the area (Scholz 1984). 

Atop the Bayfield Peninsula and Apostle Islands, which 

were relatively high topography, glacial ice was thinner. 

The thinner ice melted faster than thicker ice in adjacent 

areas and provided a basin for some of the meltwater 

streams that flowed down the St. Croix spillway. 

Eventually, this channel drained Lake Superior, which 

then stood about 145 m (475 ft) higher than its present 

level. A high-water stage of Lake Superior followed 

each glacial advance and retreat. The outlet for Glacial 

Lake Duluth (the best developed of the high-level stages 

and beaches of Lake Superior) was near Winneboujou, 

Wisconsin. A 30-m- (100-ft-) deep channel incised 

when the lake drained about 9,900 years ago via the 

Bois-Brule River outlet into the St. Croix River (Clayton 

1984; Ojakangas et al. 2011). At the time, water flowed 

south through the channel. Today, the modern Brule 

River flows as an underfit stream (a stream too small to 

Glacial Advance Years Before Present Surface Elevation Associated Glacial Lakes Associated Surficial 

Geologic Units

St. Croix 20,000 Unknown None documented Copper Falls Formation 

(Qsc, Qsg, Qsu, Qsuc, and 

Qgg) Tiger Cat <18,000? Unknown None documented

Hayward 16,000? 375 m (1,230 ft) Grantsburg, Upham

Swiss 14,000? 306 m (1,004 ft) Grantsburg, Upham

Airport 12,300 Unknown None documented

Lake Ruth 11,500 380–390 m (1,250–1,280 

ft)

Nemadji?

Porcupine 11,000 Unknown Nemadji, Ontonagon, 

Upham

Miller Creek Formation 

(Qw, Qb, Qgh, Qgl, Qgw, 

and Qou) Lake View <10,000 Unknown Nemadji, Ontonagon, 

Ashland, Brule

Table 2. Pleistocene glacial advances recorded in the Apostle Islands region.

Sources: Clayton (1984, 1985) and Nuhfer and Dalles (2004)
52

have carved its current valley) through this large notch 

northward into Lake Superior. 

The lake was at its highest level, 335 m (1,100 ft), 9,500 

years ago (Ojakangas et al. 2011). At least five lower 

levels occurred, but evidence of them was obscured 

by wave action. At each temporary shoreline, wave-cut 

lake terraces formed, recording previous elevations: 

145 m (475 ft), 116 m (380 ft), 87 m (285 ft), 67 m (220 

ft), 37 m (120 ft), and 6 m (20 ft) above the present 

normal elevation of Lake Superior at 183 m (602 ft) 

above sea level (Clayton 1984; Nuhfer and Dalles 2004). 

Approximately 8,000 years ago, during the Houghton 

stage, Lake Superior’s elevation was nearly 73 m (240 ft) 

below the present lake level (Farrand 1960; Nuhfer and 

Dalles 2004). During the height of the Nipissing stage 

about 4,500 years ago, the water level was about 186 

m (610 ft) above sea level. At this time, its outlet (Port 

Huron) was downcut, and the level of Lake Superior fell 

until about 2,200–2,100 years ago when water in Lake 

Huron dropped below Sault Ste. Marie and became 

a separate basin. This occurred as a result of uneven 

isostatic rebout of the St. Marys River outlet to Lake 

Superior (Hansel et al. 1985; Bona 1990; Larsen 1999). 

The Apostle Islands weathered in response to relentless 

wave energy from Lake Superior. As lake levels 

continuously fluctuate, the record of shoreline features 

at different levels are either perched on the islands or 

remain submerged offshore. As described in the “Lake 

Superior Shoreline” section, the modern shorelines 

(fig. 33E) are in a near constant state of flux as bedrock 

and glacial-till bluffs are worn away and sediment 

transported around, among, and beyond the Apostle 

Islands. The double tombolo joining Presque Isle with 

Stockton Island may have begun forming as recently 

as 2,500 years ago; however, pollen records from the 

bog there extend back 6,000 years (Kraft 1982; Swain 

and Winkler 1983; Bona 1990). Dated cores from bogs 

and drowned trees reveal longstanding lake-level highs 

at about 1,100, 400, and 200 radiocarbon years before 

present and low episodes at about 650 and 350 years 

before present (Larsen et al. 1999). Islands such as 

Gull Island have nearly washed away in strong storms. 

Radiocarbon ages of peat in bog-filled swales on Long 

Island are as old as 520±70 years before present (Bona 

1990). This age is likely younger than the true age of the 

barrier system (Bona 1990), but is a testament to the 

constancy of change and dynamic nature of features at 

Apostle Islands National Lakeshore. 
53

Geologic Maps

Geologic maps facilitate an understanding of an area’s 

geologic framework and the evolution of its present 

landscape. Using designated colors and symbols, 

these maps portray the spatial distribution and 

temporal relationships of rocks and unconsolidated 

deposits. Geologic maps can be divided into two 

primary types: surficial and bedrock. Surficial 

geologic maps typically encompass deposits that are 

unconsolidated and formed during the past 2.6 million 

years (the Quaternary Period). Surficial map units 

are differentiated by geologic process or depositional 

environment. Bedrock geologic maps encompass 

older, typically more consolidated sedimentary, 

metamorphic, and/or igneous rocks. Bedrock map units 

are differentiated based on age and/or rock type.

Geologic maps often depict geomorphic features, 

structural interpretations (such as faults or folds), 

and locations of past geologic hazards that may be 

susceptible to future activity. Anthropogenic features 

such as mines or quarries, as well as observation or 

collection locations, may be indicated on geologic 

maps. The American Geosciences Institute website, 

http://www.americangeosciences.org/environment/

publications/mapping (accessed 10 April 2015), 

provides more information about geologic maps and 

their uses.

There are two sets of GRI GIS data for Apostle 

Islands National Lakeshore. One is a bedrock geology 

map (apis_geology.mxd) and the other is a surficial 

“Pleistocene” geology map (aipg_geology.mxd).

Source Maps

The GRI team digitizes paper maps and converts digital 

data to conform to the GRI GIS data model. The GRI 

digital geologic map product includes essential elements 

of the source maps such as map unit descriptions, a 

correlation chart of units, a map legend, map notes, 

references, and figures. The GRI team used the 

following sources to produce the digital geologic data 

set for Apostle Islands National Lakeshore. These 

sources also provided information for this report.

Cannon, W. F., L. G. Woodruff, S. W. Nicholson, C. A. 

Hedgman, and R. D. Barber-Delach. 1999. Digital bedrock 

geologic map of the Ashland and northern part of the 

Ironwood 30’ × 60’ quadrangles, Wisconsin and Michigan 

(scale 1:100,000). Open-File Report OF 99-546. US 

Geological Survey, Reston Virginia. http://pubs.er.usgs.gov/

publication/ofr99546 (accessed 12 May 2014).

Clayton, L. 1985. Pleistocene geology of the Superior 

region, Wisconsin (scale 1:250,000). Regional Map Series 

3. Wisconsin Geological and Natural History Survey, 

Madison, Wisconsin. 

Nicholson, S. W., W. F. Cannon, L. G. Woodruff, and C. L. 

Dicken. 2004a. Bedrock geologic map of the Port Wing, 

Solon Springs, and parts of the Duluth and Sandstone 30’ 

× 60’ quadrangles, Wisconsin (scale 1:100,000). Open-File 

Report 2004-1303. US Geological Survey, Reston, Virginia. 

http://pubs.er.usgs.gov/publication/ofr20041303 (accessed 

12 May 2014).

Nicholson, S. W., C. L. Dicken, M. P. Foose, and J. Mueller. 

2004b. Integrated geologic map databases for the United 

States, the upper Midwest states: Minnesota, Wisconsin, 

Michigan, Illinois, and Indiana (scale 1:500,000). Open-File 

Report 2004-1355. US Geological Survey, Reston, Virginia. 

http://pubs.er.usgs.gov/publication/ofr20041355 (accessed 

12 May 2014).

GRI GIS Data

The GRI team implements a GIS data model that 

standardizes map deliverables. The data model is 

included on the enclosed CD and is also available at 

http://science.nature.nps.gov/im/inventory/geology/

GeologyGISDataModel.cfm (accessed 10 April 2015). 

This data model dictates GIS data structure, including 

layer architecture, feature attribution, and relationships 

within ESRI ArcGIS software. The GRI team digitized 

the data for Apostle Islands National Lakeshore using 

data model version 2.1. The GRI Geologic Maps 

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

geo_maps.cfm (accessed 10 April 2015), provides more 

information about GRI map products. 

Geologic Map Data

This section summarizes the geologic map data available for Apostle Islands National Lakeshore. 

Posters (in pocket) display the map data draped over imagery of the park and surrounding area. 

The Map Unit Properties Table (in pocket) summarizes this report’s content for each geologic map 

unit. Complete GIS data are included on the accompanying CD and are also available at the GRI 

publications website: http://go.nps.gov/gripubs.
54

GRI digital geologic data are included on the 

attached CD and are available through the NPS 

Integrated Resource Management Applications 

(IRMA) portal (https://irma.nps.gov/App/Reference/

Search?SearchType=Q; accessed 10 April 2015). Enter 

“GRI” as the search text and select a park from the unit 

list.

The following components are part of the data set:

● A GIS readme file (apis_gis_readme.pdf) 

that describes the GRI data formats, naming 

conventions, extraction instructions, use 

constraints, and contact information;

● Data in ESRI geodatabase GIS format;

● Layer files with feature symbology (tables 3 and 4);

● Federal Geographic Data Committee (FGDC)–

compliant metadata;

● An ancillary map information document (apis_

geology.pdf) that contains information captured 

from source maps such as map unit descriptions, 

geologic unit correlation tables, legends, crosssections, and figures;

● ESRI map documents (apis_geology.mxd and 

aipg_geology.mxd) that display the digital geologic 

data; and

● KML/KMZ versions of the data viewable in Google 

Earth (tables 3 and 4).

GRI Map Posters

Two posters (in pocket) illustrate each of the GRI 

GIS data sets draped over aerial imagery of the park 

and surrounding area. Not all GIS feature classes are 

included on the posters (tables 3 and 4). Geographic 

information and selected park features have been 

added to the posters. Digital elevation data and added 

geographic information are not included in the GRI 

GIS data set, but are available online from a variety of 

sources. Contact GRI for assistance locating these data.

Map Unit Properties Tables

The Map Unit Properties Tables list the geologic time 

division, symbol, and a simplified description for each 

of the map units in the GRI GIS data sets. Following 

the structure of the report, the tables summarize the 

geologic features, processes, resource management 

issues, and history associated with each map unit. 

There are separate tables for the bedrock and surficial 

(“Pleistocene”) geologic map data.

Use Constraints

Graphic and written information provided in this 

report is not a substitute for site-specific investigations. 

Ground-disturbing activities should neither be 

permitted nor denied based upon the information 

provided here. Please contact GRI with any questions.

Minor inaccuracies may exist regarding the locations 

of geologic features relative to other geologic or 

geographic features on the posters. Based on the source 

map scales (1:100,000, 1:250,000, and 1:500,000) and 

US National Map Accuracy Standards, geologic features 

represented in the geologic map data are horizontally 

within 51 m (167 ft), 127 m (417 ft), and 254 m (833 ft), 

respectively, of their true locations.

Data Layer On 

Poster?

Google Earth 

Layer?

Geologic Cross Section Lines No No

Geologic Attitude and Observation 

Points (strike and dip)

No No

Mine Point Features (drill holes) Yes No

Faults Yes Yes

Outcrops Yes Yes

Geologic Contacts Yes Yes

Geologic Units Yes Yes

Table 3. Data layers in the Apostle Islands National 

Lakeshore bedrock geology GIS data (apis_geology.mxd; 

apis_geology.kmz).

Data Layer On 

Poster?

Google Earth 

Layer?

Glacial Feature Points (direction of 

glacial stream flow)

Yes No

Glacial Feature Lines (drumlins and 

glacial ridges)

Yes Yes

Geologic Line Features (abandoned 

beach and wave-cut bluffs; river 

channel cutbanks)

Yes Yes

Geologic Contacts Yes Yes

Geologic Units Yes Yes

Table 4. Data layers in the Apostle Islands National 

Lakeshore Pleistocene geology GIS data (aipg_geology.

mxd; aipg_geology.kmz).
55

Glossary

This section contains brief definitions of selected geologic terms relevant to this report. Definitions 

are based on those in the American Geosciences Institute Glossary of Geology (5th edition; 2005). 

Additional terms are defined at http://geomaps.wr.usgs.gov/parks/misc/glossarya.html.

accretion (streams). The filling-up of a stream channel as a 

result of such factors such as silting or wave action.

adit. A horizontal passage into a mine from the surface.

aeolian. Describes materials formed, eroded, or deposited 

by or related to the action of wind. 

alluvial fan. A low, relatively flat to gently sloping, fanshaped mass of loose rock material deposited by a stream, 

especially in a semiarid region, where a stream issues from 

a canyon onto a plain or broad valley floor.

alluvium. Stream-deposited sediment.

anticline. A fold, generally convex upward (“A”-shaped) 

whose core contains the stratigraphically older rocks. 

aquiclude. A saturated geologic unit that is incapable of 

transmitting significant quantities of water under ordinary 

hydraulic gradients. Replaced by the term “confining 

bed.”

aquifer. A rock or sedimentary unit that is sufficiently 

porous to hold water, sufficiently permeable to allow 

water to move through it, and saturated to some level.

aragonite. A carbonate (carbon + oxygen) mineral of 

calcium, CaCO3; the second most abundant cave mineral 

after calcite, differing from calcite in its crystal structure.

arenite. A general term for sedimentary rocks composed of 

sand-sized fragments.

arête. A sharp-edged rocky ridge, commonly present above 

snowline in rugged, glacially sculpted mountains; results 

from the continued backward growth of the walls of 

adjoining cirques.

argillaceous. Pertaining to, largely composed of, or 

containing clay-size particles or clay minerals.

arkose. A commonly coarse-grained, pink or reddish 

sandstone consisting of abundant feldspar minerals.

artesian. Describes groundwater confined under 

hydrostatic pressure.

artesian pressure. Hydrostatic pressure of artesian water, 

often expressed in terms of pounds per square inch at 

the land surface, or in terms of the height (in feet above 

the land surface) of a column of water that would be 

supported by the pressure.

artesian spring. A spring from which water flows under 

artesian pressure, usually through a fissure or other 

opening in the confining bed above the aquifer.

artesian system. Any system incorporating the following: 

a water source, a body of permeable rock bounded by 

bodies of distinctly less permeable rock, and a structure 

enabling water to percolate into and become confined in 

the permeable rock under pressure distinctly greater than 

atmospheric.

artesian well. A well that taps confined groundwater. Water 

in the well rises above the level of the top of the aquifer 

under artesian pressure.

asth

rigid lithosphere where isostatic adjustments take place, 

magmas may be generated, and seismic waves are strongly 

attenuated; part of the upper mantle.

enosphere. Earth’s relatively weak layer below the 

astronomical tide. The periodic rise and fall of a body of 

water resulting from gravitational interactions between 

the Sun, Moon, and Earth. Synonymous with “tide,” but 

used to emphasize the absence of atmospheric influences.

back barrier. The landward side of a barrier island.

backwasting. Wasting (gradual erosion) that causes a slope 

to retreat without changing its gradient.

backwater. A body of water that is parallel to a river but is 

stagnant or little affected by the river’s currents.

bank. A submerged ridge of sand in the sea, a lake, or a river, 

usually exposed during low tide or low water.

barrier island. A long, low, narrow island consisting of a 

ridge of sand that parallels the coast.

basalt. A volcanic rock that is characteristically dark in 

color (gray to black), contains approximately 53% silica or 

less, and is rich in iron and magnesium.

base flow. Streamflow supported by groundwater and not 

attributed to direct runoff from precipitation or snow 

melt.

basement. The undifferentiated rocks, commonly igneous 

and metamorphic, that underlie rocks exposed at the 

surface, commonly sedimentary. In many regions the 

basement is of Precambrian age, but it may be much 

younger. Also, Earth’s crust below sedimentary deposits 

that extends down to the Mohorovicic discontinuity.

basin (structural). A doubly plunging syncline in which 

rocks dip inward from all sides.

basin (sedimentary). Any depression, from continental to 

local scale, into which sediments are deposited.

bathymetry. The measurement of ocean or lake depths and 

the charting of the topography of the ocean or lake floor. 

beach. The unconsolidated material at the shoreline that 

covers a gently sloping zone, typically with a concave 

profile, extending landward from the low-water line to 

the place where there is a definite change in material 

or physiographic form (e.g., a cliff), or to the line of 

permanent vegetation (usually the effective limit of the 

highest storm waves).

beach face. The section of the beach normally exposed to 

the action of wave uprush.

bed. The smallest sedimentary stratigraphic unit, commonly 

ranging in thickness from about 1 cm (0.4 in) to 1 to 2 m 

(40 to 80 in) and distinguishable from beds above and 

below.

bedding. Depositional layering or stratification of 

sediments.

bedrock. Solid rock that underlies unconsolidated 
56

sedimentary deposits and soil. sequence in time.

berm. A low, impermanent, nearly horizontal or landward- clast. An individual constituent, grain, or fragment of a rock 

sloping bench, shelf, or ledge above the high-water line on or unconsolidated deposit, produced by the mechanical 

a beach. or chemical disintegration of a larger rock mass.

block (fault). A crustal unit bounded completely or partially clastic. Describes rocks or sediments made of fragments of 

by faults. preexisting rocks.

blowout. A general term for a small saucer-, cup-, or clay. Minerals and sedimentary fragments that are less than 

trough-shaped hollow or depression formed by wind 1/256 mm (0.00015 in) across.

erosion on a preexisting dune or other sand deposit, claystone (sedimentary). An indurated rock with more especially in an area of shifting sand or loose soil, or than 67% clay-sized minerals. where protective vegetation is disturbed or destroyed.

cleavage. The tendency of a mineral to break along planes body fossil. Evidence of past organisms such as bones, of weak bonding. teeth, shells, or leaf imprints. 

coarse-grained. Describes a crystalline rock and texture braided stream. A sediment-clogged stream that forms in which the individual minerals are relatively large, multiple channels that divide and rejoin. specifically an igneous rock whose particles have an 

breakdown. Collapse of a cave ceiling or walls of a cave. average diameter greater than 5 mm (0.2 in). Also, 

Also, the accumulation of debris thus formed. describes sediment or sedimentary rock and texture in 

which the individual constituents are easily seen with the breakwater. An offshore, generally shore-parallel structure unaided eye, specifically sediment or rock whose particles that breaks the force of the waves. have an average diameter greater than 2 mm (0.08 in).

breccia. A coarse-grained, generally unsorted sedimentary colluvium. A loose, heterogeneous, and incoherent mass rock consisting of cemented angular clasts more than 2 of rock fragments and soil material deposited via surface mm (0.08 in) across. runoff or slow continuous downslope creep; usually 

burrow. A tubular or cylindrical hole or opening, made in collects at the base of a slope or hillside, but includes 

originally soft or loose sediment by a mud-eating worm, loose material covering hillsides.

mollusk, or other invertebrate; may be later filled with clay compression. A decrease in volume of material (including or sand and preserved. Earth’s crust) as it is pressed or squeezed together. calcareous. Describes a substance that contains calcium confined aquifer. An aquifer bounded above and below carbonate. When applied to a rock name it implies that as by confining beds. An aquifer containing confined much as 50% of the rock is calcium carbonate. groundwater.

calcium carbonate. CaCO3. A solid occurring in nature as 

primari

mineral.

3; calcium car

confined groundwater. Groundwater under pressure ly calcite and ar

bonat

ag

e. It is the most abundant ca

onite. 

ve 

significantly greater than that of the atmosphere. Its upper 

calcite. A carbonate (carbon + oxygen) mineral of calcium, surface is the bottom of a confining bed.

CaCO confining bed. A body of relatively impermeable or 

distinctly less permeable material stratigraphically carbonate. A mineral group composed of carbon and adjacent to one or more aquifers. Replaced the term 

oxygen plus an element or elements; for example calcite, “aquiclude.”

CaCO3; and dolomite, CaMg(CO3)2. conglomerate. A coarse-grained, generally unsorted, 

carbonate rock. A rock, for example, limestone, calcite, and sedimentary rock consisting of cemented, rounded clasts 

dolomite, that consist primarily of carbonate minerals. larger than 2 mm (0.08 in) in diameter.

cement (sedimentary). Mineral material, usually contact. The surface between two types or ages of rocks.

chemically precipitated, that occurs in the spaces among continental drift. A term for the process by which the individual grains of a sedimentary rock, thus binding continents move relative to one another; it is a the grains together. consequence of plate tectonics.

cementation. The process by which clastic sediments continental crust. Earth’s crust that is rich in silica become lithified or consolidated into hard, compact and aluminum and underlies the continents and the rocks, usually through deposition or precipitation of continental shelves; ranges in thickness from about 25 minerals in the spaces among the individual grains of the km (15 mi) to more than 70 km (40 mi) under mountain sediment; may occur simultaneously with sedimentation ranges, averaging about 40 km (25 km) thick. or at a later time.

continental shield. A large area of exposed basement chemical weathering. Chemical breakdown of minerals at at the interior of a continent, commonly with a very Earth’s surface via reaction with water, air, or dissolved gently convex surface, surrounded by sediment-covered substances; commonly results in a change in chemical platforms. The rocks of virtually all shield areas are composition providing more stability in the current Precambrian. environment.

coral. Any of a large group of bottom-dwelling, sessile, chert. An extremely hard sedimentary rock with conchoidal marine invertebrate organisms (polyps) that belong to fracturing, consisting mostly of interlocking crystals of the class Anthozoa (phylum Cnidaria), characterized by quartz. production of an external skeletons of calcium carbonate; 

chronology. The arrangement of events in their proper may exist as solitary individuals or grow in colonies. 
57

Range: Abundant in the fossil record in all periods later drainage. The manner in which the waters of an area flow 

than the Cambrian. off in surface streams or subsurface conduits; also, the 

processes of surface drainage of water from an area by core (drill). A cylindrical section of rock or sediment. streamflow and sheet flow, and the removal of excess usually 5–10 cm (2–4 in) across and up to several meters water from soil by downward flow. long, taken as a sample of the interval penetrated by a core 

bit, and brought to the surface for geologic examination drainage basin. A region or area bounded by a drainage 

and/or laboratory analysis. divide and occupied by a drainage system, specifically 

the tract of country that gathers water originating as craton. The relatively old and geologically stable interior of precipitation and contributes it to a particular stream a continent. channel or system of channels, or to a lake, reservoir, or 

creep. The slow, imperceptible downslope movement of other body of water.

mineral, rock, and soil particles under gravity. dredge (engineering). A floating machine for excavating cross-bed. A single bed, inclined at an angle to the main sedimentary material from the bottom of a body of water.

planes of stratification; the term is commonly restricted to drift. All rock material (clay, silt, sand, gravel, and boulders) a bed that is more than 1 cm (0.4 in) thick. transported and deposited by a glacier, or by running 

cross-bedding. Uniform to highly varied sets of inclined water emanating from a glacier. 

beds deposited by wind or water that indicate flow drumlin. A low, smoothly rounded, elongated oval hill, conditions such as direction and depth. mound, or ridge of till that formed under the ice margin 

cross section. A graphic interpretation of geology, structure, and was shaped by glacial flow; the long axis is parallel to 

or stratigraphy based on mapped and measured geologic the direction of ice movement.

extents and attitudes, depicted in a vertical plane (i.e., a dune. A low mound or ridge of sediment, usually sand, cut or profile view). deposited by the wind. 

crust. Earth’s outermost layer or shell. ephemeral stream. A stream that flows briefly, only in 

crystalline. Describes a regular, orderly, repeating geometric direct response to precipitation, and whose channel is 

structural arrangement of atoms. always above the water table. 

debris flow. A moving mass of rock fragments, soil, and erosion. The general process or group of processes that 

mud, with more than half of the particles larger than sand loosen, dissolve, wear away, and simultaneously move size. Slow debris flows may move less than 1 m (3 ft) per from one place to another, the materials of Earth’s crust; 

year; rapid ones reach 160 km per hour (100 mi per hour). includes weathering, solution, abrasive actions, and 

transportation, but usually excludes slope movements. deformation. The process of folding, faulting, shearing, or 

fabric development in rocks as a result of Earth stresses. erratic. A rock fragment carried by glacial ice deposited 

at some distance from the outcrop from which it was delta. The low, nearly flat, alluvial tract of land at or near derived, and generally, though not necessarily, resting on the mouth of a river, commonly forming a triangular or bedrock of different lithology. fan-shaped plain of considerable area; resulting from 

the accumulation of sediment supplied by the river in escarpment. A steep cliff or topographic step resulting from such quantities that it is not removed by tides, waves, and vertical displacement on a fault or as a result of slope 

currents. movement or erosion. Synonymous with “scarp.”

differential erosion. Erosion that occurs at irregular or estuary. The seaward end or tidal mouth of a river where 

varying rates due to differences in the resistance and freshwater and seawater mix.

hardness of surface material: softer and weaker rocks exfoliation. The spalling, peeling, or flaking of layers or are rapidly worn away; harder and more resistant rocks concentric sheets from an exposed rock mass caused by a remain to form ridges, hills, or mountains. change in heat or a reduction in pressure when overlying 

dip. The angle between a bed or other geologic surface and rocks erode away.

the horizontal plane. extension. Deformation of Earth’s crust whereby rocks are 

discharge. The rate of flow of surface water or groundwater pulled apart. 

at a given moment, expressed as volume per unit of time. extrusion. The emission of lava onto Earth’s surface; also, 

displacement. The relative movement of the two sides of a the rock so formed.

fault; also, the specific amount of such movement. fault. A break in rock characterized by displacement of one 

divergent plate boundary. A boundary between two plates side relative to the other.

that are moving apart, characterized by mid-ocean ridges feldspar. A group of abundant silicate (silicon + oxygen) at which sea-floor spreading occurs. minerals, comprising more than 60% of Earth’s crust and 

dolomite (mineral). A carbonate (carbon + oxygen) mineral occurring in all types of rocks. 

of calcium and magnesium, CaMg(CO3

)2. feldspathic. Describes a rock containing feldspar.

dolomite (rock). A carbonate sedimentary rock containing felsic. Derived from feldspar + silica to describe an igneous more than 50% of the mineral dolomite (calcium- rock having abundant light-colored minerals such as magnesium carbonate). quartz, feldspars, or muscovite; also, describes those 

downcutting. Stream erosion in which cutting is directed minerals.

primarily downward, as opposed to laterally. fetch. The area of the ocean surface over which a constant 
58

and uniform wind generates waves. by meltwater streams flowing into lakes bordering a 

glacier. fine-grained. Describes sediment or sedimentary rock and 

texture in which the individual constituents are too small Gondwana. The late Paleozoic continent of the Southern 

to distinguish with the unaided eye, specifically sediment Hemisphere and counterpart of Laurasia of the Northern 

or rock whose particles have an average diameter less Hemisphere; both were derived from the supercontinent 

than 1/16 mm (0.002 in), that is, silt-size particles and Pangaea.

smaller. Also, describes a crystalline or glassy rock and gradient. A degree of inclination (steepness of slope), or texture in which the individual minerals are relatively a rate of ascent or descent, of an inclined part of Earth’s small, specifically an igneous rock whose particles have an surface with respect to the horizontal; expressed as a ratio average diameter less than 1 mm (0.04 in). (vertical to horizontal), a fraction (such as m/km or ft/

fissure. A fracture or crack in rock along which there is a mi), a percentage (of horizontal distance), or an angle (in 

distinct separation; commonly filled with mineral-bearing degrees).

materials. granite. A coarse-grained, intrusive igneous rock in which 

fissure (volcanic). An elongated fracture or crack at the quartz constitutes 10%–50% of the felsic (“light-colored”) 

surface from which lava erupts. components and the alkali feldspar/total feldspar ratio is 

generally restricted to the range of 65% to 90%; perhaps fissure vent. A volcanic conduit having the form of a crack the best known of all igneous rocks. or fissure at Earth’s surface.

graben. An elongated, downdropped trough or basin, floodplain. The surface or strip of relatively smooth land bounded on both sides by high-angle normal faults that composed of alluvium and adjacent to a river channel, dip toward one another. constructed by the present river in its existing regimen 

and covered with water when the river overflows its gravel. An unconsolidated, natural accumulation of 

banks. A river has one floodplain and may have one or typically rounded rock fragments resulting from erosion; 

more terraces representing abandoned floodplains. consists predominantly of particles larger than sand; that 

is, greater than 2 mm (1/12 in) across. fluvial. Of or pertaining to a river or rivers.

groundwater. That part of subsurface water that is in the fluvial channel. A natural passageway or depression zone of saturation, including underground streams. produced by the action of a stream or river. 

hanging wall. The upper wall of a fault. fold. A curve or bend in an originally flat structure, such 

as a rock stratum, bedding plane, or foliation; usually a hematite. An oxide mineral composed of oxygen and iron, 

product of deformation. Fe2O3.

footwall. The lower wall of a fault. heterogeneous. Consisting of dissimilar or diverse 

ingredients or constituents. foreshore. The part of the beach face that lies between the 

berm crest and the low-water line; it is regularly covered hoodoo. A bizarrely shaped column, pinnacle, or pillar of 

and uncovered by the rise and fall of the tide and is rock, commonly produced in a region of sporadic heavy 

shaped by tides and waves. rainfall by differential weathering or erosion of horizontal 

strata, facilitated by layers of varying hardness and joints. formation. Fundamental rock-stratigraphic unit that is 

mappable, lithologically distinct from adjoining strata, horst. An elongated, uplifted block that is bounded on both 

and has definable upper and lower contacts. sides by normal faults that dip away from one another. 

fossil. A remain, trace, or imprint of a plant or animal that hot spot. A volcanic center, 100–200 km (60–120 mi) across, 

has been preserved in the Earth’s crust since some past persistent for at least a few tens of millions of year, with a 

geologic time; loosely, any evidence of past life. surface expression, commonly at the center of a plate, that 

indicates a rising plume of hot mantle material. fracture. The breaking of a mineral other than along planes 

of cleavage. Also, any break in a rock such as a crack, joint, hydrostatic pressure. The pressure exerted by the water at 

or fault. any given point in a body of water at rest. The hydrostatic 

pressure of groundwater is generally due to the weight of frost wedging. A type of mechanical disintegration, water at higher levels in the saturated zone. splitting, or breakup of a rock by which jointed rock is 

pried and dislodged by ice acting as a wedge. hydrology. The study of liquid and solid water properties, 

circulation, and distribution, on and under the Earth’s geodetic surveying. Surveying that takes into account surface and in the atmosphere. the figure and size of Earth, with corrections made for 

curvature; used where the areas or distances involved are igneous. Describes a rock or mineral that solidified 

so great that the desired accuracy and precision cannot from molten or partly molten material; also, describes 

be obtained by plane (ordinary field and topographic) processes leading to, related to, or resulting from the 

surveying. formation of such rocks. One of the three main classes or 

rocks—igneous, metamorphic, and sedimentary. geomorphology. The study of the general configuration of 

surface landforms and their relationships to underlying indurated. Describes a rock or soil hardened or 

structures, and of the history of geologic changes as consolidated by pressure, cementation, or heat.

recorded by these surface features. intrusion. The process of emplacement of magma into 

glaciolacustrine. Pertaining to, derived from, or deposited preexisting rock. Also, the igneous rock mass formed. 

in glacial lakes, especially referring to deposits and intrusive. Pertaining to intrusion, both the process and the landforms composed of suspended material transported 
59

rock body. responsible for most geologic activity.

isostasy. The condition of equilibrium, comparable loam. A rich permeable soil composed of a mixture of clay, 

to floating, of the units of the lithosphere above the silt, sand, and organic matter.

asthenosphere. loess. Windblown silt-sized sediment.

joint. A break in rock without relative movement of rocks longshore current. A current parallel to a coastline caused on either side of the fracture surface. by waves approaching the shore at an oblique angle.

kame. A mound, knob, or short irregular ridge, composed lowstand. The interval of time during one or more cycles of of stratified sand and gravel deposited by a subglacial relative sea-level change when sea level is below the edge stream as a fan or delta at the margin of a melting glacier. of the continental shelf in a given area. 

kame delta. A flat-topped, steep-sided hill of well-sorted magma. Molten rock beneath Earth’s surface capable of sand and gravel deposited by a meltwater stream flowing intrusion and extrusion. into a proglacial or other ice-marginal lake; the proximal 

margin of the delta was built in contact with a glacier. magmatism. The development and movement of magma, 

and its solidification as igneous rock. karst. A type of topography that is formed on limestone, 

gypsum, and other soluble rocks, primarily by dissolution. magnetite. Iron oxide. An oxide mineral composed of 

It is characterized by sinkholes, caves, and underground oxygen and iron, Fe3O4; commonly contains manganese, 

drainage. nickel, chromium, and titanium. A very common and 

widely distributed accessory mineral in rocks of all kinds lacustrine. Describes a process, feature, or organism and as a “heavy mineral” in sand. pertaining to, produced by, or inhabiting a lake.

mantle. The zone of the Earth below the crust and above lagoon. A narrow body of water that is parallel to the the core. shore and between the mainland and a barrier island; 

characterized by minimal or no freshwater influx and mantle plume. A vertical cylindrical part of Earth’s mantle, 

limited tidal flux, which cause elevated salinities. Also, a hotter than its surroundings, within which larger-thanshallow body of water enclosed or nearly enclosed within normal amounts of heat are conducted upward to form a 

an atoll. hot spot at the Earth’s surface.

lamina. The thinnest recognizable unit layer of original mass wasting. Dislodgement and downslope transport 

deposition in a sediment or sedimentary rock, differing of a mass of rock and/or unconsolidated material under 

from other layers in color, composition, or particle size; the direct influence of gravity. In contrast to “erosion,” 

specifically a sedimentary layer less than 1 cm (0.4 in) the debris removed is not carried within, on, or under 

thick and commonly 0.05–1.00 mm (0.002–0.04 in) thick. another medium. Synonymous with “slope movement.”

Synonymous with “lamination”; lamination is also the matrix. The fine-grained material between coarse grains formation of such layers. in an igneous or sedimentary rock. Also refers to rock or 

landslide. A collective term covering a wide variety of sediment in which a fossil is embedded.

slope-movement landforms and processes that involve the meander. One of a series of sinuous curves, bends, or turns downslope transport of soil and rock material en masse in the course of a stream, produced by a mature stream under the influence of gravity. swinging from side to side as it flows across its floodplain 

Laurasia. The late Paleozoic continent of the Northern or shifts its course laterally toward the convex side of an 

Hemisphere and counterpart of Gondwana of the original curve.

Southern Hemisphere; both were derived from the mechanical weathering. The physical breakup of rocks supercontinent Pangaea. without change in composition. 

lava. Molten or solidified magma that has been extruded medium-grained. Describes an igneous rock and texture though a vent onto Earth’s surface. in which the individual crystals have an average diameter 

left-lateral fault. A strike-slip fault on which the side in the range of 1 to 5 mm (0.04 to 0.2 in.). Also, describes 

opposite the observer has been displaced to the left. sediment or sedimentary rock and texture in which the 

Synonymous with “sinistral fault.” individual particles have an average diameter in the range 

of 1/16 to 2 mm (0.002 to 0.08 in), that is, sand size. light detection and ranging/LiDAR. A method and 

instrument that measures distance to a reflecting object member. A lithostratigraphic unit with definable contacts; a 

by emitting timed pulses of light and measuring the time subdivision of a formation.

between emission and reception of reflected pulses; the metamorphic rock. Any rock derived from preexisting measured interval is converted to distance. rocks that was altered in response to marked changes 

limestone. A carbonate sedimentary rock consisting of in temperature, pressure, shearing stress, and chemical 

more than 95% calcite and less than 5% dolomite. environment. One of the three main classes of rock—

igneous, metamorphic, and sedimentary. lithology. The physical description or classification of a 

rock or rock unit based on characteristics such as color, metamorphism. The mineralogical, chemical, and structural 

mineral composition, and grain size. changes of solid rocks, generally imposed at depth below 

the surface zones of weathering and cementation. lithosphere. Earth’s relatively rigid outer shell that consists 

of the entire crust plus the uppermost mantle. It is broken meteoric water. Water of recent atmospheric origin.

into about 20 plates, and according to the theory of plate mid-ocean ridge. The continuous, generally submarine tectonics, movement and interaction of these plates is and volcanically active mountain range that marks the 
60

divergent tectonic margins in Earth’s oceans. parting. A plane or surface along which a rock readily 

separates. mineral. A naturally occurring inorganic crystalline solid 

with a definite chemical composition or compositional peat. An unconsolidated deposit of semicarbonized plant 

range. remains in a water-saturated environment, such as a bog 

or fen, and of persistently high moisture content (at least moraine. A mound, ridge, or other distinct accumulation 75%). It is an early stage or rank in the development of of unsorted, unstratified glacial drift, predominantly till, coal; carbon content is about 60% and oxygen content is deposited mostly by direct action of a glacier. about 30% (moisture-free). 

mud crack. A crack formed in clay, silt, or mud by shrinkage pebble. A small rounded rock, especially a waterworn stone, during dehydration at Earth’s surface. between 4 and 64 mm (0.16 and 2.5 in) across.

mud flat. A relatively level area of fine silt along a shore or perched aquifer. An aquifer that is separated from around an island, alternately covered and uncovered by (“perched” above) the water table by an unsaturated zone. the tide, or covered by shallow water; a muddy tidal flat, 

barren of vegetation. period. The fundamental unit of the worldwide geologic 

time scale. It is lower in rank than era and higher than mudflow. A general term for a landform and process epoch. The geochronologic unit during which the rocks of characterized by a flowing mass of predominantly fine- the corresponding system were formed. grained earth material possessing a high degree of fluidity 

during movement. permeability. A measure of the relative ease with which 

a fluid moves through the pore spaces of a rock or mollusk. A solitary invertebrate such as gastropods, bivalves, unconsolidated deposit. and cephalopods belonging to the phylum Mollusca. 

Range: Lower Cambrian to Holocene. plate boundary. A zone of seismic and tectonic activity 

along the edges of lithospheric plates, resulting from the native elements. A mineral group that is composed of a relative motion among plates. single element, for example, gold, Au; or copper, Cu.

plate tectonics. A theory of global tectonics in which the normal fault. A fault in which the hanging wall appears to lithosphere is divided into about 20 rigid plates that have moved downward relative to the footwall; the angle interact with one another at their boundaries, causing of dip is usually 45°–90°. seismic and tectonic activity along these boundaries.

oblique fault. A fault in which motion includes both dip- platform. Any level or nearly level surface. slip and strike-slip components.

plume. A persistent, pipe-like body of hot material moving oceanic crust. Earth’s crust that underlies the ocean basins upward from Earth’s mantle into the crust. and is rich in iron and magnesium; ranges in thickness 

from about 5 to 10 km (3 to 6 mi). pluton. A deep-seated igneous intrusion. 

orogeny. A mountain-building event. plutonic. Describes an igneous rock or intrusive body 

formed at great depth beneath Earth’s surface. outcrop. Any part of a rock mass or formation that is 

exposed or “crops out” at Earth’s surface. progradation. The seaward building of land as a result of 

sedimentary deposition. outwash. Glacial sediment transported and deposited by 

meltwater streams. quartz. Silicon dioxide, SiO2. The only silicate (silicon + 

oxygen) mineral consisting entirely of silicon and oxygen. oxidation. The process of combining with oxygen. Synonymous with “crystalline silica.”

oxide. A mineral group composed of oxygen plus an quartzite. Metamorphosed quartz sandstone. A medium- element or elements, for example, iron in hematite, grained, nonfoliated metamorphic rock composed mostly Fe2O3; or aluminum in corundum, Al2O3. of quartz.

paleogeography. The study, description, and radioactivity. The spontaneous decay or breakdown of reconstruction of the physical landscape in past geologic unstable atomic nuclei. periods.

radiocarbon age. An isotopic age expressed in years and paleontology. The study of the life and chronology of calculated from the quantitative determination of the Earth’s geologic past based on the fossil record. amount of carbon-14 remaining in an organic material. 

Pangaea. A supercontinent that existed from about 300 Synonymous with “carbon-14 age.” million to about 200 million years ago and included most rebound. Upward flexing of Earth’s crust. Synonymous of the continental crust of the Earth, from which the with “upwarping.” present continents were derived by fragmentation and 

continental drift. During an intermediate stage of the recharge. The addition of water to the saturated zone below 

fragmentation—between the existence of Pangaea and the water table.

that of the present continents—Pangaea split into two reflection shooting. A type of seismic survey based on large fragments, Laurasia in the Northern Hemisphere measurement of the travel times of seismic waves that and Gondwana in the Southern Hemisphere. originate from an artificially produced disturbance 

parent material. The unconsolidated organic and mineral and are reflected back at near-vertical incidence from 

material from which soil forms. subsurface boundaries, resulting in a separation of media 

of different densities and/or elastic-wave velocities. parent rock. Rock from which soil, sediment, or other rock 

is derived. regolith. From the Greek “rhegos” (blanket) + “lithos” 
61

(stone), the layer of unconsolidated rock material that particles to the site of deposition, the actual deposition or 

forms the surface of the land and overlies or covers settling of the particles, the chemical and other changes 

bedrock; includes rock debris of all kinds, volcanic ash, occurring in the sediment, and the ultimate consolidation 

glacial drift, alluvium, loess, and aeolian deposits, vegetal of the sediment into solid rock. 

accumulations, and soil. seiche. An oscillation of a body of water in an enclosed or 

regression. Long-term seaward retreat of the shoreline or semi-enclosed basin that varies in period—depending on 

relative fall of sea level. the physical dimensions of the basin—from a few minutes 

to several hours, and in height from several centimeters reverse fault. A contractional high-angle (greater than 45°) to a few meters; primarily caused by local changes in dip-slip fault in which the hanging wall moves up relative atmospheric pressure, aided by winds, tidal currents, and to the footwall. earthquakes. 

rift. A region of Earth’s crust where extension results in seismic. Pertaining to an earthquake or Earth vibration, formation of many related normal faults, commonly including those that are artificially induced. associated with volcanic activity.

seismicity. The phenomenon of movements in the Earth’s rift valley. A depression formed by grabens along the crest crust. Synonymous with “seismic activity.” of a mid-ocean ridge or in a continental rift zone.

sequence. A succession of geologic events, processes, right-lateral fault. A strike-slip fault on which the side or rocks, arranged in chronologic order to show their opposite the observer has been displaced to the right. relative position and age with respect to geologic history 

ripple marks. The undulating, approximately parallel and as a whole. Also, a rock-stratigraphic unit that is traceable 

usually small-scale pattern of ridges formed in sediment over large areas and defined by sediment associated with a 

by the flow of wind or water. major sea level transgression–regression.

riprap. A layer of large, durable rock fragments placed in an shale. A clastic sedimentary rock made of clay-sized 

attempt to prevent erosion by water and thus preserve the particles and characterized by fissility.

shape of a surface, slope, or underlying structure. sheetwash. A sheetflood occurring in a humid region. Also, 

roche moutonnée. A glacially sculpted, elongated bedrock the material transported and deposited by the water of a 

knob or hillock. sheetwash.

rockfall. The most rapid type of slope movement in which a shoal. A relatively shallow place in a stream, lake, sea, or newly detached fragment of bedrock of any size falls from other body of water. a cliff or other very steep slope, traveling straight down or shoaling. To become shallow gradually, to fill up or block off in a series of leaps and bounds down a slope. with a shoal, or to proceed from a greater to a lesser depth 

roundness. The relative amount of curvature of the of water.

“corners” of a sediment grain. shoreface. The zone between the seaward limit of the shore 

sand. A clastic particle smaller than a granule and larger and the more nearly horizontal surface of the offshore 

than a silt grain, with a diameter ranging from 1/16 to 2 zone; typically extends seaward to storm wave depth or 

mm (0.0025 to 0.08 in). about 10 m (30 ft).

sandstone. Clastic sedimentary rock composed of silicate. A mineral group composed of silicon (Si) and 

predominantly sand-sized grains. oxygen (O) plus an element or elements, for example, 

quartz, SiO2

; olivine, (Mg, Fe)2SiO4; and pyroxene, saturated zone. A subsurface zone in which all the (Mg,Fe)SiO ; as well as the amphiboles, micas, and interstices are filled with water under pressure greater 3

feldspars. than that of the atmosphere; separated from the 

unsaturated zone (above) by the water table. silt. Clastic sedimentary material intermediate in size 

between fine-grained sand and coarse clay, 0.0039 to scarp. A steep cliff or topographic step resulting from 0.063 mm (0.00015 to 0.0025 in) across. displacement on a fault or as a result of slope movement 

or erosion. Synonymous with “escarpment.” silting. The accumulation of silt suspended throughout a 

body of standing water or in some considerable portion scour. The powerful and concentrated clearing and digging of it. In particular, the choking, filling, or covering action of flowing water, air, or ice. with stream-deposited silt behind a dam or other place sediment. An eroded and deposited, unconsolidated of retarded flow, or in a reservoir. Synonymous with accumulation of rock and mineral fragments. “siltation.”

sedimentary. Pertaining to or containing sediment. siltstone. A clastic sedimentary rock composed of silt-sized 

sedimentary rock. A rock resulting from the consolidation grains.

of loose sediment that has accumulated in layers; it may slabbing. Rock splitting into slabs along closely spaced be “clastic,” consisting of mechanically formed fragments parallel fissures, commonly falling under the force of of older rock; “chemical,” formed by precipitation from gravity from a cliff face. solution; or “organic,” consisting of the remains of plants slope. The inclined surface of any part of Earth’s surface, and animals. One of the three main classes of rock— such as a hillslope. Also, a broad part of a continent igneous, metamorphic, and sedimentary. descending into an ocean. sedimentation. The process of forming or accumulating slope movement. The gradual or rapid downslope sediment into layers, including the separation of rock movement of soil or rock under gravitational stress. particles from parent rock, the transportation of these 
62

Synonymous with “mass wasting.” striations (glacial). One of a series of long, delicate, finely 

cut, commonly straight and parallel furrows or lines slump. A generally large, coherent slope movement with a inscribed on a bedrock surface by the rasping and rubbing concave failure surface and subsequent backward rotation of rock fragments embedded at the base of a moving relative to the slope. glacier, usually oriented in the direction of ice movement; soil. The unconsolidated portion of the Earth’s crust also form on the rock fragments transported by a glacier.

modified through physical, chemical, and biotic processes strike. The compass direction of the line of intersection of into a medium capable of supporting plant growth. an inclined surface with a horizontal plane.

sorted. Describes an unconsolidated sediment consisting of strike-slip fault. A fault with measurable offset where the particles of essentially uniform size. relative movement is parallel to the strike of the fault. 

sorting. The dynamic process by which sedimentary Described as left-lateral (sinistral) when relative motion of 

particles having some particular characteristic (such as the block opposite the observer is to the left, and right- similarity of size, shape, or specific gravity) are naturally lateral (dextral) when relative motion is to the right. 

selected and separated from associated but dissimilar structure. The attitudes and relative positions of the rock particles by the agents of transportation. masses of an area resulting from such processes as 

spalling. The process by which scales, plates, or flakes of faulting, folding, and igneous intrusion.

rock, from less than a centimeter to several meters thick, submergence. A rise of water level in relation to land, so successively fall from the bare surface of a large rock mass; that areas of formerly dry land become inundated; results a form of exfoliation. from either a sinking of the land or rise of water level. 

speleothem. Any secondary mineral deposit that forms in a subsidence. The sudden sinking or gradual downward cave. settling of part of Earth’s surface.

spring. A place where groundwater flows naturally from syncline. A generally concave upward fold of which the core a rock or the soil onto the land surface or into a body of contains the stratigraphically younger rocks. surface water. 

system (stratigraphy). The fundamental unit of stage. A major subdivision of a glacial epoch, particularly chronostratigraphic classification of Phanerozoic rocks; one of the cycles of growth and disappearance of the each unit represents a time span and an episode of Earth Pleistocene ice sheets. history sufficiently great to serve as a worldwide reference stalactite. A conical or cylindrical speleothem that hangs unit. It is the temporal equivalent of a period. 

from the ceiling or wall of a cave, deposited from drops of tectonic. Describes a feature or process related to large- water and usually composed of calcite but may be formed scale movement and deformation of Earth’s crust. of other minerals.

terrace. Any long, narrow, relatively level or gently inclined stalagmite. A conical or cylindrical speleothem that is surface (i.e., a bench or steplike ledge) that is bounded developed upward from the floor of a cave by the action along one edge by a steeper descending slope and along of dripping water, usually formed of calcite but may be the other edge by a steeper ascending slope, thus breaking formed of other minerals. the continuity of the slope; commonly occurs along the 

storm surge. An abnormal, sudden rise of sea level along margin and above the level of a body of water, marking a 

an open coast during a storm, caused primarily by strong former water level.

winds offshore, or less frequently, a drop in atmospheric terrestrial. Describes a feature, process, or organism related pressure, resulting in water piled up against the coast. It is to land, Earth, or its inhabitants. most severe during high tide.

thrust fault. A dip-slip fault with a shallowly dipping (less strata. Tabular or sheetlike layers of sedimentary rock than 45°) fault surface where the hanging wall moves up that are visually distinctive from other layers above and and over relative to the footwall. below. The singular form of the term is stratum, but is less 

commonly used. till. Unstratified drift deposited directly by a glacier without 

reworking by meltwater and consisting of a mixture of stratification. The accumulation or layering of sedimentary clay, silt, sand, gravel, and boulders ranging widely in size rocks as strata. Tabular, or planar, stratification refers to and shape. essentially parallel surfaces. Cross-stratification refers to 

strata inclined at an angle to the main stratification. topography. The general morphology of Earth’s surface, 

including relief and locations of natural and human-made stratigraphy. The geologic study of the origin, occurrence, features. distribution, classification, correlation, and age of rock 

layers, especially sedimentary rocks. trace (structural geology). The intersection of a geological 

surface with another surface, for example, the trace stream. Any body of water moving under gravity flow in a of bedding on a fault surface, or the trace of a fault or clearly confined channel. outcrop on the ground.

stream channel. A long, narrow depression shaped by the trace fossil. A fossilized feature such as a track, trail, concentrated flow of stream water. burrow, or coprolite (dung), that preserves evidence of an 

stream terrace. A planar surface alongside a stream valley organism’s life activities, rather than the organism itself. representing the remnants of an abandoned floodplain, Compare to “body fossil.”

stream bed, or valley floor produced during a former stage transform fault. A strike-slip fault that links two other faults of erosion or deposition. or plate boundaries such as two segments of a mid-ocean 
63

ridge. movement that raises the rocks.

transgression. Landward migration of the sea as a result of upwarping. Upward flexing of Earth’s crust on a regional 

a relative rise in sea level. scale as a result of the removal of ice, water, sediments, or 

lava flows. trend. The direction or bearing of an outcrop of a geologic 

feature such as an ore body, fold, or orogenic belt. vadose water. Water of the unsaturated zone.

type locality. The place where a geologic feature such as an varve. Any cyclic, genetically related paired sedimentary 

ore occurrence, a particular kind of igneous rock, or the layer, generally occurring in a repeating series, such as 

type specimen of a fossil species was first recognized and seasonally deposited glacial varves, but also laminated 

described. nonglacial shale, evaporite, and other sediments. 

unconfined groundwater. Groundwater that has a water varve (glacial). A sedimentary layer deposited in a body of 

table; water not confined under pressure beneath a still water within one year’s time, specifically a thin pair 

confining bed. of graded glaciolacustrine layers seasonally deposited, 

usually by meltwater streams into a glacial lake or other unconformable. Describes strata that do not succeed the body of still water in front of a glacier. underlying rocks in immediate order of age or in parallel 

position, especially younger strata that do not have vent. Any opening at Earth’s surface through which magma 

the same dip and strike as the underlying rocks. Also, erupts or volcanic gases are emitted.

describes the contact between unconformable rocks. volatile. Readily vaporizable.

unconformity. A substantial break or gap in the geologic volcanic. Pertaining to the activities, structures, or rock record where a rock unit is overlain by another that is not types of a volcano. A synonym of extrusive. next in stratigraphic succession, resulting from either a 

change that caused deposition to cease for a considerable volcanism. The processes by which magma and its 

span of time or erosion with loss of the previously formed associated gases rise into Earth’s crust and are extruded 

record. onto the surface and into the atmosphere.

undercutting. The removal of material at the base of a steep water table. The surface between the saturated zone and 

slope by the erosive action of water (such as a meandering the unsaturated zone. Synonymous with “groundwater 

stream), sand-laden wind in a desert, or waves along a table” and “water level.”

coast. weathering. The physical, chemical, and biological 

underfit stream. A stream that appears to be too small to processes by which rock is broken down, particularly at 

have eroded the valley in which it flows. Also, a stream Earth’s surface.

whose volume is greatly reduced or whose meanders wind tide. The part of the tide produced by wind, rather show a pronounced shrinkage in radius. than astronomical forces; specifically the change in water 

unsaturated zone. A subsurface zone between the land level as a result of wind moving across the surface.

surface and the water table that includes air, gases, and Wisconsinan. Pertaining to the classical fourth glacial stage water held by capillary action. Synonymous with “vadose of the Pleistocene Epoch in North America, following zone” and “zone of aeration.” the Sangamonian interglacial stage and preceding the 

uplift. A structurally high area in Earth’s crust produced by Holocene Epoch.

65

Literature Cited

This section lists references cited in this report. Contact the Geologic Resources Division for assistance 

in obtaining these documents.

Anderson, L. 2003. Evaluation of shoreline erosion extent and processes on Wisconsin’s Lake Superior shoreline. Thesis. University 

of Wisconsin, Madison, Wisconsin.

Behrendt, J. C., A. G. Freen, W. F. Cannon, D. R. Hutchinson, M. W. Lee, B. Milkereit, W. F. Agena, and C. Spencer. 1988. Crustal 

structure of the Midcontinent Rift System: results from GLIMPCE deep seismic reflection profiles. Geology 16:81–85.

Bennett, J. P., and C. M. Wetmore. 2005. Lichens of the U. S. National Parks. The Bryologist 108(4):544–553.

Black, R. F. 1974. Geology of Ice Age National Scientific Reserve of Wisconsin. Scientific Monograph Series Number Two. National 

Park Service, Washington, DC. 

Blewett, W. 2006. Understanding ancient shorelines in the national parks of the Great Lakes; an interpretative guidebook developed 

as a ‘geoscientist-in-the-park’ at Michigan’s Pictured Rocks National Lakeshore. Geological Society of America Abstracts with 

Programs 38(7). 

Bona, L. J. 1990. Geomorphology and recent geologic history of Long Island Apostle Islands National Lakeshore, Wisconsin. Thesis. 

University of Wisconsin, Madison, Wisconsin.

Bona, L. J. and D. S. Kiesel. 1990. Geomorphology and Holocene geologic history of Long Island, Apostle Islands National 

Lakeshore, Wisconsin. Geological Society of America Abstracts with Programs 22(5):4.

Braile, L.W. 2009. Seismic monitoring. Pages 229–244 in R. Young, R. and L. Norby, editors. Geological Monitoring. Geological 

Society of America, Boulder, Colorado. http://nature.nps.gov/geology/monitoring/seismic.cfm (accessed 6 June 2014).

Brown, B. 2006. Building and ornamental stone from the Precambrian rocks of Wisconsin; a review of historic production and 

future prospects. Geological Society of America Abstracts with Programs 38(4). 

Brunner, J., J. Kenworthy, and V. Santucci. 2009. Unauthorized fossil collecting from National Park Service shorelines: Servicewide 

policy and perspectives. Pages 319-323 in S. Weber, editor. Rethinking Protected Areas in a Changing World: Proceedings of 

the 2009 George Wright Society Biennial Conference on Parks, Protected Areas, and Cultural Sites. The George Wright Society, 

Hancock, Michigan. http://www.georgewright.org/0956brunner.pdf (accessed 7 January 2015).

Burghardt, J. E., E. S. Norby, and H. S. Pranger, II. 2014. Abandoned mineral lands in the National Park System: comprehensive 

inventory and assessment. Natural Resource Technical Report NPS/NRSS/GRD/NRTR—2014/906. National Park Service, Fort 

Collins, Colorado. https://irma.nps.gov/App/Reference/Profile/2192198 (accessed 3 June 2014).

Busch, J. C. 2008. People and places: a human history of the Apostle Islands. Historic resource study of the Apostle Islands National 

Lakeshore. Midwest Regional Office, National Park Service, Omaha, Nebraska. http://www.nps.gov/apis/historyculture/upload/

Historic%20Resource%20Study.pdf (accessed 6 June 2014). 

Bush, D. M. 2009. Marine features and processes. Pages 163–188 in R. Young and L. Norby, editors. Geological monitoring. 

Geological Society of America, Boulder, Colorado. http://nature.nps.gov/geology/monitoring/marine.cfm (accessed 7 April 2015).

Bush, D. M., and R. Young. 2009. Coastal features and processes. Pages 47–67 in R. Young and L. Norby, editors. Geological 

Monitoring. Geological Society of America, Boulder, Colorado. http://nature.nps.gov/geology/monitoring/coastal.cfm (accessed 6 

June 2014).

Caffrey, M. 2007. Visualization lake level fall. Unpublished simulation. Internal document, Geologic Resources Division, National 

Park Service, Denver, Colorado. 

Cannon, W. F. 1994. Closing of the Midcontinent rift—a far-field effect on Grenvillian compression. Geology 22(2):155–158. http://

pubs.er.usgs.gov/publication/70017864 (accessed 9 April 2015).

Cannon, W. F., and S. W. Nicholson. 1992. Revisions of stratigraphic nomenclature within the Keweenawan Supergroup of northern 

Michigan. Pages A1–A8 (chapter A) in W. F. Cannon and S. W. Nicholson, editors. Contributions to the geology and mineral 

resources of the Midcontinent rift system. Bulletin 1970-A, B. US Geological Survey, Reston, Virginia. http://pubs.usgs.gov/

bul/1970a-b/report.pdf (accessed 7 April 2015).
66

Cannon, W. F., L. G. Woodruff, S. W. Nicholson, and C. A. Hedgman. 1996. Bedrock geologic map of the Ashland and the northern 

part of the Ironwood 30’ × 60’ quadrangles, Wisconsin and Michigan (scale 1:100,000). Miscellaneous Geologic Investigations 

Map I-2566. US Geological Survey, Reston, Virginia. http://ngmdb.usgs.gov/Prodesc/proddesc_13052.htm (accessed 15 June 

2014). 

Cannon, W. F., L. G. Woodruff, S. W. Nicholson, C. A. Hedgman, and R. D. Barber-Delach. 1999. Digital bedrock geologic map 

of the Ashland and northern part of the Ironwood 30’ × 60’ quadrangles, Wisconsin and Michigan (scale 1:100,000). Open-File 

Report OF 99-546. US Geological Survey, Reston Virginia. http://pubs.er.usgs.gov/publication/ofr99546 (accessed 12 May 2014).

Cary, S. J., P. F. McDowell, and L. J. Graumilch. 1979. Soils and surficial geology of four Apostle Islands. Wisconsin Academy of 

Sciences, Arts and Letters 67:14–30.

Clayton, L. 1984. Pleistocene geology of the Superior region, Wisconsin. Information Circular Number 46. Wisconsin Geological 

and Natural History Survey, Madison, Wisconsin. http://wisconsingeologicalsurvey.org/pdfs/IC46.pdf (accessed 12 June 2014).

Clayton, L. 1985. Pleistocene geology of the Superior region, Wisconsin (scale 1:250,000). Regional Map Series 3. Wisconsin 

Geological and Natural History Survey, Madison, Wisconsin. 

Climate Change Response Program. 2012. Climate change case study: Apostle Islands National Lakeshore. Unpublished report. 

National Park Service, Climate Change Response Program, Fort Collins, Colorado. 

Coburn, A. S., A. D. Griffith, and R. S. Young. 2010. Inventory of coastal engineering projects in coastal national parks. Natural 

Resource Technical Report NPS/NRPC/GRD/NRTR—2010/373. National Park Service, Fort Collins, Colorado. https://irma.nps.

gov/App/Reference/Profile/2165163 (accessed 20 March 2015).

Culler, R.L., and P. Berendsen. 1998. The provenance and chemical variation of sandstones associated with the Midcontinent rift 

system, USA. European Journal of Mineralogy 10:987–1002.

Curdts, T. 2011. Shoreline length and water area in the ocean, coastal and Great Lakes parks: updated statistics for shoreline miles 

and water acres (rev1b). Natural Resource Report NPS/WASO/NRR—2011/464. National Park Service, Fort Collins, Colorado. 

https://irma.nps.gov/App/Reference/Profile/2180595/ (accessed 26 December 2013).

Davis, D. W., and J. B. Paces. 1990. Time resolution of geologic events on the Keweenaw Peninsula and implications for development 

of the Midcontinent rift system. Earth and Planetary Science Letters 97:54–64.

Davis, M. B., C. Douglas, and R. Calcote. 1998. Holocene paleoenvironments in the western Great Lakes parks. Pages 10–52 in

Holocene paleoenvironments in western Great Lakes Parks. Final report to the National Park Service. US Geological Survey, 

Biological Resources Division, Western Great Lakes Global Change Research Program, University of Missouri, Columbia, 

Missouri. 

Desai, A. R., J. A. Austin, V. Bennington, and G. A. McKinley. 2009. Stronger winds over a large lake in response to weakening air-tolake temperature gradient. Nature Geoscience Letters 2:855–858. http://www.nature.com/ngeo/journal/v2/n12/full/ngeo693.html 

(accessed 7 April 2015). 

Dott, R. H., and J. W. Attig. 2004. Roadside geology of Wisconsin. Mountain Press Publishing Co., Missoula, Montana. 

Eichenlaub, V. L. 1979. Weather and climate of the Great Lakes region. University of Notre Dame Press, Notre Dame, Indiana. 

Engstrom, W. N. 1974a. Spatial variations in beach height and width in the Apostle Islands, northern Wisconsin. The Yearbook 

(Association of Pacific Coast Geographers) 36:146–147. 

Engstrom, W. N. 1974b. Beach foreshore sedimentology and morphology in the Apostle Islands of northern Wisconsin. Journal of 

Sedimentary Petrology 44(1):190–206.

Engstrom, W. N. 1985. Recent beach erosion along spits and cuspate forelands of Apostle Islands National Lake Shore. Shore and 

Beach 53:22–28.

Farrand, W. R. 1960. Former shorelines in western and northern Lake Superior basin. Dissertation. University of Michigan, Ann 

Arbor, Michigan. 

Gable, D. J., and T. Hatton. 1983. Maps of vertical crustal movements in the conterminous United States over the last 10 million 

years. Miscellaneous Investigations Series Map MI 1315. US Geological Survey, Reston, Virginia. 

Gafvert, U. 2009. Monitoring land cover and land use in Great Lakes Network parks. Unpublished report. National Park Service, 

Ashland, Wisconsin.
67

Galston, L. 2008. Re-evaluation of the Proterozoic Devils Island Sandstone, Keweenawan rift, northern Wisconsin. Geological 

Society of America Abstracts with Programs 40(5):76. https://gsa.confex.com/gsa/2008NC/finalprogram/abstract_138108.htm

(accessed 9 April 2015).

Great Lakes Network. 2007. Climate Change, impacts of Midwest warming. Resource brief. National Park Service, Great Lakes 

Network, Ashland, Wisconsin. 

Great Lakes Network. 2009. Climate Change. Resource brief. National Park Service, Great Lakes Network, Ashland, Wisconsin. 

Great Lakes Network. 2011a. Monitoring diatoms in lake bottom sediments. Resource brief. National Park Service, Great Lakes 

Network, Ashland, Wisconsin.

Great Lakes Network. 2011b. Water quality monitoring in the Apostle Islands. Resource brief. National Park Service, Great Lakes 

Network, Ashland, Wisconsin.

Great Lakes Network. 2012a. Climate monitoring in Great Lakes National Parks. Resource brief. National Park Service, Great Lakes 

Network, Ashland, Wisconsin.

Great Lakes Network. 2012b. Monitoring landscape dynamics in the Apostle Islands. Resource brief. National Park Service, Great 

Lakes Network, Ashland, Wisconsin.

Great Lakes Network. 2013. Climate trends at Apostle Islands National Lakeshore. Resource brief. National Park Service, Great 

Lakes Network, Ashland, Wisconsin.

Hansel, A. K., D. M. Mickelson, A. F. Schneider, and C. E. Larsen. 1985. Late Wisconsinan and Holocene history of the Lake 

Michigan basin. Pages 39–53 in P. F. Karrow and P. E. Calkin, editors. Quaternary evolution of the Great Lakes. Special Paper 30. 

Geological Association of Canada, Toronto, Ontario.

Hart, M., and U. Gafvert. 2006. Data management plan: Great Lakes Inventory and Monitoring Network. Document number 

GLKN/2006/20. National Park Service, Great Lakes Network, Ashland, Wisconsin. 

Havholm, K. 2007. Investigator’s annual report. Unpublished report. Apostle Islands National Lakeshore, Bayfield, Wisconsin.

Highland, L. M., and P. Bobrowsky. 2008. The landslide handbook—a guide to understanding landslides. Circular 1325. US 

Geological Survey, Reston, Virginia. http://pubs.usgs.gov/circ/1325/ (accessed 6 June 2014).

Hinze, W. J., D. J. Allen, L. W. Braile, and J. Mariano. 1997. The Midcontinent rift system: a major Proterozoic continental rift. Pages 

7–35 in R. W. Ojakangas, A. B. Dickas, and J. C. Green, editors. Middle Proterozoic to Cambrian rifting, central North America. 

Special Paper 312. Geological Society of America, Boulder, Colorado. 

Holmes, R. R. Jr., L. M. Jones, J. C. Eidenshink, J. W. Godt, S. H. Kirby, J. J. Love, C. A. Neal, N. G. Plant, M. L. Plunkett, C. S. 

Weaver, A. Wein, and S. C. Perry. 2013. US Geological Survey natural hazards science strategy—Promoting the safety, security, 

and economic well-being of the nation. Circular 1383–F. US Geological Survey, Reston, Virginia. http://pubs.usgs.gov/circ/1383f/

(accessed 6 June 2014).

Huff, M. D., and J. Swenson. 2002. Sedimentation in western Lake Superior. Geological Society of America Abstracts with Programs 

34(2):12. 

Hunt, R. K., J. P. Kenworthy, and V. L. Santucci. 2008. Paleontological resource inventory and monitoring—Great Lakes Network. 

Natural Resource Technical Report NPS/NRPC/NRTR—2008/120. National Park Service, Fort Collins, Colorado. 

Jordahl, H. C., Jr. 1994. A unique collection of islands. The influence of history, politics, policy, and planning on the establishment of 

the Apostle Islands National Lakeshore. University of Wisconsin, Madison, Wisconsin.

Kraft, G. 1982. Soil and recreational use on the Apostle Islands National Lakeshore: I. Soil suitability for trails and campsites, II. 

Bank erosion in the Presque Isle campground. Dissertation. University of Wisconsin, Stevens Point, Wisconsin. 

LaBerge, G. L. 1994. Geology of the Lake Superior region. Geoscience Press, Phoenix, Arizona. 

Lafrançois, T., M. B. Edlund, L. Ferrington Jr., A. Egan, A. Fedie, and J. Glase. 2010. Mapping and biological studies of shoreline 

rock pools in three Lake Superior national parks (part of GLRI project 90). Summary trip report for 2010 fieldwork in Apostle 

Islands National Park. Unpublished report. National Park Service.

Lafrançois B. M., S. C. Riley, D. S. Blehert, and A. E. Ballmann. 2011. Links between type E botulism outbreaks, lake levels, and 

surface water temperatures in Lake Michigan, 1963–2008. Journal of Great Lakes Research 37:86–91.
68

Lancaster, N. 2009. Aeolian features and processes. Pages 1–25 in R. Young and L. Norby, editors. Geological monitoring. 

Geological Society of America, Boulder, Colorado. http://nature.nps.gov/geology/monitoring/aeolian.cfm (accessed 6 June 2014).

Larsen, C. E. 1999. Submergence along the south and western shore of Lake Superior since 2100 yrs B.P. Program and Abstracts of 

the Conference of the International Association for Great Lakes Research 42:A-66.

Larsen, C. E., C. C. Cameron, and E. I. Robbins. 1999. The reconstructed Late Holocene lake-level history of the Apostles Islands 

National Lakeshore, Lake Superior. Program and Abstracts of the Conference of the International Association for Great Lakes 

Research 42:A-66–67.

Lord, M. L., D. Germanoski, and N. E. Allmendinger. 2009. Fluvial geomorphology: Monitoring stream systems in response to a 

changing environment. Pages 69–103 in R. Young and L. Norby, editors. Geological monitoring. Geological Society of America, 

Boulder, Colorado. http://nature.nps.gov/geology/monitoring/fluvial.cfm (accessed 6 June 2014).

Martin, L. 1916. The physical geography of Wisconsin. Bulletin No. XXXVI, Educational Series No. 4. Wisconsin 

Geological and Natural History Survey, Madison, Wisconsin. http://archive.org/stream/physicalgeograph00martrich/

physicalgeograph00martrich_djvu.txt (accessed 12 June 2014). 

Melillo, J. M., T. C. Richmond, and G. W. Yohe, editors. 2014. Climate change impacts in the United States: the third national climate 

assessment. US Global Change Research Program. http://nca2014.globalchange.gov/downloads (accessed 12 August 2014).

Miller, M. Date unknown. Mineral occurrence at Apostle Islands National Lakeshore. Unpublished report. National Park Service, 

Geologic Resources Division, Denver, Colorado. 

Minnesota Sea Grant. 2014. Lake Superior’s natural processes. Minnesota Sea Grant, University of Minnesota, Duluth, Minnesota. 

http://www.seagrant.umn.edu/superior/processes (accessed 21 October 2014).

National Oceanic and Atmospheric Administration. 2014. Water levels of the Great Lakes. National Oceanic and Atmospheric 

Administration, Ann Arbor, Michigan. http://www.glerl.noaa.gov/pubs/brochures/lakelevels/lakelevels.pdf (accessed 6 November 

2014). 

National Park Service. 2004. Final wilderness study: Apostle Islands. Apostle Islands National Lakeshore, Bayfield, Wisconsin. 

http://www.nps.gov/apis/parkmgmt/wild-study.htm (accessed 11 September 2014).

National Park Service. 2005. Geology fieldnotes: Apostle Islands National Lakeshore, Wisconsin. Online information. National Park 

Service, Geologic Resources Division, Lakewood, Colorado. http://www2.nature.nps.gov/geology/parks/apis/ (accessed 15 June 

2014). 

National Park Service. 2011. Final General management plan / wilderness management plan / environmental impact statement for 

Apostle Islands National Lakeshore. Apostle Islands National Lakeshore, Bayfield and Ashland counties, Wisconsin. http://www.

nps.gov/apis/learn/management/gmp.htm (accessed 3 April 2015).

Natural Resources Research Institute. 2001. Pre-settlement vegetation. University of Minnesota, The Lake Superior Decision 

Support Project, Duluth, Minnesota. http://www.nrri.umn.edu/lsgis/fservice.htm (accessed 30 June 2014). 

Natural Resources Research Institute. 2006. Important habitat sites and areas in the Lake Superior basin 2006. University of 

Minnesota, The Lake Superior Decision Support Project, Duluth, Minnesota. http://www.nrri.umn.edu/lsgis/importanthabitat.

htm (accessed 30 June 2014). 

Nicholson, S. W., W. F. Cannon, L. G. Woodruff, and C. L. Dicken. 2004a. Bedrock geologic map of the Port Wing, Solon Springs, 

and parts of the Duluth and Sandstone 30’ × 60’ quadrangles, Wisconsin (scale 1:100,000). Open-File Report 2004-1303. US 

Geological Survey, Reston, Virginia. http://pubs.er.usgs.gov/publication/ofr20041303 (accessed 12 May 2014).

Nicholson, S. W., C. L. Dicken, M. P. Foose, and J. Mueller. 2004b. Integrated geologic map databases for the United States, the 

upper Midwest states: Minnesota, Wisconsin, Michigan, Illinois, and Indiana (scale 1:500,000). Open-File Report 2004-1355. US 

Geological Survey, Reston, Virginia. http://pubs.er.usgs.gov/publication/ofr20041355 (accessed 12 May 2014).

Nicholson, S. W., W. F. Cannon, L. G. Woodruff, and C. L. Dicken. 2006. Bedrock geologic map of the Port Wing, Solon Springs, and 

parts of the Duluth and Sandstone 30’ × 60’ quadrangles, Wisconsin and Minnesota (scale 1:100,000). Scientific Investigations 

Map 2869. US Geological Survey, Reston, Virginia. http://pubs.usgs.gov/sim/2869/report.pdf (accessed 18 August 2014).

Nuhfer, E. B., and M. P. Dalles. 1994. Lake Superior’s Apostle Islands National Lakeshore as a resource for geologic education. 

Geological Society of America Abstracts with Programs 26(5):56.
69

Nuhfer, E. B., and M. P. Dalles. 2004. A guidebook to the geology of Lake Superior’s Apostle Islands National Lakeshore and nearby 

areas of the Bayfield Peninsula of Wisconsin. Revised edition. Eastern National, Fort Washington, Pennsylvania. 

Ojakangas, R. W., D. Cramer, and T. Fitz. 2011. Trip 3—geology of the Bayfield Peninsula: Keweenawan Bayfield Group and 

Pleistocene deposits. Pages 51–77 in T. Fitz, A. Mills, K. Wilson, C. Bodette, and D. Cramer, editors. Proceedings Volume 57. 

Institute on Lake Superior Geology, Ashland, Michigan. http://www.d.umn.edu/prc/lakesuperiorgeology/Volumes/ILSG11%20

Guidebook-fixed.pdf (accessed 7 April 2015).

Ojakangas, R. W., A. B. Dickas, and J. C. Green. 1997. Introduction: Middle Proterozoic to Cambrian rifting, central North America. 

Pages 1–6 in Ojakangas, R. W., A. B. Dickas, and J. C. Green, editors. Middle Proterozoic to Cambrian rifting, central North 

America. Special paper 312. Geological Society of America, Boulder, Colorado. 

Pendleton, E. A., E. R. Thieler, and S. J. Williams. 2007. Coastal change-potential assessment of Sleeping Bear Dunes, Indiana Dunes, 

and Apostle Islands National Lakeshore to lake-level changes. Open-File Report 2005-1249. US Geological Survey, Woods Hole, 

Massachusetts. http://pubs.usgs.gov/of/2005/1249/index.html (accessed 18 June 2014). 

Peterson, W. L. 1986. Late Wisconsinan glacial history of northeastern Wisconsin and western Upper Michigan. Bulletin 1652. US 

Geological Survey, Reston, Virginia. http://pubs.er.usgs.gov/publication/b1652 (accessed July 4, 2014). 

Petersen, M. D., A. D. Frankel, S. C. Harmsen, C. S. Mueller, K. M. Haller, R. L. Wheeler, R. L. Wesson, Y. Zeng, O. S. Boyd, D. 

M. Perkins, N. Luco, E. H. Field, C. J. Wills, and K. S. Rukstales. 2008. Documentation for the 2008 update of the United States 

national seismic hazard maps. Open-file report 2008–1128. US Geological Survey, Reston, Virginia. http://pubs.usgs.gov/

of/2008/1128/ (accessed 15 January 2014).

Pranger, H. 2005. Recommendations for Disturbed Land Restoration at Ironwood Island, Apostle Islands National Lakeshore, 

Wisconsin. Unpublished report. National Park Service, Geologic Resources Division, Denver, Colorado. 

Robinson, J. E., C. R. Bacon, and C. Wayne. 2012. Under trees and water at Crater Lake National Park, Oregon. Map sheet/poster. 

Scientific Investigations Map 3223. US Geological Survey, Washington, DC. http://pubs.usgs.gov/sim/3223/ (accessed 1 April 

2015).

Rodman, K. 2014. Harsh winter causes greatest Great Lakes ice coverage since 1979. Online article (8 March 2014). Accuweather.

com, State College, Pennsylvania. http://www.accuweather.com/en/weather-news/nearly-frozen-lake-superior-ma/23439393

(accessed 1 April 2015). 

Rolfhus, K., J. Wiener, M. Sandheinrich, and R. Haro. 2005. Characterization of mercury contamination of aquatic food webs in 

lagoons in the Apostle Islands National Lakeshore (Lake Superior). Unpublished document. On file at Apostle Islands National 

Lakeshore, Bayfield, Wisconsin.

Rose, W. J. 1987. Water resources of the Apostle Islands National Lakeshore, northern Wisconsin. Water-Resources Investigations 

Report 87-4220. US Geological Survey, Madison, Wisconsin. http://pubs.er.usgs.gov/publication/wri874220 (accessed 8 April 

2015).

Santucci, V. L., J. P. Kenworthy, and A. L. Mims. 2009. Monitoring in situ paleontological resources. Pages 189–204 in R. Young 

and L. Norby, editors. Geological monitoring. Geological Society of America, Boulder, Colorado. http://nature.nps.gov/geology/

monitoring/paleo.cfm (accessed 6 June 2014).

Scholz, C. 1984. Late and post-glacial lacustrine sediment distribution in western Lake Superior from seismic reflection profiles. 

Proceedings and Abstracts for the Annual Institute on Lake Superior Geology 30:61–62.

Stucker, G. 1974. Shores of the Apostles, once again the Manitou treads the land of Hiawatha. National Parks and Conservation 

Magazine 48(7):4–9.

Swain, A. M., and M. Winkler. 1983. Forest and disturbance history at Apostle Islands National Lakeshore. Park Science 3(4):3–5.

Syverson, K. M., and P. M. Colgan. 2004. The Quaternary of Wisconsin: a review of stratigraphy and glaciation history. Pages 

295–311 in J. Ehlers and P. L. Gibbard, editors. Quaternary glaciations—extent and chronology, Part II: North America. Elsevier 

Publishing, Amsterdam, Netherlands.

Thornberry-Ehrlich, T. L. 2013. Geologic Resources Inventory scoping summary Apostle Islands National Lakeshore. National Park 

Service, Geologic Resources Division, Denver, Colorado. http://www.nature.nps.gov/geology/inventory/gre_publications.cfm

(accessed 12 May 2014).
70

Thwaites, F. T. 1912. Sandstones of the Wisconsin coast of Lake Superior. Bulletin 25, Scientific Series 8. Wisconsin Geological and 

Natural History Survey, Madison, Wisconsin. 

Toomey, R. S. III. 2009. Geological monitoring of caves and associated landscapes. Pages 27–46 in R. Young and L. Norby, editors. 

Geological monitoring. Geological Society of America, Boulder, Colorado. http://nature.nps.gov/geology/monitoring/cavekarst.

cfm (accessed 6 June 2014).

Tynan, T. 2012. The bayous of the north: the Kakagon and Bad River sloughs. Online article and video (posting date unknown). 

http://vimeo.com/32147202 (accessed 11 August 2013).

US Global Change Research Program. 2000. US National assessment of the potential consequences of climate variability and 

change. US Climate Change Science Program/US Global Change Research Program, Washington, DC.

Varnes, D. J. 1978. Slope movement types and processes. Pages 11–33 in R. L. Schuster and R. J. Krizek, editors. Landslides: analysis 

and control. Special Report 176. National Academy of Science, Transportation and Road Research Board, Washington, DC.

Volkert, R. A., and A. A. Drake Jr. 1999. Geochemistry and stratigraphic relations of middle Proterozoic rocks of the New Jersey 

Highlands. Professional Paper 1565-C. US Geological Survey, Reston, Virginia. http://pubs.er.usgs.gov/publication/pp1565C

(accessed 4 September 2013).

Wieczorek, G. F., and J. B. Snyder. 2009. Monitoring slope movements. Pages 245–271 in R. Young and L. Norby, editors. Geological 

monitoring. Geological Society of America, Boulder, Colorado. http://nature.nps.gov/geology/monitoring/slopes.cfm (accessed 6 

June 2014).

Wilson, L. R. 1935. The Nipissing flora of the Apostle Islands region. Bulletin of the Torrey Botanical Club 62(9)533–535.

Wright, M. 1997. Apostle Islands National Lakeshore: sea caves. Site bulletin. Geoscientists-in-the-Parks document 1997-APIS. 

National Park Service, Denver, Colorado. 

Wyckoff, J. 1999. Reading the Earth—landforms in the making. Adastra West, Inc. Mahwah, New Jersey. 

Young, R. and L. Norby, editors. 2009. Geological monitoring. Geological Society of America, Boulder, Colorado. http://nature.nps.

gov/geology/monitoring/index.cfm (accessed 7 April 2015).
71

Additional References

This section lists additional references, resources, and websites that may be of use to resource managers. 

Web addresses are valid as of May 2015. Refer to Appendix B for laws, regulations, and policies that 

apply to NPS geologic resources.

Geology of National Park Service Areas

● NPS Geologic Resources Division (Lakewood, 

Colorado) Energy and Minerals; Active Processes 

and Hazards; Geologic Heritage:

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

● NPS Geologic Resources Inventory: http://www.

nature.nps.gov/geology/inventory/index.cfm.

● NPS Geoscientist-In-the-Parks (GIP) internship 

and guest scientist program:

http://www.nature.nps.gov/geology/gip/index.cfm

● NPS Views program (geology-themed modules are 

available for Geologic Time, Paleontology, Glaciers, 

Caves and Karst, Coastal Geology, Volcanoes, and a 

variety of geologic parks):

http://www.nature.nps.gov/views/

NPS Resource Management Guidance and 

Documents

● 1998 National parks omnibus management act: 

http://www.gpo.gov/fdsys/pkg/PLAW-105publ391/

pdf/PLAW-105publ391.pdf

● Management Policies 2006 (Chapter 4: Natural 

resource management):

http://www.nps.gov/policy/mp/policies.html

● NPS-75: Natural resource inventory and 

monitoring guideline:

http://www.nature.nps.gov/nps75/nps75.pdf

● NPS Natural resource management reference 

manual #77: http://www.nature.nps.gov/Rm77/

● Geologic monitoring manual (Young, R., and 

L. Norby, editors. 2009. Geological monitoring. 

Geological Society of America, Boulder, Colorado): 

http://nature.nps.gov/geology/monitoring/index.

cfm

● NPS Technical Information Center (TIC) (Denver, 

Colorado; repository for technical documents): 

http://www.nps.gov/dsc/technicalinfocenter.htm

Climate Change Resources

● NPS Climate Change Response Program Resources: 

http://www.nps.gov/subjects/climatechange/

resources.htm

● US Global Change Research Program:

http://globalchange.gov/home

● Intergovernmental Panel on Climate Change:

http://www.ipcc.ch/

Geological Surveys and Societies

● Wisconsin Geological and Natural History Survey: 

http://wgnhs.uwex.edu/

● US Geological Survey: http://www.usgs.gov/

● Geological Society of America:

http://www.geosociety.org/

● American Geophysical Union: http://sites.agu.org/

● American Geosciences Institute:

http://www.americangeosciences.org/

● Association of American State Geologists:

http://www.stategeologists.org/

US Geological Survey Reference Tools

● National geologic map database (NGMDB):

http://ngmdb.usgs.gov/

● Geologic names lexicon (GEOLEX; geologic unit 

nomenclature and summary):

http://ngmdb.usgs.gov/Geolex/geolex_home.html

● Geographic names information system (GNIS; 

official listing of place names and geographic 

features): http://gnis.usgs.gov/

● GeoPDFs (download searchable PDFs of any 

topographic map in the United States):

http://store.usgs.gov (click on “Map Locator”)

● Publications warehouse (many publications 

available online): http://pubs.er.usgs.gov

● Tapestry of time and terrain (descriptions of 

physiographic provinces):

http://tapestry.usgs.gov/Default.html

73

Appendix A: Scoping Participants

The following people attended the GRI scoping meeting for Apostle Islands National Lakeshore, held 

on 20-21 July 2010, or the follow-up report writing conference call, held on 19 June 2014. Discussions 

during these meetings supplied a foundation for this GRI report. The scoping summary document is 

available on the GRI publications website: http://go.nps.gov/gripubs.

2010 Scoping Meeting Participants

Name Affiliation Position

Peggy Burkman NPS Apostle Islands National Lakeshore Biologist

Eric Carson Wisconsin Geological and Natural History Survey Geologist

Jim Chappell Colorado State University Geologist

Ulf Gafvert NPS Great Lakes Network GIS Specialist

Bruce Heise NPS Geologic Resources Division Geologist

Jason Kenworthy NPS Geologic Resources Division Geologist, GRI reports coordinator

Richard Ojakangas University of Minnesota (Duluth) Geologist

Jamie Robertson Wisconsin Geological and Natural History Survey Geologist

Brandon Seitz NPS Grand Portage National Monument Biological science technician

Trista Thornberry-Ehrlich Colorado State University Geologist, GRI report author

Julie Van Stappen NPS Apostle Islands National Lakeshore Chief, Planning and Resource Management

Laurel Woodruff US Geological Survey Geologist

2014 Conference Call Participants

Name Affiliation Position

Peggy Burkman NPS Apostle Islands National Lakeshore Biologist

Jim Chappell Colorado State University Geologist

Ulf Gafvert NPS Great Lakes Network GIS Specialist

Jason Kenworthy NPS Geologic Resources Division Geologist, GRI reports coordinator

Dale Pate NPS Geologic Resources Division Cave and Karst Program coordinator

Hal Pranger NPS Geologic Resources Division Chief, Geologic Features and Systems Branch

Trista Thornberry-Ehrlich Colorado State University Geologist, GRI report author

Julie Van Stappen NPS Apostle Islands National Lakeshore Chief, Planning and Resource Management

Laurel Woodruff US Geological Survey Geologist

75

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Caves and Karst Systems

Federal Cave Resources Protection Act 

of 1988, 16 USC §§ 4301 – 4309 requires 

Interior/Agriculture to identify “significant 

caves” on Federal lands, regulate/

restrict use of those caves as appropriate, 

and include significant caves in land 

management planning efforts. Imposes 

civil and criminal penalties for harming 

a cave or cave resources. Authorizes 

Secretaries to withhold information about 

specific location of a significant cave from 

a Freedom of Information Act (FOIA) 

requester. 

National Parks Omnibus Management 

Act of 1998, 16 USC § 5937 protects the 

confidentiality of the nature and specific 

location of cave and karst resources.

36 CFR § 2.1 prohibits possessing/ 

destroying/disturbing...cave resources...in 

park units.

43 CFR Part 37 states that all NPS caves 

are “significant” and sets forth procedures 

for determining/releasing confidential 

information about specific cave locations to 

a FOIA requester.

Section 4.8.1.2 requires NPS to maintain 

karst integrity, minimize impacts.

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Section 4.8.2.2 requires NPS to protect 

caves, allow new development in or on 

caves if it will not impact cave environment, 

and to remove existing developments if 

they impair caves.

Section 6.3.11.2 explains how to manage 

caves in/adjacent to wilderness.

Lechuguilla Cave Protection Act of 

1993, Public Law 103-169 created a cave 

protection zone (CPZ) around Lechuguilla 

Cave in Carlsbad Caverns National Park. 

Within the CPZ, access and the removal 

of cave resources may be limited or 

prohibited; existing leases may be cancelled 

with appropriate compensation; and lands 

are withdrawn from mineral entry.

Paleontology

National Parks Omnibus Management 

Act of 1998, 16 USC § 5937 protects the 

confidentiality of the nature and specific 

location of paleontological resources and 

objects.

Paleontological Resources Preservation 

Act of 2009, 16 USC § 470aaa et 

36 CFR § 2.1(a)(1)(iii) prohibits destroying, 

injuring, defacing, removing, digging or 

disturbing paleontological specimens or 

parts thereof.

Prohibition in 36 CFR § 13.35 applies 

even in Alaska parks, where the surface 

collection of other geologic resources is 

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Section 4.8.2.1 emphasizes Inventory and 

Monitoring, encourages scientific research, 

directs parks to maintain confidentiality of 

paleontological information, and allows 

seq. provides for the management and 

protection of paleontological resources on 

federal lands.

permitted.

Regulations in association with 2009 PRPA 

are being finalized (May 2015).

parks to buy fossils only in accordance with 

certain criteria.

Appendix B: Geologic Resource Laws, Regulations, and Policies

The NPS Geologic Resources Division developed this table to summarize laws, regulations, and policies 

that specifically apply to National Park Service minerals and geologic resources. The table does not 

include laws of general application (e.g., Endangered Species Act, Clean Water Act, Wilderness Act, 

National Environmental Policy Act, or National Historic Preservation Act). The table does include the 

NPS Organic Act when it serves as the main authority for protection of a particular resource or when 

other, more specific laws are not available. Information is current as of May 2015. Contact the NPS 

Geologic Resources Division for detailed guidance.
76

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Rocks and Minerals

NPS Organic Act, 16 USC § 1 et seq.

directs the NPS to conserve all resources 

in parks (including rock and mineral 

resources), unless otherwise authorized by 

law.

Exception: 16 USC § 445c (c) – Pipestone 

National Monument enabling statute. 

Authorizes American Indian collection of 

catlinite (red pipestone).

36 CFR § 2.1 prohibits possessing, 

destroying, disturbing mineral resources...

in park units.

Exception: 36 CFR § 7.91 allows limited 

gold panning in Whiskeytown.

Exception: 36 CFR § 13.35 allows some 

surface collection of rocks and minerals 

in some Alaska parks (not Klondike 

Gold Rush, Sitka, Denali, Glacier Bay, 

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

or Katmai) by non-disturbing methods 

(e.g., no pickaxes), which can be stopped 

by superintendent if collection causes 

significant adverse effects on park 

resources and visitor enjoyment.

Section 9.1.3.3 clarifies that only the 

NPS or its agent can extract park-owned 

common variety minerals (e.g., sand and 

gravel), and:

-only for park administrative uses;

-after compliance with NEPA and other 

federal, state, and local laws, and a finding 

of non-impairment;

-after finding the use is park’s most 

reasonable alternative based on 

environment and economics;

-parks should use existing pits and create 

new pits only in accordance with park-wide 

borrow management plan;

-spoil areas must comply with Part 6

Park Use of Sand and Gravel

Materials Act of 1947, 30 USC § 601

does not authorize the NPS to dispose of 

mineral materials outside of park units.

Exception: 16 USC §90c 1(b) the nonwilderness portion of Lake Chelan National 

Recreation Area, where sand, rock and 

gravel may be made available for sale to 

the residents of Stehekin for local use as 

long as such sale and disposal does not 

have significant adverse effects on the 

administration of the National Recreation 

Area. 

None applicable.

standards; and

-NPS must evaluate use of external 

quarries.

Any deviation from this policy requires a 

written waiver from the Secretary, Assistant 

Secretary, or Director.
77

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Coastal Features and Processes

NPS Organic Act, 16 USC § 1 et. 

seq. authorizes the NPS to promulgate 

regulations to protect park resources and 

values (from, for example, the exercise of 

mining and mineral rights).

Coastal Zone Management Act, 16 USC 

§ 1451 et. seq. requires Federal agencies 

to prepare a consistency determination for 

every Federal agency activity in or outside 

of the coastal zone that affects land or 

water use of the coastal zone.

Clean Water Act, 33 USC § 1342/Rivers 

and Harbors Act, 33 USC 403 require that 

dredge and fill actions comply with a Corps 

of Engineers Section 404 permit.

Executive Order 13089 (coral reefs) 

(1998) calls for reduction of impacts to 

coral reefs.

Executive Order 13158 (marine 

protected areas) (2000) requires every 

federal agency, to the extent permitted by 

law and the maximum extent practicable, 

to avoid harming marine protected areas.

36 CFR § 1.2(a)(3) applies NPS regulations 

to activities occurring within waters subject 

to the jurisdiction of the US located 

within the boundaries of a unit, including 

navigable water and areas within their 

ordinary reach, below the mean high water 

mark (or OHW line) without regard to 

ownership of submerged lands, tidelands, 

or lowlands.

36 CFR § 5.7 requires NPS authorization 

prior to constructing a building or other 

structure (including boat docks) upon, 

across, over, through, or under any park 

area.

Section 4.1.5 directs the NPS to reestablish natural functions and processes 

in human-disturbed components of natural 

systems in parks unless directed otherwise 

by Congress.

Section 4.4.2.4 directs the NPS to allow 

natural recovery of landscapes disturbed by 

natural phenomena, unless manipulation of 

the landscape is necessary to protect park 

development or human safety.

Section 4.8.1 requires NPS to allow natural 

geologic processes to proceed unimpeded. 

NPS can intervene in these processes 

only when required by Congress, when 

necessary for saving human lives, or when 

there is no other feasible way to protect 

other natural resources/ park facilities/

historic properties.

Section 4.8.1.1 requires NPS to:

-Allow natural processes to continue 

without interference,

-Investigate alternatives for mitigating the 

effects of human alterations of natural 

processes and restoring natural conditions, 

-Study impacts of cultural resource 

protection proposals on natural resources, 

-Use the most effective and natural-looking 

erosion control methods available, and 

-Avoid putting new developments in areas 

subject to natural shoreline processes 

unless certain factors are present.
78

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Upland and Fluvial Processes

Rivers and Harbors Appropriation 

Act of 1899, 33 USC § 403 prohibits the 

construction of any obstruction on the 

waters of the United States not authorized 

by congress or approved by the USACE.

Clean Water Act 33 USC § 1342 requires 

a permit from the USACE prior to any 

discharge of dredged or fill material 

into navigable waters (waters of the US 

[including streams]).

Executive Order 11988 requires federal 

agencies to avoid adverse impacts to 

floodplains. (see also D.O. 77-2) 

Executive Order 11990 requires plans for 

potentially affected wetlands (including 

riparian wetlands). (see also D.O. 77-1)

None applicable. Section 4.1 requires NPS to manage 

natural resources to preserve fundamental 

physical and biological processes, as well 

as individual species, features, and plant 

and animal communities; maintain all 

components and processes of naturally 

evolving park ecosystems.

Section 4.1.5 directs the NPS to reestablish natural functions and processes 

in human-disturbed components of natural 

systems in parks, unless directed otherwise 

by Congress.

Section 4.4.2.4 directs the NPS to allow 

natural recovery of landscapes disturbed by 

natural phenomena, unless manipulation of 

the landscape is necessary to protect park 

development or human safety.

Section 4.6.4 directs the NPS to (1) 

manage for the preservation of floodplain 

values; [and] (2) minimize potentially 

hazardous conditions associated with 

flooding.

Section 4.6.6 directs the NPS to manage 

watersheds as complete hydrologic systems 

and minimize human-caused disturbance 

to the natural upland processes that deliver 

water, sediment, and woody debris to 

streams.

Section 4.8.1 directs the NPS to allow 

natural geologic processes to proceed 

unimpeded. Geologic processes...include...

erosion and sedimentation...processes.

Section 4.8.2 directs the NPS to protect 

geologic features from the unacceptable 

impacts of human activity while allowing 

natural processes to continue.
79

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Soils

Soil and Water Resources Conservation 

Act, 16 USC §§ 2011–2009 provides for 

the collection and analysis of soil and 

related resource data and the appraisal of 

the status, condition, and trends for these 

resources.

Farmland Protection Policy Act, 7 USC 

§ 4201 et. seq. requires NPS to identify 

and take into account the adverse effects 

of Federal programs on the preservation 

of farmland; consider alternative actions, 

and assure that such Federal programs 

are compatible with State, unit of local 

government, and private programs 

and policies to protect farmland. NPS 

actions are subject to the FPPA if they 

may irreversibly convert farmland (directly 

or indirectly) to nonagricultural use and 

are completed by a Federal agency or 

with assistance from a Federal agency. 

Applicable projects require coordination 

with the Department of Agriculture’s 

Natural Resources Conservation Service 

(NRCS).

7 CFR Parts 610 and 611 are the US 

Department of Agriculture regulations for 

the Natural Resources Conservation Service.

Part 610 governs the NRCS technical 

assistance program, soil erosion predictions, 

and the conservation of private grazing 

land.

Part 611 governs soil surveys and 

cartographic operations. The NRCS 

works with the NPS through cooperative 

arrangements.

Section 4.8.2.4 requires NPS toprevent unnatural erosion, removal, and 

contamination;-conduct soil surveys;-

minimize unavoidable excavation; 

and-develop/follow written prescriptions 

(instructions).

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.

NPS 633/128695, May 2015
EXPERIENCE YOUR AMERICATM

Natural Resource Stewardship and Science 

1201 Oak Ridge Drive, Suite 150 

Fort Collins, Colorado 80525

www.nature.nps.gov

National Park Service

U.S. Department of the Interior



APIS Surficial (“Pleistocene”) Geologic Map Unit Properties Table, Page 1 of 4

Surficial (“Pleistocene”) Geologic Map Unit Properties Table: Apostle Islands National Lakeshore

Map unit colors correspond to GRI map poster (in pocket). Bold text refers to sections in report.

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY

(Holocene)

Postglacial deposits, 

shoreline sediment

(Qc)

Qc consists of well-sorted sand and gravel. Qc occurs 

along the shoreline of Lake Superior and no more than 

a few meters above the present lake level. 

Long Island is composed entirely of Qc. Qc is also 

mapped on Rocky, Outer, Michigan, and Stockton 

islands.

Sandscapes—Qc is the primary unit composing the 

dynamic sandscapes within the lakeshore.

Recent Surficial Deposits—Qc is still collecting and 

being reworked on the shores of the Apostle Islands by 

coastal processes. 

Sedimentary Rock Features—modern features in Qc

such as ripple marks reflect the ancient features in 

solidified bedrock of the Bayfield Group (PCdi, PCch, and 

PCo). 

Shoreline Response to Lake-Level Change—Qc is 

vulnerable to considerable erosion. Sandscapes would 

decline with rising lake levels. High water caused loss of 

beach area at Outer Island in spring 2014.

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qc reflects 

the ongoing processes of weathering and erosion along 

the shores of Lake Superior. Qc was deposited after about 

5,000 years ago.

Miller Creek Formation, 

windblown sediment

(Qw)

Qw consists of well-sorted sand. Deposits of Qw can 

reach several meters thick. When enough sand is 

present, transverse dunes with northward-sloping slip

faces form locally.

Qw is not mapped within Apostle Islands National 

Lakeshore.

Aeolian Processes—Qw forms as wind transports away 

the finer-grained components. Individual sand grains move 

via saltation (series of little hops) in the wind. 

Shoreline Response to Lake-Level Change—Qw, 

where present along the shoreline, is prone to erosion 

when lake levels drop.

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qw

reflects the ongoing processes of weathering and erosion 

along the shores of Lake Superior. Qw was deposited after 

about 9,500 years ago following the final retreat of 

glaciers from the Lake Superior basin and prior to 

significant vegetation.

QUATERNARY

(Holocene and Pleistocene)

Miller Creek Formation, 

shoreline sediment, 

offshore sediment

(Qb)

Qb consists of well-sorted sand and gravel in deposits 

between 1 and 10 m (3 and 30 ft) thick. Ob appears as 

a series of beach ridges (i.e., lines in the GRI GIS 

geologic map data).

Qb is not mapped within Apostle Islands National 

Lakeshore.

Glacial Features—Qb is well-sorted beach material that 

collected on the shores of a series of proglacial lakes. None reported.

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qb was 

deposited between about 11,500 and 9,500 years ago as 

beach deposits on a series of proglacial lakes. Qb was 

deposited after about 9,500 years ago following the final 

retreat of glaciers from the Lake Superior basin and prior 

to significant vegetation.

this row is blank to maintain print formatting
Map unit colors correspond to GRI map poster (in pocket). Bold text refers to sections in report.

APIS Surficial (“Pleistocene”) Geologic Map Unit Properties Table, Page 2 of 4

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY

(Holocene and Pleistocene)

Miller Creek Formation, 

till—valley sides

(Qgh)

Till is unsorted clay, silt, sand, gravel, and boulders 

(glacial sediment) deposited directly from glacial ice. In 

general, the Miller Creek Formation is redder in color 

and richer in clay content than the Copper Falls 

Formation. 

Qgh is mapped where steep slopes (10° to 15°) cut 

into the Miller Creek till. 

Deposits of Qgl are of low relief with some hummocky 

and collapsed (soft-sediment deformation) areas. Qgl is 

commonly 1 to 20 m (3 to 6 ft) thick. 

Qgw occurs on higher and steeper part of the Superior 

lowland than Qgl and was subjected to less wave 

energy. In less-eroded areas, 1 m (3 ft) of nearshore 

sand overlies Qgw. 

Qgl and Qgw are the primary surficial geologic units 

mapped within Apostle Islands National Lakeshore. 

Glacial Features—in areas where broad hillsides are 

exposed as Qgh, Miller Creek till, offshore and stream 

sediment, and Copper Falls till are visible. The hummocky 

topography of Qgl was subdued by wave action, or by 

being deposited in water-saturated conditions with softsediment collapse. Drumlins are mapped in Qgl.

Slope Movements—Qgh features slope wash and slopemovement deposits at the base of hillslopes. Landslides are 

common.

Shoreline Response to Lake-Level Change—Qgl and 

Qgw weather to provide sediment to the sandscapes 

within Apostle Islands National Lakeshore. 

Slope Movement Hazards and Risks—Qgl, Qgh, and

Qgw are prone to slope wash and landslides. 

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qgh, Qgl, 

and Qgw were deposited between about 11,500 and 

9,500 years ago. Wave action from Lake Superior modified 

Qgl. 

Miller Creek Formation, 

till—lake-modified glacial 

topography

(Qgl)

Miller Creek Formation, 

till—wave-planed 

topography

(Qgw)

Miller Creek Formation, 

uncollapsed offshore 

sediment

(Qou)

Qou consists of finer-grained, offshore sediments with 

horizontal bedding. Qou exhibits flat topography.

Qou is not mapped within Apostle Islands National 

Lakeshore.

Fluvial Features—the Sand River valley cuts through Qou

on its course to Lake Superior.

Glacial Features—Qou was deposited on solid ground 

and was not subject to soft sediment deformation caused 

by collapse of any underlying ice melting.

Slope Movement Hazards and Risks—Qou, as part of 

the Miller Creek Formation, is prone to slope wash and 

landslides.

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qou was 

deposited between about 11,500 and 9,500 years ago. 

Postglacial deposits, 

organic sediment

(Qp)

Qp consists of peat or mixtures of organic deposits and 

fine-grained material in layers between less than 1 m (3 

ft) and a few meters thick. Bedding is typically flat. 

Qp is mapped on Sand and Stockton islands. 

Recent Surficial Deposits—Qp accumulated in low-lying 

swamps, bogs, and marshes.

Paleontological Resources—Qp may contain fossil 

pollen recording conditions since the last major glaciation 

of the Pleistocene Epoch. 

None reported.

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qp

accumulated in marsh areas since about 10,000 years ago. 
Map unit colors correspond to GRI map poster (in pocket). Bold text refers to sections in report.

APIS Surficial (“Pleistocene”) Geologic Map Unit Properties Table, Page 3 of 4

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY

(Holocene and Pleistocene)

Postglacial deposits, stream 

sediment

(Qsm)

Qsm consists of sand and gravel in overlapping 

channel deposits (i.e., trough-shaped deposits in cross 

section). Most deposits of Qsm are more than 1 m (3 

ft) thick and may be overlain by silt and clay that were 

overbank deposits. This pattern is typical of 

meandering streams and adjacent floodplains. 

Qsm is mapped along the shoreline of the mainland 

unit of Apostle Islands National Lakeshore at the 

mouth of the Sand River. 

Fluvial Features—Qsm is being deposited at the mouth 

of the Sand River. Deposits compose terraces and 

floodplains along the active channel. 

Recent Surficial Deposits—Qsm still accumulates in the 

channels and adjacent floodplains of modern streams. 

Fluvial and Other Surface Water Issues—Qsm is 

deposited by the Sand River within Apostle Islands 

National Lakeshore. Qsm is unconsolidated and prone to 

erosion as the river meanders across its floodplain. 

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qsm has 

accumulated in stream channels and floodplains since 

about 10,000 years ago.

QUATERNARY

(Pleistocene)

Copper Falls Formation, 

collapsed proglacial stream 

sediment

(Qsc)

Qsc formed as mixed stream sediments deposited atop 

glacial ice. As the ice melted, the sediments collapsed, 

resulting in faulted bedding and hummocky collapse 

topography. Surface boulders are rare in Qsc. 

Qsg is collapsed stream sediments mixed with till or 

windblown or offshore sediments and underlying 

stream sediments. Surface boulders are common in 

Qsg. 

Qsu and Qsuc consist of poorly sorted, coarse 

sediments with undisturbed bedding surfaces. Qsu and 

Qsuc exhibit flat topography and no surface boulders. 

None of these units is mapped within Apostle Islands 

National Lakeshore.

Glacial Features—Qsc was deposited on stagnant ice and 

collapsed when the underlying ice melted. Proglacial 

streams deposited Qsg. Qsu and Qsuc were deposited on 

solid ground and were not subject to soft sediment 

deformation or collapse. 

None reported

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qsc, Qsg, 

Qsu, and Qsuc were deposited between about 16,000 

and 11,500 years ago. Several stability surfaces are present 

as part of Qsu: Lost Lake, Iron River, Valhalla, Lake Ruth, 

Airport, Swiss, Hayward, and Tiger Cat surfaces. 

Copper Falls Formation, 

hummocky stream 

sediment overlain by silty 

material

(Qsg)

Copper Falls Formation, 

uncollapsed proglacial 

stream sediment 

(Qsu)

Copper Falls Formation, 

uncollapsed proglacial 

stream sediment—Valhalla 

surface

(Qsuc)

Copper Falls Formation, till 

– thick mass-movement 

deposits

(Qgg)

Different types of tills reflect the method of their 

deposition. Lodgement till is deposited by plastering of 

glacial debris from the sliding glacial ice. Massmovement tills are tills that were subject to slope 

movements after initial deposition. Melt-out tills are 

released by the melting of stagnant or slowly moving 

ice without subsequent sedimentary transport or softsediment deformation. Qgg occurs as mass-movement 

till, some melt-out till, and lodgement till that covers 

and obscures older till deposits. In general, the Copper 

Falls Formation is reddish to brownish sandy sediment. 

Topography of Qgg is hummocky with abundant 

surface boulders. 

Qgg is not mapped within Apostle Islands National 

Lakeshore.

Glacial Features—Qgg displays hummocky topography 

and three different types of glacial till. None reported

66 Million Years Ago to Present (Cenozoic Era)—

Erosion, Ice Age Glaciation, and Rebound—Qgg was 

deposited between about 16,000 and 11,500 years ago. 
Map unit colors correspond to GRI map poster (in pocket). Bold text refers to sections in report.

APIS Surficial (“Pleistocene”) Geologic Map Unit Properties Table, Page 4 of 4

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

CAMBRIAN–PROTEROZOIC

Bedrock

(CPCrs)

CPCrs includes sandstone, mudstone, and 

conglomerate of PCch, PCdi, PCo, and PCf (see 

Bedrock Geologic Map Unit Properties Table). 

CPCrs is mapped at Bear and Rocky islands.

See Bedrock Geologic Map Unit Properties Table. See Bedrock Geologic Map Unit Properties Table. See Bedrock Geologic Map Unit Properties Table.
APIS Bedrock Geologic Map Unit Properties Table, Page 1 of 1

Bedrock Geologic Map Unit Properties Table: Apostle Islands National Lakeshore

Map unit colors correspond to GRI map poster (in pocket). Bold text refers to sections in report.

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

MESOPROTEROZOIC ERA

Keweenawan Supergroup

Bayfield Group

Chequamegon 

Sandstone

(PCch)

Arkose is a feldspar-rich sandstone. PCch contains red, 

brown, and white arkosic sandstone. Scant beds of red 

shale and conglomerate occur locally. Feldspar grains 

are common. Sedimentary bedding is generally thick. 

PCch is about 150 m (490 ft) thick. 

PCch dominates the bedrock within the park, and is 

mapped on all islands except Eagle, Sand, and Devils. 

Sedimentary Rock Features—PCch contains planar 

crossbedding, thin channel cross sections, scour-and-fill 

structures, mud cracks, ripple marks, convoluted bedding 

and other features associated with braided-stream 

deposition. Arkose typically forms as a result of granitic 

rocks breaking down. The presence of large fragments and 

feldspar commonly means the sediments did not travel far 

from their point of origin and/or were deposited rapidly 

because feldspars typically decompose faster than quartz. 

Slope Movement Hazards and Risks—PCch, as part of 

the bedrock that composes the cliff shorelines at the 

lakeshore, is prone to erosion and blockfall.

Caves and Associated Landscape Management—PCch

contains sea caves. Climbing in sea caves is restricted at 

Apostle Island National Lakeshore.

Abandoned Mineral Lands and Disturbed Land

Restoration—Brownstones of PCch were quarried on 

three islands at four sites and are now interpretive targets.

1.6 Billion to 1.0 Billion Years Ago (Mesoproterozic 

Era)—Continental Rifting, Volcanism, and Sandstone 

Deposition—PCch, PCdi, PCo, and PCf were deposited 

in one of the world’s great continental rifts. PCch is the 

youngest member of the Bayfield Group deposited atop 

PCdi in a series of braided streams after the local basin 

subsided again. The sediment was derived from 

weathering granites along the flanks of the Midcontinent 

rift.

Devils Island 

Sandstone

(PCdi)

PCdi consists of white to tan quartz sandstone arenite 

(consolidated sedimentary rock composed of sandsized fragments). Individual beds within PCdi are 

generally thin. PCdi is about 100 m (330 ft) thick.

PCdi crops out at Devils and Sand islands, and along 

the shoreline in the mainland unit of Apostle Islands 

National Lakeshore. 

Sedimentary Rock Features—PCdi contains crossbedding and ripple marks. PCdi is also well sorted. 

Devils Island Sandstone Type Locality—the remarkable 

exposures of PCdi at Devils Island are the geologic type 

locality.

Paleontological Resources—PCdi contains potential 

trace fossils as burrows. Microbial fossils are known from 

rocks similar in age, elsewhere in the region.

Slope Movement Hazards and Risks—PCdi, as part of 

the bedrock that composes the cliff shorelines at Apostle 

Islands National Lakeshore, is prone to erosion and 

blockfall.

Caves and Associated Landscape Management—PCdi

contains sea caves. Climbing in sea caves is restricted at 

Apostle Island National Lakeshore.

Paleontological Resource Inventory, Monitoring, and 

Protection—If fossils are identified from PCdi, they 

should be documented and an appropriate monitoring 

protocol established. 

1.6 Billion to 1.0 Billion Years Ago (Mesoproterozic 

Era)—Continental Rifting, Volcanism, and Sandstone 

Deposition—PCch, PCdi, PCo, and PCf were deposited 

in one of the world’s great continental rifts. PCdi is the 

thin, middle member of the Bayfield Group deposited atop 

PCo as a blanket of beach sand flanking numerous 

shallow ponds and lakes after the basin was partially filled 

with sediment.

Orienta 

Sandstone

(PCo)

Arkose is a feldspar-rich sandstone. PCo is red, brown, 

and white, arkosic sandstone. Scant beds of red shale 

and conglomerate occur locally. Feldspar grains are 

common. PCo is approximately 1,000 m (3,300 ft) 

thick. 

PCo is mapped at Eagle and Sand islands. 

Sedimentary Rock Features—PCo contains planar 

crossbedding, thin channel cross sections, scour-and-fill 

structures, mudcracks, ripple marks, convoluted bedding 

and other features associated with braided-stream 

deposition. PCo is also poorly sorted. Arkose typically 

forms as a result of granitic rocks breaking down. The 

presence of large fragments, poor sorting, and feldspar 

commonly means the sediments did not travel far from 

their point of origin and/or were deposited rapidly because 

feldspars typically decompose faster than resistant quartz.

Slope Movement Hazards and Risks—PCo, as part of 

the bedrock that composes the cliff shorelines at the 

lakeshore, is prone to erosion and blockfall. 

Caves and Associated Landscape Management—PCo

contains sea caves. Climbing in sea caves is restricted at 

Apostle Island National Lakeshore.

1.6 Billion to 1.0 Billion Years Ago (Mesoproterozic 

Era)—Continental Rifting, Volcanism, and Sandstone 

Deposition—PCch, PCdi, PCo, and PCf were deposited in 

one of the world’s great continental rifts. PCo is the oldest 

member of the Bayfield Group deposited atop PCf in a 

series of braided streams. The sediment was derived from 

weathering granites along the flanks of the Midcontinent 

rift.

Oronto Group

Freda Sandstone

(PCf)

PCf contains fine- to medium-grained, red, brown, and 

tan sandstone arenite (consolidated sedimentary rock 

composed of sand-sized fragments) with lesser shale 

and conglomerate layers. Feldspar and mica flakes are 

common. Bedding is well defined. 

PCf is not mapped within Apostle Islands National 

Lakeshore.

Sedimentary Rock Features—PCf contains crossbedding. None reported.

1.6 Billion to 1.0 Billion Years Ago (Mesoproterozic 

Era)—Continental Rifting, Volcanism, and Sandstone 

Deposition—PCch, PCdi, PCo, and PCf were deposited 

in one of the world’s great continental rifts. PCf is the 

youngest member of the Oronto Group to form from 

eroding volcanic rocks of the Keweenawan volcanics.