The Geologic Story of

By N. King Huber

Geological Survey Bulletin 1309
ROY/VLE

Prepared in cooperation with the National Park Service 

1975
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Huber, Norman King 1926-

The geologic story of Isle Royale National Park.

(Geological Survey Bulletin 1309)

Bibliography: p. 64-66

Supt.of Docs. No.: 119.3:1309

1. Geology-Michigan-Isle Royale National Park-Guide-books. I. Title. II. Series: United

States. Geological Survey. Bulletin 1309. 

QE75.B9 [QE126.I8] 557.3'08s [557.74'997] 75-619126

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D. C. 20402

Stock Number 024-001-02672-4
CONTENTS

The island in history______________________ 2

A look at the landscape __________________ 5

The building blocks rocks and minerals __________ 8

Volcanic rocks ____________________ 8

Sedimentary rocks_________ 13

Pyroclastic rocks________________________ 14

The way the rocks are stacked up___ ____ 14

A volcanic pile the Portage Lake Volcanics ______ 14

Rock Harbor-Tobin Harbor area __________ 17

Washington Harbor area_______________ 20

An ancient alluvial fan the Copper Harbor

Conglomerate ________ 21

Tilting and breaking the rocks _____________ 29

Isle Royale a small piece of the big puzzle ____ 34

What happened when?___________________ 35

Time as a geologic concept ________________ 36

What was there before the pre-Keweenawan ____ 37

The rocks of Isle Royale the Keweenawan ______ 38

The missing chapters ____________________ 40

The glaciers take over ____________________ 41

Glacial effects on the land _____________ 41

Glacial effects on lake levels ____________ 47

Some minerals of special interest ____________ 55

Native copper widespread but not abundant ____ 56

Chlorastrolite Michigan's state gem __________ 58

Prehnite the little pink pebbles _____________ 59

Agate an array of colors _________________ 60

What the future holds? ________________'._____ 61

A word of thanks ________________________ 61

Definition of terms ________________________ 62

References and additional reading ______________ 64
To describe a biota there is no substitute for a sample. 

But it is logical to ask, what might one want to know which 

would require the preservation of a sample? Whether such 

a question is asked at all is a reflection of the stage of 

intellectual maturity of a civilization. We take it for granted 

that there is some social gain in the erection and 

maintenance of a museum of fine arts, a museum of 

natural history, or even a historical museum. Sooner or 

later we ought to be mature enough to extend this concept 

to another kind of museum, one which you might call the 

museum of land types, consisting of samples as 

uninfluenced as possible by man.

Luna Leopold, "The Meaning of Wilderness to Science," 

1960: San Francisco, Sierra Club, p. 33.
The 

GeoloqiG^tory

Hie National Park

N. King Huber

I,.sle Royale is an outstanding 

example of relatively undisturbed 

northwoods lake wilderness. But more 

than simple preservation of such an 

environment is involved in its inclusion 

in our National Park System. Its isolation from the mainland provides an 

almost untouched laboratory for research in the natural sciences, especially those studies whose very nature 

depends upon such isolation.

One excellent example of such research is the intensive study of the 

predator-prey relationship of the 

timber wolf and moose, long sponsored 

by the National Park Service and Purdue University. In probably no other 

place in North America are the necessary ecological conditions for such a 

study so admirably fulfilled as on Isle 

Royale. The development of a natural 

laboratory with such conditions is ultimately dependent upon geologic processes and events that although not 

unique in themselves, produced in 

their interplay a unique result, the island archipelago as we know it today, 

with its hills and valleys, swamps and 

bogs the ecological framework of the 

plant and animal world.

Even the most casual visitor can 

hardly fail to be struck by the fiordlike 

nature of many of the bays, the chains 

of fringing islands, the ridge-andvalley topography, and the linear nature of all these features. The distinctive topography of the archipelago is, 

of course, only the latest manifestation 

of geologic processes in operation since 

time immemorial. Fragments of geologic history going back over a billion 

years can be read from the rocks of the 

island, and with additional data from 

other parts of the Lake Superior region, we can fill in some of the story of 

Isle Royale. After more than a hundred 

years of study by man, the story is still 

incomplete. But then, geologic stories 

are seldom complete, and what we do 

know allows a deeper appreciation of 

one of our most naturally preserved 

parks and whets our curiosity about the 

missing fragments.
THE ISLAND IN 

HISTORY

G,reology influenced the role that 

Isle Royale played in both pre-historic 

and historic activities of man in the 

Lake Superior region. Native copper 

implements, weapons, and ornaments 

have been found in Indian mounds 

and habitats throughout central and 

eastern North America, some of which 

probably date back to 3,000 B.C. Undoubtedly, most of this copper came 

from the Lake Superior region, and 

perhaps a significant part from Isle 

Royale. Even today, hundreds of Indian mining pits can be seen scattered 

throughout the park. The major concentration of these pits is near 

McCargoe Cove, where many were 

reexcavated later by miners and archaeologists. A charred log recovered 

from one of these pits yielded a 

radiocarbon age of about 1,500 B.C., 

attesting to the antiquity of these early 

mining operations.

The existence of native copper in 

North America was known to the 

European explorers as early as 

Carder's second voyage in 1535-36. It 

was said to have come from the direction of the Lake Superior region. Not 

until the Jesuit missionaries penetrated 

the region more than a hundred years 

later, however, did more definite information as to the source of the copper become available, but even much of 

that was inaccurate. In the "Jesuit Relation" for 1669-70, a Father Dablon 

cites Indian legends regarding "an island called Menong [Minong], celebrated for its copper," including a 

rather extravagant description of the 

abundance of the metal. Minong was 

the Indian general term for island but 

was specifically applied to Isle Royale; it

has been perpetuated on the island as 

Minong Ridge. As exploration continued, additional information about 

the sources of the copper was accumulated, and actual prospecting on Isle 

Royale began in 1843, the year in which 

the Chippewas relinquished their 

claims to the island. By 1850 copper 

mining by Europeans was established 

on Keweenaw Peninsula and had been 

attempted on Isle Royale (fig. 1).

In 1847 the U.S. Congress authorized a survey of the vast Lake 

Superior Land District to determine its 

mineral potential, and the result of this 

survey was the first series of comprehensive reports on the geology of 

the region. Charles T. Jackson? s report 

of 1849 contains the first map of Isle 

Royale to show in a very general way 

the different rock types present on the 

island. Two following reports, both by 

John W. Foster and Josiah D. Whitney 

in 1850 and 1851, include an improved 

geologic map of Isle Royale and a 

wealth of information on the early mining activities on the island. Whitney 

later became the first State Geologist of 

California, and the highest mountain 

in the conterminous United States is 

named for him. These three reports 

show an amazingly sophisticated understanding of the general geology of 

the region, especially considering the 

limited amount of time that was spent 

exploring such a vast and nearly trackless wilderness. Foster and Whitney 

lucidly describe some of the difficulties 

in working on Isle Royale:

The physical obstructions to a successful 

exploration of the island are greater than we 

encountered in any other portion of the 

mineral district. The shores are lined with 

dense but dwarfed forests of cedar and 

spruce, with their branches interlocking and 

wreathed with long and drooping festoons 

of moss. While the tops of the trees flourish 

luxuriantly, the lower branches die off and
REGIONAL SETTING of Isle Royale National Park. (Fig. 1)

stand out as so many spikes, to oppose the 

progress of the explorer. So dense is the 

interwoven mass of foliage that the noonday 

sunlight hardly penetrates it. The air is 

stiffled; and at every step the explorer starts 

up swarms of musquitoes, which, the very 

instant he pauses, assail him.

And so it remains today, but at least the 

modern explorer has relatively effective mosquito repellent available to 

him. Among the more astute conclusions reached by Jackson and by Foster 

and Whitney is that Isle Royale and the 

Keweenaw Peninsula may be regarded 

as geologic counterparts on the opposite sides of a structural basin occupied 

by Lake Superior. They also predicted 

correctly that "the island, for mining 

purposes, may be regarded as infinitely 

less valuable than Keweenaw Point."

The first period of mining activity on 

Isle Royale, from 1843 to 1855, ended 

with much disappointment, as no

economic deposits were found. Mining 

was renewed during the period from 

1873 to 1881 with the development of 

the Minong and Island mines. Although these two mines were the most 

productive ever to be opened on the 

island, they also were uneconomic, and 

this mining revival was short lived (fig. 

3). Finally, from 1889 to 1893, an extensive exploration program was undertaken on the west end of the island, 

east of Washington Harbor. No economic deposits of copper were located, 

however, and the efforts were abandoned.

It was during this latest period of 

exploration that the most detailed and 

comprehensive study of Isle Royale to 

date was made by Alfred C. Lane of the 

Geological Survey of Michigan. His 

geologic map and report were published in 1898. The timing of this study 

was fortunate indeed, as samples ob-
HOIST AND BOILER REMAINS at Island mine, abandoned in 1877. (Fig. 2)

tained with diamond drills during the 

mineral exploration of this period were 

made available to Lane and permitted 

him to work out the succession of rock 

strata the stratigraphy of the island 

in much greater detail than would have 

been possible from the very incomplete 

sequence to be seen in natural rock exposures. The samples also permitted 

him to make stratigraphic correlations 

between Isle Royale and the Keweenaw 

Peninsula, which are for the most part 

still valid today. Lane's report was so 

comprehensive that until very recentlyonly a few additional studies of limited 

geologic scope have been made on the 

island; most of these have been related 

to the glacial and post-glacial geologic 

history. Many other studies throughout the Lake Superior area, however,

have given us a better understanding of 

the regional geologic setting within 

which we must view Isle Royale.

Recent field studies undertaken by 

myself, and Roger G. Wolff, have resulted in a series of technical publications on the geology of Isle Royale, 

which are listed at the end of this report. Included is a new geologic map 

(Huber, 1973c), published separately, 

which can profitably be used as a companion to this report. The map is at a 

sufficiently large scale (1 in.= l mi.) to 

show details of the geology of Isle 

Royale not possible to show at the scale 

of maps in this report. In fact some of 

the geologic units described in this report are shown only on the larger map, 

which is also recommended for place 

names.
A LOOK AT THE 

LANDSCAPE

V^/ne's first impression of Isle 

Royale, when approaching by boat, is 

of an inhospitable rocky shoreline behind which a verdant mantle of vegetation softens what appears to be an irregular, hilly landscape. From this 

lake-level view, no topographic pattern 

is readily apparent (fig. 3). But if one 

approaches by air, a rather striking pattern becomes apparent, and one sees 

the island as a series of long parallel 

ridges and valleys, which give it a washboard aspect. This pattern is most conspicuous at the northeast end of the 

island where the ridges project into 

Lake Superior as so many rocky fingers 

and the drowned valleys between them 

have a distinct fiordlike appearance 

(fig. 4). The pattern is also dramatically 

displayed by the chains of elongate islands that fringe the main island no 

more than the protruding tops of almost completely submerged ridges.

To the hiker traveling across the 

grain of the country, the series of 

ridges seems never ending; seldom 

walking level, he is monotonously 

going up and down. He also quickly 

becomes aware of another aspect of the 

topography the southeast-facing 

slopes are relatively gentle compared 

with the northwest-facing slopes, which 

are often precipitous; the ridge-andvalley structure is decidedly unsymmetrical. And to his discomfort, the 

hiker finds that even when traveling 

with the grain of the island, he is still 

spending an inordinate amount of time 

going up and down, for the linear 

ridges are interrupted at frequent intervals by crosscutting ravines or depressions.

Another aspect of the topography 

that is readily apparent to the hiker is 

the generally poor drainage on the isROCKY SHORELINE on Isle Royale. (Fig. 3)
FINGERLIKE PROMONTORIES on northeast end of Isle Royale. Photograph by Alan Eliason,

National Park Service. (Fig. 4)

land. Every valley seems to be at least 

partly occupied by swamps and bogs, as 

well as numerous lakes, all adding to 

the difficulty of traversing across the 

grain of the island. For those willing to 

expend the effort, however, hiking on

Isle Royale can be an enormously rewarding experience, tor the very topographic factors that make hiking arduous also provide numerous vantage 

points for some of the most scenic vistas 

in the Lake Superior region (fig. 5).

VISTA FROM LOOKOUT LOUISE looking across Duncan Bay. (Fig. 5)
And the sight of a bog mat tinged 

magenta with miniature orchids or a

bull moose feeding in a beaver pond is 

not soon to be forgotten (fig. 6).

BULL MOOSE feeding in beaver pond. (Fig. 6)

The landforms of Isle Royale, like 

landforms everywhere, are the result 

of geologic processes and are dependent upon the type of underlying 

rocks, the structure of the rocks, and 

their erosional history. The bedrock 

sequence on Isle Royale consists of a 

thick pile of lava flows and sedimentaryrocks that has been tilted toward the 

southeast, and the linear ridges of the 

island are the eroded edges of individual layers of the pile (hg. 7). Many 

geologic processes in combination 

worked together to create Isle Royale 

as we see it today, not only its landforms, but the rocks and minerals 

themselves. These processes and their 

consequences are discussed at some 

length on the pages that follow in an 

attempt to unravel the geologic story 

involved at Isle Royale National 

Park.

RIDGE-AND-VALLEY TOPOGRAPHY related to tilted rock structure. Circled areas indicate 

sedimentary rocks; blank areas indicate lava flows. (Fig. 7)
THE BUILDING

BLOCKS 

ROCKS AND

MINERALS

VOLCANIC ROCKS

When we think of volcanic activity, 

we usually conjure up visions of a Vesuvius spewing forth billowing clouds 

of steam and ash or a Kilauea with fiery 

fountains of molten lava reaching skyward. This is partly because most people are familiar with the type of volcanism that builds imposing cones, 

such as Vesuvius, Kilauea, Mount 

Rainier, Mount Fuji, and many others 

and partly because many of these volcanoes have erupted in historic times, 

sometimes with disastrous results. Individual eruptions of such volcanoes, 

while spectacular, generally involve relatively small volumes of molten lava. 

In other words, such volcanoes are constructed slowly by frequent eruptions 

of relatively small quantities of lava 

spewed out in all directions from a generally localized vent. The lava seldom 

travels far from the vent, and the 

tongue-shaped flows accumulate one 

upon another to form a conical pile 

(fig. 8).

VOLCANIC CONE cross section. (Fig. 8)

However, in some of the world's 

most extensive volcanic areas, true volcanoes are rare or absent. In such areas 

tremendous volumes of lava, sometimes measured in tens or even hundreds of cubic miles, welled out 

through fissures not connected with 

volcanic cones to form extensive sheets 

of flood basalts or plateau basalts. 

These sheets of basaltic lava, piled up 

flow upon flow to thicknesses of 

thousands of feet, are the most extensive of all volcanic deposits, and such 

deposits have occurred at various 

places and times in the geologic past. 

Such eruptions are rare at the present 

time, however; the only known historic 

eruptions of this type occurred in Iceland. There, at Laki, in 1783, an estimated volume of 3 cubic miles of lava 

flowed from a fissure 20 miles long to 

cover an area of 215 square miles. But 

even this was small compared with similar eruptions during earlier geologic 

periods.

The volcanic rocks of Isle Royale are 

flood basalts, but before discussing 

their details we can look at a younger 

sequence of Hood basalts that is still relatively undisturbed geologically, allowing us to deduce more about their 

origin. In the northwestern United 

States, between 18 and 10 million years 

ago, enormous amounts of basalt lava 

welled up through deep fissures in the 

earth's crust and flooded much of 

present-day Washington, Oregon, and 

Idaho to form the great Columbia 

River Plateau. These basalt flows are 

known collectively as the Columbia 

River basalt (fig. 9).

Much of the Columbia River basalt 

has been eroded away, but it is estimated that the lava Hood once covered 

an area of 150,000 square miles, 

equivalent to the combined areas of 

Washington and Oregon. It was probably over f mile thick in places and had
COLUMBIA RIVER BASALT today. (Fig. 9)

an average depth of 1,800 feet. The 

tremendous amount of basalt contained in even a single flow can be illustrated by the Roza lava How in 

Washington (fig. 10). This flow, which 

can be traced over an area of 8,800 

square miles, averages 100 feet in 

thickness. Its volume of at least 160 

cubic miles is roughly seven times the

FLOOD BASALTS of the Columbia River 

Plateau form these cliffs along the Yakima 

River. The Roza lava flow lies near the top of 

the hill. Photograph by R. S. Fiske. (Fig. 10)

volume of Mount Rainier, one of the

great volcanoes of the Cascade Range.

A schematic diagram of an eruption

of flood basalt is shown in figure 11.

FLOOD BASALT ERUPTION schematic diagram. (Fig. 11)
The Java wells out of long fissures that 

cut through earlier, cooled and 

solidified flows, and it is erupted in 

such a great volume that it rapidly flows 

away from the source fissures when it 

reaches the earth's surface. Instead of 

building volcanic cones, the eruption 

forms a vast lava lake, and the resulting 

flow, when solidified, tends to have 

nearly level surfaces. A sequence of 

such eruptions results in the accumulation of a series of basalt sheets covering 

vast areas much like a layer cake 

having quite a different geometry from 

the outward-dipping concentric structure that results from the eruptions of 

the more familiar cone volcanoes.

At first glance the volcanic rocks on 

Isle Royale seem to have a monotonous 

similarity -just nondescript darkcolored rocks. When studied more 

closely, however, numerous variations 

can be seen, some readily apparent and 

others more subtle. These variations 

are more significant than might be 

supposed. Some, for example, result in 

differences in rock hardness, which in 

turn affects resistance to erosion and 

thus the development of landforms. 

Other differences help to identify individual lava flows, thus enabling one to 

separate them from their neighbors 

and to trace them across the countryside. This recognition is critical to 

the preparation of a geologic map and 

to the unscrambling of the geologic history.

The compositional classification of 

volcanic rocks is largely based upon 

their silica (silicon dioxide) content, 

with a range from basalt, with a silica 

content of roughly 50 percent or less, to 

rhyolite, with a silica content of 70 percent or more. Rhyolite and volcanic 

rocks of intermediate composition 

usually have an excess of silica, and so 

some of the silica occurs in the free or 

uncombined state as quartz, as well as 

in other more complex silicate miner10

als. In basalt, however, all silica generally is needed to form silicate minerals, 

and quartz is uncommon.

The volcanic rocks on Isle Royale are 

nearly all basalt, and their mineral constituents are plagioclase feldspar, 

pyroxene, and lesser amounts of 

olivine, magnetite, and other minerals. 

A few of the rocks are somewhat more 

silicic than basalt and would be 

classified as andesite. Volcanic rocks as 

high in silica as rhyolite are not exposed 

on the island. In fact, the basalts of Isle 

Royale are so much alike chemically 

that we cannot readily use chemical 

composition by itself to distinguish one 

volcanic rock on the island from 

another.

When lava cools and solidifies 

quickly, it forms glass. If it cools more 

slowly, it partly or completely crystallizes to mineral grains. The glass, in 

time, will also crystallize to mineral 

grains, but they generally will be finer 

than those formed during slow cooling. 

Differences in the cooling history, as 

well as in chemical composition, of individual lava flows result in differences 

in rock texture, a property that chiefly 

reflects the grain size, shape, and distribution of the minerals in the rock. 

The basalts of Isle Royale, and elsewhere in the Lake Superior region, do 

exhibit variations in texture; consequently, a rock classification based 

upon textures has developed through 

the years and is widely used in the 

Michigan copper district. Only an experienced geologist can estimate the 

chemical compositions of rocks in the 

field, but anyone can learn to recognize 

the different textures; remembering 

their names is the major hurdle. The 

textural classification is extremely useful because it can be applied directly in 

the field, at the outcrop, and thus is a 

powerful aid in identifying individual 

flows for geologic mapping. I must 

emphasize, however, that these tex-
tural terms are not always used with the 

same connotation outside the Lake 

Superior region, and some geologists 

have abandoned them altogether because of conflicting usage. But they will 

be found in any further readings on the 

geology of the volcanic rocks of the 

Lake Superior region. The rock textures are illustrated in figure 12.

Ophite. Rock with a mottled texture produced by crystals of pyroxene 

surrounded by a slightly darker matrix 

of finer grained minerals (fig. 12/4). 

Pyroxene has good cleavage the tenCENTIMETRES 

B

dency to break along definite planes, 

producing smooth surfaces. Such surfaces reflect light better than the rock 

matrix, and on freshly broken specimens of ophite, the flashing of pyroxene cleavage surfaces in the sunlight is 

a distinctive feature. Cleavage

CENTIMETRES 

D

VOLCANIC ROCK TEXTURES found on Isle Royale. (Fig. 12)

11
reflections are dulled by chemical processes involved in weathering, but 

weathering actually accentuates the 

texture by increasing the color contrast 

between the pyroxene crystals and the 

rock matrix and by producing a knobby 

surface (fig. 13). The size of the pyroxene crystals varies from less than 1 millimetre to more than 2 centimetres, but 

crystal sizes are usually rather uniform 

in any individual specimen. Within a 

given lava flow, the pyroxene crystals 

are progressively larger toward the 

flow interior. Ophite is the most abundant volcanic rock type on Isle Royale.

COARSE-GRAINED OPHITE showing 

weathered, knobby surface (Fig. 13)

Porphyrite. Rock with a texture 

produced by well-defined plagioclase 

crystals scattered through a finer 

grained groundmass. The term is applied to two distinct varieties of such 

porphyritic rocks. One variety has 

small, blocky millimetre-sized plagioclase crystals rather uniformly distributed through the groundmass (fig. 

12fi); when the plagioclase crystals 

tend to clot together, the term glomeroporphyrite is used (fig. 12C). The 

other variety of porphyrite has larger, 

tabular-shaped crystals more sparselydistributed in the rock and commonly 

occurring in clots (fig. 12.D); the large 

crystals are often as much as 2 centimetres long.

Pegmatite. Rock with a texture in 

which all of the minerals, especially the

plagioclase, are larger when compared 

with those in most of the other rock 

types; the elongate plagioclase laths 

give the rock a matted appearance

(fig. 12E).

Trap. Fine-grained dark-colored 

massive rock showing none of the distinctive textures mentioned (fig. 12F). 

Trap commonly breaks with a curved 

fracture.

Felsite. Fine-grained light-colored 

volcanic rock, generally reddish. Texturally it is most similar to trap, although commonly containing scattered 

crystals, but is applied to more siliceous 

volcanic rocks such as rhyolite. Felsite 

does not occur in lava flows exposed on 

Isle Royale but does occur as pebbles in 

most of the conglomerate on the island.

All the rock textures are more easily 

recognized on weathered than on fresh 

surfaces because color contrasts between many minerals are increased 

during weathering and most lava flows 

can readily be characterized as ophite, 

porphyrite, or trap. In some flows the 

texture is obscure, but in nearly all 

these flows the rock is ophite. Pegmatite is a unique rock found only in layers 

in the interior of some ophitic flows, 

generally the thicker ones; its formation involves late-stage crystallization 

of a liquid remaining in the central part 

of a flow after the bulk of the flow has 

already solidified.

Two other terms are useful in describing the volcanic rocks: vesicle and 

amygdule. As a lava flow cools, small 

circular cavities are formed by the expansion of bubbles of gas or steam during the solidification of the rock; these 

cavities are called vesicles. When the 

cavities are later filled with minerals 

deposited from solutions circulating 

through the rock, the vesicle fillings are 

called amygdules. A rock with abundant arnygdules is called an amygdaloidal rock or simply an amygdaloid 

(fig. 14); Amygdaloid Island, for ex12
CENTIMETRES

AMYGDALOID with calcite amygdules and 

calcite vein at top. (Fig. 14)

ample, is named for a rock of this 

character. The rocks on Isle Royale are 

very old and have been saturated by 

mineral-bearing solutions long enough 

so that no vesicles remain all have 

been filled and converted to amygdules. The uppermost part of most individual lava flows is conspicuously 

amygdaloidal, containing as much as 

50 percent amygdules. The abundance 

of amygdules decreases downward toward the amygdule-poor massive basalt 

of the middle and lower part of the 

flow, with another usually much thinner amygdaloidal zone reappearing at 

the base of the flow (fig. 15). The

LAVA FLOWS cross section showing distribution of amygdules. (Fig. 15)

amygdaloidal zones are less resistant 

and more easily eroded than the flow 

interiors and normally do not crop out 

as well. Because of this, the actual surface of a flow is only rarely observable 

(fig. 16).

ANCIENT LAVA FLOW with ropy surface 

(above) on Isle Royale compared with a 

"modern" lava flow (below) at Craters of the 

Moon National Monument. (Fig. 16)

SEDIMENTARY ROCKS

The sedimentary rocks of Isle 

Royale, sandstone and conglomerate, 

are easily recognized as being the 

lithified or consolidated equivalents of 

sand and gravel. These rocks exhibit a 

wide range in coarseness from a very 

fine grained sandstone to conglomerate with boulders 2 feet in diameter. 

Most of the erosional debris that 

formed the original deposits of sand 

and gravel was derived from volcanic 

rocks with a wide range in composition. 

A striking rusty-red color is characteristic of all the sedimentary rocks a

13
color that reflects the presence of a 

significant quantity of hematite (iron 

oxide) in the fine groundmass in which 

the larger fragments are imbedded. 

The fact that many of the volcanic rock 

fragments themselves have a reddish 

tone enhances this coloration.

PYROCLASTIC ROCKS

In contrast to the relatively quiet 

fissure eruptions of flood basalts, at various times more violent eruptions of 

rhyolitic lava hurled vast quantities of 

ash and other volcanic material into the 

air. The resulting rain of volcanic debris covered extensive areas and 

formed deposits that because of their 

fragmental character, superficially resemble sedimentary deposits. Rocks resulting from the consolidation of such 

volcanic deposits are known as pyroclastic rocks; those formed from very 

fine material, such as ash, are called 

tuffs. Pyroclastic rocks are not abundant on Isle Royale but do occur sandwiched between some of the flood 

basalt flows.

THE WAY 

THE ROCKS ARE 

STACKED UP 

nr

A he bedrock sequence on Isle 

Royale consists of a thick pile of lava 

flows and sedimentary rocks that has 

been tilted toward the southeast. Having already looked briefly at the different varieties of rocks that occur in the 

pile, we can now examine the rock sequence in more detail and get to know 

the subtleties of its character and the 

relative position or succession of its 

component parts its stratigraphy.

In geology, a formation is an assemblage of rocks that have some character

in common, whether of origin or composition. It is thus a way of dividing 

rocks into manageable units for discussion or for depiction on a geologic 

map. The rock sequence on Isle Royale 

has been divided into two formations, 

one deposited above the other. The 

lower and thus older one has been 

named the Portage Lake Volcanics and 

includes all the lava flows and their 

minor interbedded sedimentary and 

pyroclastic rocks. The upper formation, the Copper Harbor Conglomerate, contains only sedimentary rocks; it 

is mostly conglomerate but also includes sandstone. On the Keweenaw 

Peninsula of Michigan, where these 

formations also occur and were originally named, other formations can be 

seen both above and below the Portage 

Lake Volcanics and Copper Harbor 

Conglomerate. Some of the other formations undoubtedly also extend to the 

vicinity of Isle Royale, but they are concealed beneath the waters of Lake 

Superior.

A VOLCANIC PILETHE PORTAGE LAKE 

VOLCANICS

There are probably over 100 individual lava flows in the Portage Lake 

Volcanics on Isle Royale with a total 

thickness of over 10,000 feet quite an 

impressive pile. And the total thickness 

of the formation must be greater because although the top of the formation is exposed, the bottom is not, and 

an unknown number of additional 

flows undoubtedly lie beneath Lake 

Superior on the north side of the island.

The sedimentary rocks interbedded 

with the lava flows are exposed only 

under unusual circumstances because 

they are more easily eroded than the 

volcanic rocks and as a result are gen14
erally buried beneath surficial materials in depressions between ridges of 

more resistant volcanic rock. We know 

from holes drilled during exploration 

for copper deposits that about 25 

sedimentary layers 1 foot thick or more 

are present in the sequence; however, 

less than a third of these layers are 

known from outcrop, and all those are 

in the upper part of the sequence.

In order to construct a geologic map, 

which shows the geographic distribution of individual rock units and their 

relationships to each other, some 

method must be found of distinguishing key rock units from each other; 

usually, rock composition does the 

trick. But within a pile of similar lava 

flows, this is a distinct problem. Fortunately, some of the lava flows in the 

Portage Lake Volcanics can be distinguished from their neighbors above and 

below on the basis of their distinctive 

rock textures. These flows can then be 

used as divisions or marker units within 

the sequence of flows and can provide 

stratigraphic and structural control for 

depicting the sequence on a geologic 

map. The interbedded sedimentary 

rocks would also serve as marker units 

for mapping if they were exposed more 

frequently.

The amygdaloidal zones at the base 

and top of flows are less resistant than 

the massive flow interiors; therefore, 

the interior parts of the thicker flows 

are the dominant ridge-forming units. 

Contacts between flows, on the other 

hand, are seldom exposed as they are 

generally concealed beneath alluvial 

materials or talus in the depressions between ridges. Fortunately for geologic 

mapping, the distinguishing texture of 

an individual flow is generally better 

developed and more readily apparent 

in the nonamygdaloidal flow interiors, 

and therefore, that part of a flow most 

useful for purposes of identification is

invariably the most likely part to be exposed.

The volcanic rocks on Isle Royale are 

predominantly ophitic, and except for 

a few unusual ophites, the nonophitic 

flows are most conspicuous among 

their neighbors and make the best 

horizon markers. The finer grained 

porphyrites and traps are also generally more resistant to erosion than 

ophitic flows of similar thickness and 

thus tend to stand out topographically, 

making it easier to trace them across 

the countryside and adding to their 

usefulness in geologic mapping. There 

are 12 flows within the Portage Lake 

Volcanics on Isle Royale that are distinctive enough to be easily separated 

from the rest of the flows in the pile, 

and these have been named for ease of 

reference (fig. 17). Most of these 

named flows are less than 100 feet 

thick, but six of them can be traced the 

length of the island. The named flows 

include five porphyrites, four traps, 

and three ophites. Only four of these 

individual flows are shown on the 

geologic map in this volume (pi. 2), but 

all are shown on the larger scale 

geologic map published separately 

(Huber, 1973c).

For most visitors to Isle Royale, the 

northeast and southwest ends of the 

island are the most accessible, although 

a growing number of visitors are hiking 

the length of the island and the Greenstone Ridge Trail, which follows 

Greenstone Ridge, the backbone of the 

island. At neither end of the island can 

all the named lava flows or volcanic 

rock types be readily seen, but enough 

variety is present to provide interesting 

geological excursions from Rock Harbor Lodge and Windigo Inn via small 

boats available at both places. A somewhat less varied assortment of rocks 

can be seen on short hikes from the 

lodge areas.

15
Named lava flows:

Scoville Point Flow (porphyrite)

Edwards Island Flow (trap)

Middle Point Flow (porphyrite)

Long Island Flow (trap)

Tobin Harbor Flow (porphyrite)

Washington Island Flow (ophite)

0 <?V o 0 V^ 

0 0 

0 o Copper ° ° 

Q Harbor ° 

Conglomerate Q

° ° 0 ° 0 0

'' "/x 1 (\''l

W^Wl ijimi

'V"'^ '"' "'x

i^^^A^t SHIES

T,-,V V7

*»*,* ;;»

,, ,- 4 ,, 7

Unnamed rock sequences:

Numerous thin ophitic flows with as 

many as seven interbedded sandstone 

and conglomerate units

Several thin ophitic flows

Conglomerate known from drill records

Several thin ophitic flows

Sandstone known from drill records

Tuff -breccia

Greenstone Flow (ophite)

Grace Island Flow (porphyrite)

Minong Flow (trap)

Huginnin Flow (porphyrite)

Hill Point Flow (ophite)

Amygdaloid Island Flow (trap)

x*

E^p

l-rm

' A

rt

X i

m

TTTT

< A

4rr

i \

 o 

\'\ ^/

--^ -

QjL

II

iikjiy

Si

[WPr

x >

X X

#¥A*i

^

A 4 &d

j^ J / 1 )

;:

°

/ " /

K^

Wrh

flW

V

||

uuu

f i n i" i.i.i.i.i, i

rrWf

"VVr

c x

X X

4Vr^

Jfffi

< A

\

m

K_L

Conglomerate known from drill records

Sequence of thin to thick (more than 

100 ft) flows chiefly ophitic, with one

or more sedimentary units suggested

by drill data

Tuff- breccia

Sequence of thin to thick flows chiefly

ophitic, with one or more sedimentary 

units suggested by drill data

One or more ophitic flows present locally

Sequence of thin to moderately thick 

flows, chiefly ophitic. Several sediby drill records

Breccia

PORTAGE LAKE VOLCANICS schematic columnar section on Isle Royale. Not drawn to scale.

(Fig. 17)

16
ROCK HARBOR 

TOBIN HARBOR

AREA

Most of the rock types in the Portage 

Lake Volcanics can be seen in the Rock 

Harbor Tobin Harbor area (pi. 1). 

The main exception is the coarse porphyrite characteristic of the Huginnin 

Flow (fig. 12D), which can only be seen 

on the north side of the island.

All the volcanic rocks on the chain of 

islands on the south side of Rock Harbor are ophitic, and excellent exposures can be found on the wavebuffeted south shores of those islands. 

Ophitic texture is especially well developed on the south side of Raspberry 

Island where exposures are readily accessible via the Raspberry Island Nature Trail (fig. 13 is from this locality). 

Ophite can also be seen along the 

Commodore Kneutson Trail near the 

Rock Harbor Lodge.

Fine-grained porphyrite can be seen 

in the Scoville Point Flow at various 

points on the north shoreline of Rock 

Harbor, as at Threemile Campground, 

on Scoville Point, and on the south 

sides of Edwards and North Government Islands. Its speckled appearance 

is quite distinctive (fig. 125). Rock with 

a somewhat similar texture (fig. 12C) 

can be seen in the Tobin Harbor Flow 

at various points along the north 

shoreline of Tobin Harbor, as near the 

dock for the Lookout Louise Trail, on 

several small islands in Tobin Harbor, 

and on the south side of Porter Island. 

The trail to Mount Franklin also surmounts a bold outcrop of the Tobin 

Harbor Flow shortly after crossing 

Tobin Creek.

Fine-grained trap of the Edwards Island Flow can best be seen on the small 

promontory just north of Scoville 

Point, on Split Island, and on the north

sides of Edwards and North Government Islands. Trap of the Long Island 

Flow can be seen on several small islands opposite the dock for the Lookout Louise Trail and on Long and 

Third Islands. The Edwards Island 

Flow and to a lesser extent the Long 

Island Flow have an important secondary attribute that helps in their field 

recognition; except for the upper part 

of the Greenstone Flow, they are the 

only flows in the volcanic sequence that 

commonly exhibit well-developed columnar jointing (fig. 18).

COLUMNAR JOINTING IN EDWARDS ISLAND FLOW, Edwards Island, (Fig. 18)

Columnar joints are cracks that divide a lava flow into long vertical columns ideally tending toward a hexagonal cross section. They are formed 

during cooling of the flow under certain specific conditions. Rapid and uniform cooling rates tend to promote the 

development of columnar joints, but 

the degree of homogeneity of the solidifying rock is probably more important. It can be shown mathematically 

that the surface of a homogeneous 

medium should be divided by a crack 

system defining regular hexagons 

when it is subjected to uniform tension 

because a hexagonal system provides 

the greatest stress relief with the fewest

17
cracks. Regular hexagons are rare, 

however, because cooling stresses in 

rocks are never completely uniform 

and the columns are generally 

bounded by curved cracks forming 

irregular-shaped polygons with variable numbers of sides. "Ideal" cooling 

conditions are never reached at the 

surface of a flow. However, as a progressive zone of cooling, solidification, 

and cracking proceeds from the surface of a flow into its interior, a point 

may be reached where the shrinkage 

forces may be uniform enough for earlier irregular jointing to give way to the 

formation of columnar joints. The importance of the homogeneity of the 

cooling rock explains why in the Portage Lake Volcanics well-developed 

columnar jointing is restricted to the 

very fine grained traps and has only a 

very slight tendency to form in the 

coarser grained porphyritic and ophitic rocks.

Columnar jointing can be seen in the 

Edwards Island Flow at all the localities 

mentioned previously. Other accessible 

areas where columnar jointing can be 

observed are where the trail from Rock 

Harbor to Mount Franklin crosses the 

flow and on the south slope of Ransom 

Hill just west of the trail from Daisy 

Farm Campground to Mount Ojibway.

The Greenstone How is the thickest 

flow on the island and holds up the 

most prominent ridge running the 

length of the island, Greenstone Ridge. 

The Greenstone Ridge Trail, which follows the ridge for nearly its full length, 

provides access to numerous scattered 

outcrops, but only at the far northeast 

end of the island near Blake Point is a 

reasonably complete cross section of 

the How exposed. There the flow can be 

seen to consist of four divisions with 

approximate thicknesses as indicated: a 

lower ophitic zone (100 ft), a central 

pegmatitic zone (75 ft), an upper ophitic zone (175 ft), and an uppermost 

columnar-jointed trap (50 ft), for a 

total thickness of about 400 feet. The 

Greenstone Flow attains its greatest 

thickness in the central part of Isle 

Royale, where it is estimated to be 

nearly 800 feet thick.

The lower ophitic zone, which makes 

up the cliffs of the Palisades on the 

north side of Blake Point, is of particular interest as it is matched in coarseness only by the ophite of the Hill Point 

How; each has augite crystals as much 

as 2 centimetres in diameter. The pegmatite is best seen in the vicinity of the 

lighthouse on Blake Point. The 

columnar-jointed trap that forms the 

uppermost part of the Greenstone 

Flow is exposed on the string of small 

islands south of Merritt Lane, on Red 

Rock Point, and at a few additional isolated localities on the main island.

An excursion to Blake Point is also a 

botanical experience, as a rather unusual shrub fills a small ravine just west 

of the lighthouse devilsclub. The entire plant, including its giant leaves, is 

profusely armed with sharp spines, and 

anyone who ventures to walk through a 

thick stand of these would appreciate 

the plant's specific name Oplopanax 

horridum (fig. 19). It presence on Isle

DEVILSCLUB. (Fig. 19)

18
Royale is truly remarkable, for it appears nowhere else in the United States 

east of the Rocky Mountains; at Isle 

Royale it is found only near Blake 

Point, on Passage Island, and on a few 

of the small islands in the Rock Harbor 

area.

Pyroclastic and sedimentary rocks 

are exposed in only a few localities in 

the Rock Harbor Tobin Harbor area. 

Pyroclastic rock can be seen on the 

north shore of Tobin Harbor opposite 

Newman Island and on Porter Island 

above (southeast of) the columnarjointed upper part of the Greenstone 

Flow. The rock is a tuff-breccia, composed of angular fragments of volcanic 

rock cemented together by ash (fig. 20). 

The ash was in a hot and partly plastic 

state when deposited and was then 

fused into a coherent mass upon cooling.

An extensive wave-swept outcrop of 

conglomerate is exposed at the southwest end of Mott Island, and a few

other less accessible outcrops are scattered along the rest of the chain of islands south of Rock Harbor (fig. 21). 

Volcanic rocks of many varieties can be 

identified as pebbles in the conglomerate. Many of them are of rhyolite or 

other types different from those that 

form the bulk of the Portage Lake volcanic sequence and must have been derived elsewhere.

PYROCLASTIC ROCK note fragmental 

character. (Fig. 20)

CONGLOMERATE ON MOTT ISLAND (southwest end of island) showing pebbly surface. (Fig.

21)

19
Washington Harbor Area

The southwest end of Isle Royale is 

mantled with a much thicker blanket of 

glacial till and other surhcial materials 

than the northeast end, and consequently, outcrops are common only 

along shorelines with exposure to 

storm waves. In the Rock Harbor 

Tobin Harbor area, we were discussing 

rocks that make up the upper part of 

the volcanic sequence, that is rocks 

from the Greenstone Flow upward in 

the sequence. In the Washington Harbor area, most exposures are of the sequence downward from the Greenstone Flow (fig. 22).

Most of the rocks exposed in the 

Washington Harbor area are of ophitic 

basalt, as for example on the series of 

small promontories from North Gap 

northward. Exceptionally coarse 

ophite can be seen in the Hill Point 

Flow at the entrance to Huginnin Cove, 

accessible via the Huginnin Cove Trail 

as well as by boat. Some large loose slabs 

of the coarse porphyrite of the Huginnin Flow lie on the west shore of the

cove (fig. 23), and excellent exposures 

of that flow can be seen on the north

HUGINNIN FLOW PORPHYRITE sample of 

loose slab from Huginnin Cove. (Fig. 23)

BOTH

Island 

South * 

Rockx

Porphyrite ol\ Ophite of_Washingtc 

Tobin Harbor 

Flow

WASHINGTON HARBOR AREA selected outcrops of various rock types. (Fig. 22)

20
shore of the island about half a mile 

southwest of the cove. Working upward in the volcanic sequence, trap of 

the Minong Flow is exposed along the 

Huginnin Cove Trail and forms a small 

promontory near the northwesternmost corner of the main island.

Outcrops of the coarse porphyrite of 

the Grace Island Flow are found on 

Grace Island just south of the campground, on Booth Island, and near 

both ends of Washington Island. Beach 

cobbles of rock from this flow are readily identifiable (fig. 24).

GRACE ISLAND FLOW PORPHYRITE 

sample of beach cobble from Grace Island. 

(Fig. 24)

The rock of the Washington Island 

Flow is difficult to describe but distinctive when seen; perhaps A. C. Lane's 

application of the term "glomeroporphyritic ophite" suggests the problem. 

The rock is definitely an ophite, but the 

ophitic texture is obscure; instead the 

rock is characterized by a speckled appearance due to the rather uniform 

distribution of greenish splotches of 

chlorite (fig. 25). The Washington Island Flow is well exposed on the south 

side of Washington Island where it 

overlies the Greenstone Flow and caps 

the highest ridge.

South Rock, just southwest of 

Washington Island, is a herring gull 

rookery. It is also one of only two small 

outcrops of fine-grained porphyrite 

that are reasonably accessible near the

CENTIMETRES

WASHINGTON ISLAND FLOW OPHITE 

showing distinctive dark splotches of chlorite. 

(Fig. 25)

west end of Isle Royale. It is believed to 

be part of the Tobin Harbor Flow. 

Another very small outcrop of porphyrite of the Middle Point Flow can be 

seen on the south shore of Grace Harbor about half a mile northeast of Middle Point.

AN ANCIENT 

ALLUVIAL FANTHE COPPER HARBOR 

CONGLOMERATE

The shoreline of the southwestern 

part of Isle Royale from Cumberland 

Point to Malone Bay lacks the dark, 

forbidding cliffs typical of most of the 

island. There in many places rusty-red 

rocks slope rather gently down to the 

water (fig. 26). Pebble beaches are 

moderately abundant, whereas they 

are quite rare elsewhere (fig. 27). It is 

readily apparent that the entire appearance of the island is different here, 

and the reason is geologic. This part of 

the island is underlain by sedimentary 

rocks of the Copper Harbor Conglomerate.

21
COPPER HARBOR CONGLOMERATE sloping gently southeast into Lake Superior near Attwood Beach. (Fig. 26) .

SANDSTONE PEBBLE BEACH west of Little Boat Harbor. National Park Service boat 

D. J. Tobin provided transportation for geologic studies. (Fig. 27)

The name of this formation is somewhat misleading because on Isle Royale 

it includes an appreciable amount of 

sandstone as well as conglomerate. On 

the Keweenaw Peninsula where the 

formation was originally defined, however, conglomerate is overwhelmingly 

predominant and hence the name. We 

shall see that within this formation on 

Isle Royale there is a definite pattern to 

the distribution of the sandstone and 

conglomerate that helps us to interpret

22
where and how the original sediments 

were laid down.

The sedimentary rocks, especially 

the sandstones, commonly exhibit a 

well-developed parting parallel to the 

original layering or bedding in the rock 

(fig. 28). It is this parting, along with 

the lakeward slope of the strata, that 

permits storm waves to remove thin 

slabs of rock so as to produce a shore 

that slopes into the water. However, 

where the parting slopes away from 

rather than toward the water, as on the 

north sides of islands and promontories, cliffs form. What is probably the 

highest sheer cliff on Isle Royale is cut 

into conglomerate on the north side of 

Feldtmann Ridge (fig. 29).

The sedimentary rocks of the Copper Harbor Conglomerate are similar 

to those interbedded with the Portage 

Lake Volcanics. The pebbles and sand 

making up these rocks have predominantly been derived by erosion from volcanic rocks, many of which are similar 

to those exposed on the island, but 

many of which are more felsic (higher 

in silica, see p. 12). The felsic varieties 

can be recogni/.ed by their generally 

reddish tint. Agate pebbles that are locally abundant in the conglomerate are 

amygdules from volcanic rocks.

The rock sequence of the Copper 

Harbor Conglomerate is far from 

completely exposed on Isle Royale, as 

nearly half its total thickness is concealed beneath the mud and water, 

black spruce, white cedar, and bog mats 

of the Big Siskiwit Swamp. Nevertheless, enough can be deduced from the 

existing outcrops to reconstruct the environment in which the sediments were 

deposited and then to speculate upon 

the origin of the formation.

One clue comes from variations in 

thickness of the rock sequence. In spite 

of the fact that we cannot estimate 

the total thickness of the formation because the top lies beneath Lake 

Superior, the Copper Harbor Conglomerate has marker horizons that 

can be traced or projected for considerable distances, allowing us to calculate changes in thickness for equivalent 

parts of the formation. The thickness 

of the exposed part of the section increases rather steadily from Rainbow 

Point to the vicinity of Menagerie Island about 24 miles to the northeast 

(fig. 30). The Copper Harbor ConSLABBY SANDSTONE outcrops on Stone House Island. (Fig. 28)

23
CLIFF IN COPPER HARBOR CONGLOMERATE on north side of Feldtmann Ridge. 

(Fig. 29)

glomerate is a great wedge of sedimentary rock, and while we see it in only two 

dimensions, other clues will help to 

elucidate its nature more fully.

Another clue lies in the variable 

coarseness of the sedimentary debris 

that makes up the formation. Near the

base of the formation, on Cumberland 

Point, the rock is a very coarse boulder 

conglomerate with boulders as much as 

2 feet in diameter. Eastward the conglomerate decreases in coarseness, 

finally grading entirely into sandstone 

east of Malone Bay. Higher in the formation a similar change can be seen 

from boulder conglomerate at Rainbow Point and cobble and pebble conglomerates along the shore south of 

Feldtmann Ridge to sandstone on 

Houghton Point and on the chain of 

islands on the south side of Siskiwit 

Bay. Thus the size of the stones in the 

conglomerate decreases as the formation increases in thickness. This decrease in grain size suggests a corresponding increase in distance from the 

source of the sedimentary debris, as 

fine materials can be transported 

farther than coarse materials under the 

same conditions.

Other evidence regarding the location of the source of the sediments 

comes from structures preserved in the 

rocks themselves. Some of these structures indicate current direction and 

thus presumably the direction sand 

and gravel were carried. An example is 

a structure formed by the deflection of 

the water current around larger-thanaverage fragments (fig. 31). Analysis of 

such features in the Copper Harbor 

Conglomerate indicates that the direcVERTICAL SCALE EXAGGERATED 4X

COPPER HARBOR CONGLOMERATE section showing changes in thickness and in coarseness of grain size. (Fig. 30)

24
WATER-CURRENT FEATURE formed by 

flow deflected around shale fragments. Direction of current flow is toward the right. 

(Fig. 31) ,

tion of sediment transport was generally eastward (fig. 32).

One other clue helps to nail down the 

source area the composition of the

pebbles themselves. Felsites and some 

of the other volcanic rocks that occur as 

pebbles in the Copper Harbor Conglomerate are not found in flows in the 

exposed Portage Lake Volcanics on Isle 

Royale. However, such rock types are 

found in a somewhat older sequence of 

volcanic flows exposed on the north 

shore of Lake Superior in Minnesota, 

exactly in the direction indicated as the 

source by the current structures, the 

distribution of coarse conglomerate, 

and the thinning of the sedimentary 

wedge. As a well-known expert might 

say, happiness is having the right kind 

of rocks in the right place.

To sum up, the Copper Harbor 

Conglomerate is a vast wedge of 

sedimentary rock derived from volcanic rocks in what is now Minnesota 

and transported generally eastward to 

its present site on Isle Royale, with, as 

might be expected, the fine sediment 

particles being transported farther 

than the coarse ones. But what can we 

say about the nature of conditions at 

the time and place of deposition?

Most of the bedding structures as 

well as the lenticular form of zones of 

differing coarseness in the conglomerates are characteristic of deposits associated with the flowing water of 

streams and rivers rather than lakes or

i^ ;7~ ____Portage _Lake__yplcan_ics___________--- 

Copper Harbor Conglomerate

\

DIRECTIONS OF SEDIMENT TRANSPORT for the Copper Harbor Conglomerate. (Fig. 32)

25
oceans. Irregular scour-and-fill structure (fig. 33) is characteristic of sediment deposited by braided streams,

ones flowing in dividing, reuniting, and 

constantly shifting channels resembling the strands of a braid (fig. 34).

SCOUR-AND-FILL STRUCTURE in pebbly sandstone, Chippewa Harbor area. (Fig. 33)

BRAIDED STREAM CHANNEL of the Muddy River, Alaska. Photograph by Bradford Washburn.

(Fig. 34)

26
Some of the structures, such as ripple 

marks (fig. 35), do indicate deposition 

in standing bodies of water. But associated desiccation cracks and raindrop impressions (fig. 36) indicate that 

those bodies of water were temporary 

ones, such as might occur on river flood 

plains or mudflats.

***.^^v'*n^-jr Jl*

RIPPLE MARKINGS on sandstone, west of Atrwood Beach. Photograph by R. G. Wolff. (Fig. 35)

ANCIENT DESICCATION CRACKS (A ) AND RAINDROP IMPRESSIONS (B } on siltstone at Isle 

Royals compared with similar features (C) on modern drying mud. Photograph of modem 

features by D. M. Baird. (Fig. 36). Figure continued on following page.

21
CENTIMETRES

B

Figure 36 Continued.

The closest analogy we can find 

today of a sedimentary deposit with the 

features of the Copper Harbor Conglomerate are the massive alluvial fan 

deposits and associated finer grained 

playa deposits in arid regions of the

28
American West (fig. 37). The formation of such deposits requires very 

rapid, flash-Hood runoff after rainstorms, rather than the continuously 

flowing water of permanent streams, 

and lack of vegetation is the key element permitting this to happen. This 

analogy does not necessarily mean that 

the Copper I larbor Conglomerate was 

deposited in a desert, for at the time the 

deposit was formed vegetation clicl not 

yet exist on land. In Precambrian time 

(see p. 36), before the appearance of a 

significant plant cover, erosion and 

runoff rates were high; floods were 

large, and coarse sediments were 

spread over piedmont areas more readily and rapidly than under modern 

conditions. Thus, the Copper Harbor 

Conglomerate on Isle Royale is a massive piedmont fan deposit, with associated Hood plain or playa deposits, 

that was spread out over the Portage

Lake Volcanics from a bordering highland area 10 the west.

TILTING AND 

BREAKING THE ROCKS

We have seen that Isle Royale consists of a thick series of Hood basalt 

flows overlain by a great alluvial fan. 

Furthermore, the sequence has been 

tilted southeast toward the axis of the 

Lake Superior basin, and subsequent 

erosion has exposed the upturned 

edges of individual layers. The tilt or 

dip of individual layers varies from less 

than 10° to 55°. The clip is generally 

steeper on the north side of the island 

than on the south side and averages less 

than 20° for the island as a whole.

The only major distortion superimposed upon this generally parallel 

group of inclined beds is in the area 

between Siskiwit Lake and Rock I larALLUV1AL FANS AND PLAYAS in Death Valley, Calif. From "Geology Illustrated" by John S. 

Shelton, W. H. Freeman and Company (copyright 1966). (Fig. 37)

29
North Government 

Island

EXPLANATION

^ 

Alluvium

Conglomerate and sandstone

Scoville Point Flow (porphyrite)

El

Edwards Island Flow (trap) 

Long Island Flow (trap)

IB

Tobin Harbor Flow (porphyrite)

Greenstone Flow (ophite)

Minong Flow (trap)

Huginnin Flow (porphyrite)

Hill Point Flow (ophite)

Amygdaloid Island Flow (trap) 

Portage Lake Volcanics, undivided

Fault

Symbol on side that moved down, 

where known

Glacial strations showing-direction 

of ice movement

88° 35'

GEOLOGIC MAP OF THE NORTHEASiND OF ISLE ROY ALE (PLATE 1)
EXPLANATION

cn

Copper Harbor Conglomerate

PoHage Lake Volcanics 

Subunits shown on map:

LLiiJ Scoville Point Flow 

I . J'.'l Greenstone Flow

Minong Flow 

H Hill Point Flow 

r^"l Sandstone and conglomerate

Strike and dip of sedimentary strata or lava flow

Symbols plaited over water to avoid crowding up[>l\

to adjacent shoreline

Approximate location of holes drilled 

in exploration for copper

10 MILES 

10 KILOMETRES

SIMPLIFIED BEDROCK GEOLOGIC tf AP OF ISLE ROYALE (PLATE 2)

SO" 30'

 47° 50'
bor, where the upper part of the Portage Lake Volcanics is warped arotind a 

node of apparently uplifted and fractured rocks (pi. 1). In appearance the 

distortion is something like a knot in an 

otherwise straight-grained piece of 

wood. The area appears to have been 

domed upward slightly to create this 

structure, but the specific cause is unknown.

Groups or sets of nearly parallel fractures cutting across the linear ridge system are also superimposed upon the 

tilted strata. Erosion along these fractures sometimes creates conspicuous 

ravines, and one set is clearly visible on 

the shaded-relief map (see title page). 

This set trends northward at the west 

end of Isle Royale and changes progressively eastward to trend about 30° 

east of north at McCargoe Cove. Minor 

fault displacement has occurred along 

some of the fractures in this set, but the 

amount of movement was generally 

small. A second set of fractures, trending east-west and not as apparent on 

the shaded-relief map, commonly 

shows somewhat greater fault displacement but still no more than a few 

hundred feet. A third set of curved 

fractures is present in the structurally 

disturbed area between Siskiwit Lake 

and Rock Harbor. All the fractures

were probably formed as the beds 

broke up during tilting and warping of 

the layered rock sequence.

ISLE ROYALE 

A SMALL PIECE OF

THE BIG PUZZLE

The orientation of the beds on the 

Kewecnaw Peninsula is a mirror image 

of that on Isle Royale. The bedrock 

sequence, similar to that on Isle Royale, 

generally dips northwestward toward 

the axis of the Lake Superior basin. 

Recognition of these similarities permitted C. T.Jackson to state as early as 

1849 that "this island has the same 

geological character as Keweenaw 

Point, and is of '\hextiini' geological age." 

I le and his colleagues, J. W. Foster and 

J. I). Whitney, furthermore considered 

the rock sequences in the two areas to 

be connected beneath Lake Superior 

and thus interpreted the lake as occupying a structural basin, a syncline, 

as well as a topographic depression (fig.

38).

The later work of A. C. Lane not only 

reinforced this interpretation but also 

demonstrated that some individual 

lava (lows or groups of flows, as well as 

some sedimentary rock units, on Isle

NW THUNDER BAY 

REGION

VERTICAL SCALE 

EXAGGERATED

LAKE SUPERIOR

KEWEENAW SE 

PENINSULA

Postvolcanic sedimentary rocks, including 1 nterbedded volcanic and sedimentary rocks, Prevolcanic rocks Arrows indicate relative directions

the Copper Harbor Conglomerate including the Portage Lake Volcanics of movement along faults

LAKE SUPERIOR BASIN^ross section. (Fig. 38)

34
Royale were the same as those on the 

Keweenaw Peninsula (fig. 39). The 

continuity of the Greenstone Flow 

across the Lake Superior basin is most 

convincing, and its name is used in both 

localities; specific correlation of other 

flows is less certain, and separate names 

are applied to them on opposite sides of 

the lake.

The Portage Lake Volcanics on Isle 

Royale and on the Keweenaw Peninsula represent only the upper part of 

the total Keweenawan volcanic sequence in the Lake Superior region. 

The lower part of the sequence is exposed in the so-called South Trap 

Range of westernmost Michigan and 

along the north shoreof Lake Superior 

in Minnesota and Ontario. Different 

formation names are given to these 

rocks in different areas (fig. 39). The 

total volcanic sequence reflects a long 

period of volcanic activity with a complex history. Erosional debris derived

from the lower part of the volcanic sequence occurs in the sedimentary rocks 

interbedded with the lava flows of the 

Portage Lake Volcanics. The interpretation is that the lower part of the sequence was being eroded at the margins of a basin while the later flows were 

still being erupted into the central part 

of the basin.

WHAT HAPPENED 

WHEN?

J. he geologic story of Isle Royale as 

presented up to this point has been 

largely a description of the geology as 

we see it now. But how did it get this 

way? And when? The search for answers to these questions involves considerable interpretation of geologic observations made on Isle Royale and in 

the Lake Superior region, together

Lake 

Nipigon

ONTARIO

Sandstone of Cambrian and 

latest Keweenawan age

Copper Harbor Conglomerate

CD

Keweenawan volcanic sequence, 

including Portage Lake Volcanics

50'KILOMETRES

85-

LAKE SUPERIOR REGION distribution of selected rock units. (Fig. 39)

35
with numerous deductions based upon 

our accumulated geologic knowledge. 

Some parts of the geologic history can 

be deciphered in considerable detail, 

and other parts only incompletely because the geologic data are very spotty. 

The first step toward the answers is to 

develop a time framework within 

which to reconstruct events in the 

geologic past.

TIME AS A 

GEOLOGIC CONCEPT

A relative time scale, useful 

throughout the world, has been developed by dividing the last approximately 570 million years of geologic 

time into periods. The periods, which 

began with the Cambrian and end with 

the Quaternary, are recognized largely 

from the fossil record (fig. 40). Rocks of 

the Cambrian Period contain the earliest evidence of complex forms of life, 

which slowly evolved through the subsequent periods into the life of the 

modern world. The presence of distinctive fossils in various periods permits the correlation of rocks of similar 

age. However, the near absence of fossils in Precambrian rocks those older 

than 570 million years severely limits 

the use of fossils for the relative dating 

of the Precambrian rocks, rocks which 

cover more than 80 percent of geologic 

time. The rocks on Isle Royale are unfossiliferous and of Precambrian age, 

and it is with this poorly classified segment of geologic time that we are 

mostly concerned.

Fortunately, a means for measuring 

geologic time without fossils has been 

developed from the long-known 

natural process of radioactive decay of 

certain elements. From such radiometric age determinations, many rocks can 

now be assigned ages in years, which 

for Precambrian rocks, in the near absence of fossils, are virtually the only 

means for long-range correlations.

The uppermost, or youngest, Precambrian rocks in the Lake Superior 

region consist of a thick sequence of 

volcanic and sedimentary rocks. This 

sequence has been named the

QUATERNARY and TERTIARY 

CRETACEOUS 

JURASSIC 

TRIASSIC

100 20Q 300 100 500 1000

MILLIONS OF YEARS AGO

GEOLOGIC TIME CHART showing major divisions. (Fig. 40)

36
Keweenawan Supergroup because 

many of the formations that make it up 

were first described on the Keweenaw 

Peninsula and adjacent parts of 

Michigan and Wisconsin. The period 

of time during which the Keweenawan 

rocks were deposited can be referred to 

informally as Keweenawan time, recognizing that the term has usefulness 

only in the Lake Superior region where 

the Keweenawan rocks themselves 

occur. The Keweenawan Supergroup 

has been informally divided into lower, 

middle, and upper parts; formations 

exposed on Isle Royale, the Portage 

Lake Volcanics and the Copper Harbor 

Conglomerate, are assigned to the 

middle Keweenawan (fig. 41). We do 

not have sufficient data to determine 

the bounds of Keweenawan time, but 

radiometric age determinations indicate ages in the general range of 

1,120-1,140 million years ago for the 

Keweenawan volcanic rocks. It is from 

this point in the enormously distant 

past that we begin to unravel the 

geologic history of Isle Royale.

WHAT WAS THERE

BEFORE THE 

PRE-KEWEENAWAN

Rocks of Keweenawan age, as old as 

they are, rest upon still older Precambrian rocks that are strikingly different. The older, or basement, rocks are 

of many varieties, ranging from granite 

and other intrusive igneous rocks to 

volcanic and sedimentary rocks, and 

cover a wide range in age. They usually 

are both more highly metamorphosed, 

or altered from their original state, and 

more strongly deformed than rocks of 

the Keweenawan sequence. Basement 

rocks are not exposed on Isle Royale, 

but they are sufficiently well exposed in 

areas both north and south of Lake

Q.

3

g

O

Not differentiated here s

tc

CD

Freda Sandstone =>

8

C3

Nonesuch Shale 2

Copper Harbor Conglomerate

(including minor lava flows)

~~~7

/

Unnamed volcanic?

formation C^^

c^

Portage Lake Volcanics

Volcanic rocks of the 

South Trap Range

Bessemer Formation

(sandstone and conglomerate)

^

|

UJ

UJ

cc

UJ 

Q.

Q.

D

Z

1

Z

UJ

I

111

Q

Q

OR 1IDDLE LOWER WEENAWAN

 * UJ

ffil

5 uj

O UJ

CL

8cc

o

oc

UJ 

Q.

CO

Z

1UJ I

i

oc m

o

UJ

or

0.

KEWEENAWAN SUPERGROUP classification in Michigan. (Fig. 41)

Superior for their nature to be studied 

and their relationship to the overlying 

Keweenawan sequence to be determined.

The history of the pre-Keweenawan 

rocks is extremely complex, as might be 

expected from the wide range in rocks 

and age; however, prior to the time the 

Keweenawan rocks were deposited, 

much of what is now the Lake Superior 

region had been reduced to an area of 

fairly low relief, and shallow seas covered large parts of it. A thick sequence

37
of marine sedimentary and volcanic 

rocks was then formed in those seas. In 

addition to impure sandstone and 

shale, chemically precipitated sediments were deposited, including 

dolomite and the iron-rich rocks that 

were later to make the Lake Superior 

district world famous as a producer of 

iron ore. The material forming the 

pre-Keweenawan volcanic rocks was 

erupted under water, where quick 

quenching of the lava caused it to congeal in irregular-shaped globular or ellipsoidal masses that accumulated one 

upon the other at the bottom of the sea 

(fig. 42). These volcanic rocks are thus 

quite different from the uniformly 

layered widespread sequence of 

Keweenawan flood basalts.

ELLIPSOIDAL STRUCTURES indicative of 

extrusion of lava under water, found in Precambrian volcanic rock from northern 

Michigan. (Fig. 42)

Toward the close of pre-Keweenawan time, the rocks present were 

folded, deformed, and eroded, quite 

drastically in some areas and only 

slightly in others, and so the material 

forming the overlying Keweenawan 

volcanic and sedimentary rocks was

laid down on the edges of the older 

beds in some localities.

THE ROCKS OF

ISLE ROYALE 

THE KEWEENAWAN

The lowermost Keweenawan strata, 

which are not present on Isle Royale, 

consist of a relatively thin sequence of 

sedimentary rocks, chiefly conglomerate and sandstone, that lie on the preKeweenawan rocks. They have been interpreted as shallow-water, near-shore 

deposits and are exposed in only a few 

localities along the perimeter of the 

area of Keweenawan outcrops around 

the margins of the Lake Superior basin. 

The lowermost few of the Keweenawan 

lava flows above these sedimentary 

rocks contain ellipsoidal structures typical of those formed when lava is 

erupted into a body of water. Such 

structures are found only near the base 

of the lava sequence, however, and we 

conclude that the first few of the flood 

basalt flows were erupted into a shallow 

body of water, which was soon filled or 

disappeared for one reason or another, 

and that all later flows were on virtually 

dry land. Furthermore, as mentioned 

earlier, the sedimentary rocks interbedded with the lava flows show 

characteristics typical of those formed 

in streams rather than in lakes or 

oceans.

The source of the lavas appears to 

have been along the axis of an arcshaped trough or system of fissures 

generally following the center of 

present-day Lake Superior. The system was actually much more extensive, 

however, extending southwest at least 

as far as Kansas and southeast into the 

southern peninsula of Michigan, a total 

distance of over 1,000 miles. Great volumes of lava were erupted from

38
fissures near the axis of the trough and 

spread laterally and rapidly toward 

both margins of the trough, finally 

ponding and cooling in place to form 

vast sheets covering thousands of 

square miles. The Portage Lake Volcanics, as exposed on the Keweenaw 

Peninsula and Isle Royale, represent 

roughly the upper half of this volcanic 

pile (fig. 41).

The material forming the sedimentary rocks that are interbedded with the 

lava flows of the Portage Lake Volcanics and that are in the Copper 

Harbor Conglomerate and other 

Keweenawan formations above the 

lava sequence was transported by 

streams into the trough or basin from 

highlands around its margins. This 

evidence for inward flow of streams, 

contrasted with evidence that the lavas 

flowed toward the margins of the basin, 

shows that there were, at times, reversals of the prevailing slope over large 

areas and leads to the concept of a basin 

subsididing as it was being filled (fig. 

43). The surfaces of the lava flows were 

horizontal or sloped gently toward the 

margins of the basin as long as filling by 

lava kept pace with downwarping. 

When extrusion of the lava was interrupted, however, continued downwarping produced inward slopes that 

permitted sedimentary debris to be 

swept into the basin. Finally, with the 

gradual demise of volcanic activity, 

continued subsidence permitted the 

accumulation of the Copper Harbor 

Conglomerate and younger Keweenawan deposits to form a thick 

sedimentary sequence above the volcanic rocks in the basin.

Sandstone of latest Keweenawan or 

earliest Cambrian age is exposed along 

much of the shore of the southeastern 

part of Lake Superior and extends 

westward to the Keweenaw Peninsula. 

The basin may have been largely filled

A. Lava erupts near the center'Of the basin and spreads 

laterally toward the margins to form a sequence of. 

lava flows.

Y

8. The basin subsides, and during a lull in volcanic activity gravels are swept into the basin and'spread out 

over the uppermost lava flow.

C. Volcanic activity resumes, and the cycle starts over 

again.

FLOOD BASALTS AND SEDIMENTS showing the process of interbedding. (Fig. 43)

by this sandstone, together with similar 

sandstone exposed in the southwestern 

part of the basin (fig. 39).

The gross synclinal form of the 

Keweenawan basin resulted from subsidence coincident with filling of the 

basin rather than later folding by 

squeezing. However, Keweenawan 

strata near the margins of the basin, as 

on the Keweenaw Peninsula and Isle 

Royale, were subsequently steepened 

by upward movement on major faults, 

the Keweenaw fault and the Isle Royale 

fault, thus accentuating the synclinal 

structure (fig. 38).

39
THE MISSING 

CHAPTERS

It is somewhat disconcerting that we 

can reconstruct so much of what happened during the Precambrian and so 

little of the geologic history of the Lake 

Superior region during the last 570 

million years, including most of the 

Paleozoic, Mesozoic, and Cenozoic 

Eras. But this is the situation, for erosion has removed most of the critical 

evidence regarding that period of time. 

And, of course, much of the area is 

concealed by Lake Superior itself.

We do know, nevertheless, that the 

rocks of the Lake Superior region constitute the southermost part of an area 

known as the Canadian Shield an 

area of predominantly Precambrian 

rock and one that for the most part has 

been relatively stable geologically since 

the Precambrian (fig. 44). Beginning in 

Late Cambrian time, however, the socalled Michigan Basin, centered on 

what is now the southern peninsula of 

Michigan, began to subside, and a thick

THE CANADIAN SHIELD (stippled area) 

predominantly Precambrian rocks. (Fig. 44)

40

sequence of Paleozoic rock accumulated in the invading sea. Lower 

Paleozoic rocks at the margin of this 

basin extend up onto the shield near 

the southeast shore of Lake Superior. 

A few remnants of lower Paleozoic 

dolomite occur as far west as the base of 

the Keweenaw Peninsula; they are very 

thin and were probably deposited very 

close to the outer margin of the 

Michigan Basin.

We also know that lower Paleozoic 

rocks are present in a fairly extensive 

area centered on Hudson Bay. They 

include various types of rocks of shallow marine origin and in the vicinity of 

James Bay are locally overlain by nonmarine Cretaceous sedimentary rocks 

that include coal. Remnants of lower 

Paleozoic strata at a few other scattered 

localities on the Canadian Shield indicate that such strata were once more 

extensive than now, but how extensive 

is unknown. It is possible that they 

covered the entire shield, but we have 

no direct evidence that such rocks were 

ever present in the vicinity of Isle 

Roy ale.

Much of the Canadian Shield probably had been eroded to a relatively flat 

surface prior to the deposition of the 

lower Paleozoic strata. It remained so 

during the deposition and the subsequent erosion of much of those strata, 

right up to the Pleistocene Epoch and 

the onset of glaciation. Sometime after 

the deposition of the Jacobsville Sandstone of Precambrian or Cambrian age, 

however, movement on the Keweenaw 

fault thrust the Portage Lake Volcanics 

up over the Jacobsville Sandstone on 

the Keweenaw Peninsula. Presumably 

at about the same time, thrusting on the 

Isle Royale fault left the volcanic rocks 

of Isle Royale standing as an elongate 

ridge surrounded by softer sedimentary rocks. With this last disturbance 

the present geologic structure of the
Lake Superior region was established, 

and little has happened since except for 

erosion by both water and ice. A broad 

river valley probably developed in the 

present site of Lake Superior, as well as 

in the sites of the other Great Lakes, for 

all are located in belts of rocks less resistant to erosion than their bounding 

areas. The gross features of the preglacial landscape probably exerted a 

major influence on the direction of ice 

movement as successive lobes of glacial 

ice invaded the region during the Pleistocene and moved through the ancestral "Lake Superior valley."

THE GLACIERS TAKE 

OVER

The concept of widespread glaciation during a so-called ice age evolved 

over a relatively short period of time 

during the first half of the 19th century. It was first clearly expounded by 

Louis Agassiz beginning in 1837 but 

met with considerable skepticism for 

many years. Interestingly enough, 

Agassiz himself visited the Lake 

Superior region in 1848 and remarked 

that nowhere had he seen evidence of 

glacial phenomena in greater clarity 

than on the shores of Lake Superior. 

He traversed the north shore of the 

lake as far west as the Hudson Bay Co. 

post at Fort William, opposite Isle 

Royale on the Canadian shore, but he 

was unable to visit the island because of 

stormy weather. The next year 

Edouard Desor, who had been 

Agassiz's secretary and coworker for 

the previous 10 years, became associated with Foster and Whitney in their 

studies of the Lake Superior region 

and was the first to describe glacial 

features on Isle Royale. Mount Desor, 

the highest summit on Isle Royale, and 

Lake Desor, the second largest lake on 

the island, bear his name.

Beginning about 2 or 3 million years 

ago, the earth underwent a series of 

cyclic climatic changes, with at least 

four episodes cold or moist enough to 

permit the formation of extensive ice 

sheets. In North America, ice accumulated at several separate centers, spread 

out, and coalesced to cover a large 

part of the continent during each of 

these four episodes that together make 

up the glacial or Pleistocene Epoch 

(fig. 45).

Isle Royale was overridden by glacial 

ice during each of the four major glaciations, and each successive glaciation 

virtually obliterated all direct evidence 

of the preceding glaciations on the island. In the waning phase of the last 

major glaciation, the Wisconsin Glaciation, the frontal ice margin retreated 

northward from at least the greater 

part of the Lake Superior basin, then 

readvanced into the basin about 12,000 

years ago (fig. 46). We can attribute the 

final carving of the Isle Royale landforms to this last advance, although all 

the earlier glaciations played their part.

Glacial Effects On 

The Land

The effects of glacial erosion were 

strongly dependent upon the nature of 

the bedrock. Over much of the Canadian Shield, where the bedrock consists 

of granite and other resistant Precambrian rocks, the ice sheets probably did 

little more than remove surficial materials, weathered rock, and minor 

amounts of bedrock. However, where 

the bedrock consisted of softer, 

sedimentary rocks, as along the axes of 

each of the Great Lakes, the ice sheet 

deepened the preglacial lowlands to 

form the present lake basins. During 

the excavation of the Lake Superior 

basin, Isle Royale remained a ridge because its volcanic rocks resisted erosion.

41
? / £ k'' f / V r>l -^(A

PLEISTOCENE ICE SHEETS showing the main centers of accumulation and the maximum extent 

of the Pleistocene ice sheets. (Fig. 45)

Plucking and Abrasion

Glacial erosion of bedrock includes 

two distinct processes: plucking and 

abrasion. Glacial plucking or quarrying 

is the lifting out and removal of fragments of bedrock by the ice, whereas 

glacial abrasion involves grinding and 

polishing of the bedrock surface by 

rock fragments imbedded in the base 

of the moving ice sheet. Plucking is by 

far the more effective in altering the 

landscape.

Glacial plucking is controlled chiefly 

by bedrock structures, particularly 

preexisting joints or other fractures. 

The sedimentary rocks and amygdaloidal zones at the base and top of

flows are less resistant to abrasion than 

the massive flow interiors, and the 

sedimentary rocks and thinner flows 

have closer spaced jointing than the 

massive interiors of the thicker flows. 

The alternation of massive, poorly 

jointed rock and highly jointed rock 

that guided the development of the 

preglacial topography by stream erosion also was the major control for glacial quarrying. The main role of glacial 

erosion on Isle Royale thus was to accentuate the asymmetry of the existing 

ridge-and-valley topography that had 

been formed earlier by stream erosion 

and to interrupt the preglacial stream 

channels with a series of small lake basins scooped out of the bedrock.

42
WISCONSIN ICE RETREAT from central North America. Note major ice surge, or readvance, in 

the Lake Superior basin (from V. K. Prest, 1969, Geological Survey of Canada Map 1257A). 

(Fig. 46)

Whereas glacial quarrying modified 

the gross topography of Isle Royale, 

glacial abrasion rounded, polished, 

and striated many rock outcrops (figs. 

47, 48). The quarrying and scouring 

together were so effective, in fact, that 

deeply weathered rock, which we 

would expect to find after millions of 

years of weathering, is found only 

rarely on the island on the lee sides of a 

few outcrops where the weathered rock 

was protected from the action of mov- GLACIAL STRIATIONS near Moskey Basin 

ing ice. Campground. (Fig. 47)

43
GLACIALLY ROUNDED AND STRIATED 

OUTCROP near portage trail at Pickerel 

Cove. (Fig. 48)

Striations

Glacial striations on the bedrock not 

only indicate the former passage of glacial ice but also provide an indication of 

the general direction of movement of 

the ice that formed them (fig. 49). The 

striations indicate that the last ice 

movement on the east half of the island 

was toward the southwest, parallel to 

the bedrock ridge-and-valley topography. On the west half, striations indicate that the direction of movement 

was westward, crossing the ridges and 

valleys at an angle. The westerly direction is roughly perpendicular to a series 

of glacial moraines, or ridges of glacial 

till, that crosses the island near its west 

end; these moraines mark a pause in 

the retreat of the ice sheet. Southwesterly striations near the north side of the 

southwest end of the island presumably 

reflect the direction of the main advance down the Lake Superior basin 

prior to retreat to the position of the 

glacial moraines. Glacial striations are 

rarely preserved on the sedimentary 

rocks because the surfaces of those 

rocks weather and disintegrate relatively rapidly. Striations are best preserved on the denser, nonamygdaloidal volcanic rocks and can best be seen 

along shorelines where such rocks dip 

gently into Lake Superior or along 

shorelines of some of the larger inland 

lakes. Such locations are along the 

north shores of Tobin Harbor, Rock 

Harbor, Pickerel Cove, Lake Richie, 

Siskiwit Lake, and the west end of MosDIRECTION OF GLACIAL ICE MOVEMENT as indicated by striations and other glacial features. 

Dark areas are recessional moraines. (Fig. 49)

44
key Basin. Striations can also be seen 

along many of the inland trails, especially in places where the trail itself has 

channeled rainfall runoff so as to remove the soil cover and expose fresh 

bedrock.

Till ..;" - "\ -":"'

On the east half of Isle Royale, where 

the last ice advanced approximately 

parallel with the ridge-and-valley topography, surficial deposits other than 

a thin mantle of soil and rock debris are 

confined to valleys and smaller depressions, and conversely outcrops are relatively abundant except in the valleys; 

heterogeneous glacial deposits called 

till are not apparent. Scattered boulders of foreign rocks, called glacial erratics, do indicate the presence of glacially transported materials, but they 

are not overly abundant. Beach gravels 

along the present shoreline are predominantly derived from nearby bedrock, although they contain pebbles 

reworked from glacial materials. The 

most obviously foreign pebbles are of 

granitic rocks from the Canadian 

mainland and fossiliferous limestone 

and chert from the Hudson Bay area 

(fig. 50).

Near the west end of Siskiwit Lake, 

where the westward direction of ice 

movement crossed the trend of the 

ridges and valleys at an angle of about

FOSSIL SHELL in chert pebble. (Fig. 50)

25°, glacially deposited materials are 

more abundant; thin glacial till mantles 

many of the slopes, and outcrops decrease in size and number. Large erratics are numerous.

Till is abundant on the west end of 

the island, effectively concealing most 

of the bedrock and subduing the landforms (see title page). Large erratics 

also are abundant (fig. 51). Bedrock 

outcrops are mainly limited to small 

knobs and the steep north faces of 

some ridges. In many places, even 

these bedrock exposures have clearly 

been excavated from till by wave action 

at a time when Lake Superior was 

higher than now.

ERRATIC BOULDER of granite near crest of 

Feldtmann Ridge. (Fig. 51)

Drumlins. West of Siskiwit Lake, 

where the till becomes more abundant, 

one can see deposits of till with distinctive shapes, certain of which are called 

drumlins. A drumlin is a streamlined 

hill or ridge of glacial till elongate in the 

direction of flow of the glacier that 

formed it. The drumlins on Isle Royale 

are of the crag-and-tail variety linear 

ridges streamed out behind a bedrock 

knob (fig. 52). The drumlins vary in 

length from a few feet to nearly 2 miles; 

the larger ones can be seen on the 

shaded-relief map (see title page) and 

on aerial photographs as distinct eastwest ridges superimposed upon the

45
Direction of ice movement

CRAG-AND-TAIL DRUMLIN cross section. 

(Fig. 52)

bedrock-controlled, northeast-trending ridges (fig. 53). They are most 

abundant between the west end of Siskiwit Lake and a locale about 5 miles 

east of Cumberland Point. In an area 

west of Siskiwit Bay covering several 

square miles, the only bedrock exposed 

is at the heads of the drumlins. The 

elongation of the drumlins is useful to 

determine the direction of ice movement in areas where glacially striated 

bedrock is not exposed.

Recessional moraines. A series of 

till-built recessional moraines that was 

formed during a pause in the retreat of 

the last ice sheet crosses the island in a 

general northerly direction west of 

Lake Desor. (See fig. 49). The form of 

these deposits is distinctive and can be 

generally characterized as an arcshaped ridge roughly transverse to the 

direction of ice movement and not controlled by bedrock structure. The deposits define the former position of an 

irregular ice front controlled by topography, with tongues of ice extending 

into valleys or low areas in front of the 

main ice mass. (See fig. 59.) The ice 

front was not completely static, however, and continued to retreat by small 

increments. This intermittent retreat is

GLACIAL FEATURES FROM AIR including drumlins, abandoned shorelines, and other features 

of glacial origin west of Siskiwit Bay. (Fig. 53)

46
best illustrated by the succession of 

moraines on the south flank of Mount 

Desor that were constructed by the ice 

lobe that extended up the drainage 

basin of the Little Siskiwit River.

Glacial Effects On 

Lake Levels

Some uncertainty exists as to 

whether the last ice readvance completely filled the Lake Superior basin. 

But however far the ice did advance, 

when it retreated it left behind a sequence of lakes that progressively filled 

more and more of the basin. The retreating ice uncovered successively 

lower outlets, and thus the general 

trend of lake elevations was initially 

downward. Later lake levels were

influenced by uplift of the final lake 

outlet as the earth's crust rose in response to the removal of the weight of 

the glacial ice, weight which had previously depressed the earth's elastic crust 

when the ice advanced southward.

In between rather rapid changes in 

lake levels caused by changes in outlets, 

the water remained at stable elevations 

long enough for waves to erode cliffs, 

build beaches, and construct other recognizable shoreline features. The history of the sequence of lakes that occupied the Lake Superior basin has 

been deduced largely from matching 

their abandoned shorelines around the 

basin. Such correlation of shorelines is 

complicated by the fact that as the 

shorelines are traced toward the northeast, in the direction of ice retreat, indiFigure 53. Continued.

47
vidual shorelines gradually increase in 

elevation, owing to the rise of the land 

after the shorelines were formed (fig. 

54). A specific example can be given by 

the shoreline of Lake Minong, one of 

the postglacial lakes. The elevation of 

its uplifted and warped shoreline on 

Isle Royale increases from about 80 

feet above Lake Superior at the southwest end of the island to about 170 feet 

at the northeast end (fig. 55) a rise of 

over 2 feet per mile. Despite the complications introduced by tilting of the 

abandoned shorelines, through careful 

tracing of them, several distinct lake 

stages for the Lake Superior basin have 

been established and named. A

shoreline diagram for the most important ones is shown in figure 56, and the 

chronology of lake-level changes is 

shown in figure 57.

An important fact to remember is 

that on Isle Royale all shorelines older 

than the present one have been uplifted to a varying degree, and thus 

their present elevations above sea level 

do not reflect the elevations of the lakes 

that formed them. The original elevations of individual lakes can be determined only where the shorelines remain undeformed, south of the region 

of crustal rebound or uplift (at the far 

left of figs. 54, 56). Note, for example, 

that Lake Minong, whose shorelines

A. Glacial lake phase, A, has formed between the receding glacier margin and high ground to the left.

B. Uplift has affected the area to the right of the vertical dotted line, bending the shoreline of phase A 

to a higher position. A new lake phase, B, has formed at a lower level.

C. Renewed uplift has affected the area to the right of the vertical line, bending the shoreline of phase B 

and further warping the right-hand end of A. A third phase, C, has formed at a still lower level. The 

amount of warping is greatly exaggerated.

SHORELINE WARPING due to progressive uplift of the earth's crust as the glacier retreated (after

Flint, 1971). (Fig. 54)

48
Lake Minong shoreline, shown solid 

Lake Superior shoreline, shown dotted

Numbered lines show increase toward Ihe 

northeast of the elevation of (he Lake 

Minong shoreline in feet above the 

present level of Lake Superior

UPLIFT AND WARPING of the shoreline of postglacial Lake Minong. (Fig. 55)

0 50 KILOMETRES

POSTGLACIAL LAKE SHORELINE 

STAGES in the Lake Superior Basin. Curvature is due to crustal rebound (uplift), progressively greater to the northeast. Horizontal arrows indicate progressive retreat of the 

ice margin; vertical arrows indicate direction 

of changes in lake elevations. (Fig. 56)

are well displayed on Isle Royale above 

the present shoreline, actually existed 

at an elevation more than 150 feet.

THOUSANDS OF YEAFIS AGO

LAKE-LEVEL CHANGES in the Lake 

Superior basin. Declining lake levels are 

chiefly due to progressive uncovering of 

lower outlets as the ice retreated northward. 

Rising lake levels are due to uplift of outlets 

by crustal rebound. (Fig. 57)

lower than the present level of Lake 

Superior.

Lake Duluth was the first and highest

49
glacial lake to fill a major part of the 

Lake Superior basin during retreat of 

the last ice (fig. 58). The ice sheet was in 

a period of rather rapid retreat when 

Lake Duluth formed, but it then must 

have slowed down because the immediately following lakes did not extend much farther northeast than Lake 

Duluth did, although lake levels fell 

nearly 500 feet between the levels of 

Lake Duluth and Lake Beaver Bay.

LAKE DULUTH and its contemporary ice border, about 11,000 years ago. (Fig. 58)

The ice front forming the north 

margin of the earlier lakes probably 

remained south of Isle Royale until 

about the time of Lake Beaver Bay or a 

little later, when it retreated to a position straddling Isle Royale west of Lake 

Desor (fig. 59). Abundant glacial till 

was deposited upon the newly 

emergent west end of the island, and 

the ice front remained stable long 

enough to build across the island the 

previously described complex of recessional moraines. Shorelines formed by 

the glacial lake associated with this ice 

front are found on the west end of the 

island where they now occur about 200 

feet above the level of Lake Superior.

Subsequent renewed and complete 

retreat of the ice margin from Isle 

Royale was rapid enough that only a 

minor amount of glacial till was deposited on the central and eastern parts of 

the island. When the ice margin

Shoreline of Isle Royale Archipelago 

ZOOIeet riighet than present Lakes

GLACIAL ICE MARGIN during deposition of 

recessional moraines on Isle Royale and 

prior to complete northeastward retreat of ice 

from the island. (Fig. 59)

reached the north edge of the Lake 

Superior basin, Lake Minong was 

formed, and the entire basin was filled 

for the first time since the readvance of 

the ice sheet (fig. 60). Lake Minong 

marked a relatively stable period in the 

history of the basin, and its beaches are 

among the best developed on Isle 

Royale. In fact, it was on Isle Royale 

that evidence for the existence of Lake 

Minong was first recognized.

An even lower lake, Lake Houghton, 

followed Lake Minong. Then, during a 

long period, slowly rising lake levels 

controlled largely by crustal uplift reLAKE MINONG and contemporary ice border 

along the north shore of the Lake Superior 

basin, about 10,500 years ago. (Fig. 60)

50
suited about 5,000 years ago in the 

Nipissing lake stage at 605 feet above 

sea level (fig. 57), and finally the present Lake Superior evolved at 602 feet 

(fig. 61).

On Isle Royale, beaches of the postglacial lakes ancestral to Lake Superior 

are best developed on the southwest 

end of the island, where abundant glacial deposits provided easily worked 

materials for beach construction (fig. 

62). Most abandoned beaches are not

readily accessible to the hiker, but the 

trail from Siskiwit Bay to the Island 

mine crosses rather prominent Nipissing beaches about one-quarter mile 

from Siskiwit Bay. The trail from 

the Siskiwit Bay campground to 

Feldtmann Ridge follows a Nipissing 

beach terrace through a clearing marking the site of an abandoned Civilian 

Conservation Corps camp and then 

climbs to a Minong beach, which it follows for about 2!/z miles.

BIRTH AND GROWTH OF ISLE ROYALE during selected postglacial lake stages. (Fig. 61)

51
PROMINENT ABANDONED SHORELINE FEATURES on the southwest end of Isle Royale. (Fig.

- : ' .... .., - - :-. ,- . . 62> : :,. - -. , .. .- .- : ...

Among the best developed Minong 

and Nipissing beaches are those between Rainbow Cove and Feldtmann

Lake; these are accessible from the trail 

between the cove and the lake (figs. 63, 

64). Indeed, these beaches dam

LAKE MINONG AND LAKE NIPISSING BARRIER BEACH BARS between Rainbow Cove and

Feldtmann Lake. (Fig. 63)

52
Minong

2000

DISTANCE, IN FEET

LAKE MINONG AND LAKE NIPISSING BARRIER BEACH BARS (profile) between Rainbow 

Cove and Feldtmann Lake. (Fig. 64)

Shoreline of Isle Royale just prior to Lake Minong stage.

I__..__

Formation of barrier bar to close off Feldtmann Lake during Lake Minong stage.

Formation of barrier bar to close off Lake Halloran during Lake Nipissing stage. 

FELDTMANN AND HALLORAN LAKES development. (Fig. 65)

53
Feldtmann Lake. Just prior to the time 

of Lake Minong, the lowland between 

Rainbow Cove and Siskiwit Bay was 

covered by a glacial lake at a higher 

elevation, and Feldtmann Ridge was a 

separate island (fig. 65). As the water 

level fell to that of Lake Minong, the 

central part of this lowland emerged 

from the lake; however, Siskiwit Bay 

still extended inland about 5 miles 

more than it does today, and a similar 

bay on Lake Minong occupied the site 

of Feldtmann Lake. The Rainbow Cove 

area was exposed to the full force of 

storm waves from the southwest, and 

Lake Minong was at a stable elevation 

long enough for those waves to construct a barrier beach bar across the 

mouth of "Feldtmann Bay" and isolate 

it as a lake. Other beach bars were subsequently built at the lower levels of the 

Nipissing stage, including one that, 

formed Lake Halloran.

On the northeast end of Isle Royale, 

where glacially transported debris is 

limited and abandoned beaches are less 

evident, wave-cut features in the bedrock mark ancient shorelines (fig. 66).

WAVE-CUT SHORELINE FEATURES including cliffs, stacks, and an arch. (Fig. 66)

Prominent examples are Monument 

Rock, a stack associated with the 

Minong shoreline north of Tobin Harbor (fig. 67), and an arch cut through a

MONUMENT ROCK, a stack associated with 

the shoreline of postglacial Lake Minong. 

Adjacent wave-cut cliff is just out of photograph to right. National Park Service photograph by Tom Haas. (Fig. 67)

narrow ridge crest on Amygdaloid Island (fig. 68), probably associated with 

the shoreline of the Nipissing stage. 

Suzy's Cave, on the north side of Rock 

Harbor about 2 miles west of Rock 

Harbor Lodge, may also be associated 

with the Nipissing shoreline.

Except for wave-cut shoreline features, changes in the topography of Isle 

Royale have been very slight since the 

ice left. Materials derived from slope 

erosion are transported only very short 

distances, owing to low stream gradients and innumerable beaver dams. 

Bogs and swamps occupy much of the 

alluvial valley bottoms.

54
WAVE-CUT ARCH on Amygdaloid Island, associated with the shoreline of postglacial 

Lake Nipissing. (Fig. 68)

SOME MINERALS

OF SPECIAL

INTEREST

. any interesting minerals occur 

on Isle Royale in addition to the basic 

rock-forming minerals that make up 

the bulk of the lava flows and other 

rocks on the island. Among these are 

the native copper that played such an 

important part in the early explorations of the island and chlorastrolite, 

the official state gem of Michigan.

These minerals as a group are known 

as secondary minerals, ones introduced 

into the rocks after the rocks themselves were formed. They chiefly fill 

holes in the volcanic rocks and the conglomerates or form veins filling fractures. Some of these minerals, especially those occurring as amygdules in 

the volcanic rocks, are quite attractive 

when polished and long have been 

sought by collectors, although collection has been restricted since Isle 

Royale became a national park. Fortunately for the collector, these minerals, 

except for chlorastrolite, are equally or 

more abundant on the Keweenaw 

Peninsula or elsewhere in the Lake 

Superior region.

Considerable evidence obtained during mining activities on the Keweenaw 

Peninsula indicate that the copper and 

most other secondary minerals were 

deposited from solutions that percolated upward through the rocks. We 

can only speculate upon the ultimate 

source of the copper and other elements in the mineralizing solutions, but 

one of the more generally accepted 

theories is that those elements were 

"sweated" out of the lower part of the 

pile of volcanic rocks after those rocks 

were warped downward in the Lake 

Superior syncline, into a region of 

higher temperature and pressure. The 

elements then migrated upward and 

were deposited as minerals in open 

spaces higher in the rock sequence. If 

the source of the mineralizing solutions 

was indeed the deeper part of the volcanic pile, then the source also would 

have been closer to the Keweenaw 

Peninsula, for the axis, or deepest part 

of the Lake Superior syncline, is closer 

to the Keweenaw Peninsula than to Isle 

Royale. This asymmetry of the syncline 

may have been one of the more important factors in producing economically

55
valuable deposits of copper on the 

peninsula but not on Isle Royale.

Among the many secondary minerals on Isle Royale, those occurring most 

often are barite, calcite, chlorite, copper, datolite, epidote, laumontite, natrolite, prehnite, chlorastrolite (pumpellyite), quartz (including agate), and 

thomsonite. Only a few of special interest on Isle Royale are described 

further in this report. Information on 

the others can be obtained from many 

sources, including the booklet "Rocks 

and Minerals of Michigan" (Poindexter 

and others, 1965).

NATIVE

COPPER WIDESPREAD 

BUT NOT ABUNDANT

Copper, like most other metals, most

commonly occurs in nature bound up 

with other elements into minerals such 

as chalcocite(Cu2S), chalcopyrite 

(CuFeSg), and cuprite (Cu2O), and 

such minerals form the bulk of copper 

ores. Of all the metals that do occur in 

the native or metallic state, copper is by 

far the most common; however, major 

concentrations of native copper are 

rare, and so it is noteworthy that the 

most important deposits known in the 

world are those of the Keweenaw 

Peninsula.

Economically valuable concentrations or deposits of native copper on 

the peninsula can be divided into two 

broad groups lode deposits and 

fissure deposits. The lode deposits 

comprise mineralized conglomerate 

beds and the vesicular tops of lava 

flows; in each case the primary porosity

COPPER NUGGET weighing 5,720 pounds found at a depth of 16 V4 feet in a pit dug by prehistoric 

Indians at the site of the Minong mine. Note the uneven surface resulting from attempts to 

remove sections for implements (Burton collection, Detroit Public Library). (Fig. 69)

56
of the rock or presence and continuity 

of Open spaces was a factor in determining where the ore would be deposited. 

The fissure deposits are along fracture 

zones that generally cut across the beds. 

Some of the fissure deposits contained 

great masses of metallic copper as 

much as hundreds of tons in weight. 

Extensive Indian mining pits on the 

fissures later led prospectors to most of 

the known deposits of this type. However, such deposits, rich as they were, 

have been much less important than 

the lower grade but vastly larger lode 

deposits, which have produced about 

98 percent of the total copper mined in 

the Native Copper district, about 

5,400,000 tons.

Native copper is widely distributed 

on Isle Royale, but mineralization was 

apparently too weak to develop large 

lode deposits. Except for the Island 

mine, which was on a conglomerate 

lode, most of the prospects and shortlived mines on the island were opened 

on small fissure deposits. In the fissure 

occurrences, such as at the Minong, 

Siskiwit, and most other mines and 

prospects, native copper occurs in 

nodules and irregular masses in highly 

altered rock in fracture zones a few inches to many feet wide. Several large 

masses more than a ton in weight were 

found at the Minong mine (fig. 69); 

these were rare and most pieces mined 

were probably similar to that in figure 

70, or smaller. Although approximately 250 tons of copper were produced from the Minong mine, the copper was too sparse and widely distributed to be mined profitably.

At the Island mine, near the west end 

of Siskiwit Bay, native copper occurs in 

the matrix of a conglomerate, the only 

known occurrence of this type on Isle 

Royale. The specimen illustrated (fig. 

71), however, is much richer in copper 

than average for the mine, and this 

mine too was a financial failure.

CENTIMETRES

NATIVE COPPER MASS from the Minong 

mine. This is perhaps typical of specimens 

from the fissure deposits on Isle Royale, but 

larger than average. (Fig. 70)

In other places in the park, most notably on the chain of islands south of 

Rock Harbor, veins of quart/, prehnite, 

and calcite contain scattered grains of 

copper. Native copper also occurs in 

amygdules in lava flows, especially in 

prehnite amygdules.

57
CONGLOMERATE WITH NATIVE COPPER 

from the Island mine. The specimen has 

been sawed and slightly polished so that the 

copper reflects light and shows up as lightcolored irregular-shaped patches. Specimen 

is 8 centimetres wide. (Fig. 71)

Rakestraw (1966) has described the 

historic mining ventures on Isle Royale 

and what the visitor can see at some of 

the abandoned mine sites.

CHLORASTROLITE 

MICHIGAN'S 

STATE GEM

Isle Royale has long been famous as 

the home of chlorastrolite, known informally in rock-collecting and lapidary circles as "Isle Royale greenstone." 

This usage of "greenstone" should not 

be confused with the use of the same 

term for a volcanic rock with a greenish 

hue, such as makes up Greenstone 

Ridge on the island. In 1972 the governor of Michigan signed a bili designating chlorastrolite as the official state 

gem.

Chlorastrolite, meaning "green star 

stone," occurs as amygdules or cavity 

fillings in certain of the lava flows on 

Isle Royale. When weathered out of the 

lava flows, it can be found on some of 

the island beaches as pea-sized pebbles, 

generally greenish in color. When 

polished, either by wave action on the

58

beaches or artificially, the "greenstones" generally exhibit a distinctive 

and attractive mosaic or segmented 

pattern, sometimes referred to as "turtleback" (fig. 72). The polished stones 

also commonly are chatoyant the 

property of having a luster resembling 

the changing luster of the eye of a cat. 

Chatoyancy is probably best known in 

the gemstone called tiger eye and is a 

property of translucent material that 

contains fibrous structures capable of 

scattering light. The grouping together 

of bundles of such fibers produces the 

mosaic pattern of the "greenstones."

CHLORASTROLITE AMYGDULES showing 

characteristic segmented pattern. (Fig. 72)

Chlorastrolite was first discovered on 

Isle Royale and named and described 

by C. T.Jackson andJ. D. Whitney in 

1847. Long afterward it was found to 

be the same material as another mineral, pumpellyite, first described from 

the Keweenaw Peninsula in 1925 and 

named for Raphael Pumpelly, a 

pioneer student of the minerals of the 

Keweenawan copper deposits. The 

material from the peninsula was described in much greater detail than that 

from the island, and the name "pum-
pellyite" became deeply entrenched in 

the world mineralogical literature long 

before it was realized that the material 

from both areas was mineralogically 

the same. Consequently, pumpellyite 

has been adopted as the only valid 

name for the mineral species, although 

chlorastrolite is still useful as a term to 

designate the variety with the peculiar 

crystal habit of the Isle Royale "greenstone." Pumpellyite is common in many 

parts of the world, but the chlorastrolite variety is apparently rare outside of 

Isle Royale.

PREHNITE THE

LITTLE PINK

PEBBLES

Prehnite is an abundant secondary 

mineral in some lava flows on Isle 

Royale and elsewhere in the Lake 

Superior region. It occurs as amygdule 

fillings, crosscutting veins, and as a replacement of earlier minerals or rock. 

Most of the prehnite has a characteristic pale-green to white color, but that in 

amygdules is commonly light to dark 

pink or variously mottled in pink and 

green. The pink prehnite superficially 

resembles thomsonite, with which it has 

commonly been confused; most of the 

so-called thomsonite from Isle Royale 

is actually pink prehnite, including 

material from Thomsonite Beach on 

the north side of the island. The prehnite does not develop the spectacular 

patterns and color variations present in 

gem-quality thomsonite, which explains why "Isle Royale thomsonite" 

has always been considered to be of 

inferior quality. Nevertheless, the 

prehnite amygdules polish nicely and 

are attractive in themselves.

The pink prehnite amygdules, which 

most commonly range in size from l/% to 

1 centimeter, are more resistant to erosion than the volcanic rock matrix 

within which they have formed. As a

result, beach pebbles containing them 

often have a knobby appearance, the 

amygdules projecting above the general surface of the matrix (fig. 73). 

Where the prehnite amygdules 

weather completely free from the matrix, they may make up a fair percentage of the fine gravel on beaches near 

the prehnite-bearing outcrops. 

The amygdules in figure 74 are typiPREHNITE AMYGDULES in beach pebble 

derived from lava flow. Long dimension of 

pebble is 5 centimetres. (Fig. 73)

PREHNITE AMYGDULES note radiating 

fibrous structure with development of 

"eyes." (Fig. 74)

59
cal and illustrate the radiating fibrous 

habit, with the occasional development 

of "eyes," that has probably been a fac-. 

tor in their confusion with thomsonite. 

The pink color of the prehnite is due to 

internal reflections from finely disseminated native copper inclusions, 

and the color intensity is related to the 

distribution, abundance, and grain size 

of the inclusions.

AGATE AN

ARRAY OF

COLORS

Perhaps the best known gemstones 

of the Lake Superior region are agates. 

Although they are most plentiful near 

where they weather from the enclosing 

rocks, glacial transport has caused 

them to be widely distributed, and 

there are few pebble beaches where 

one cannot be found.

Agate is a subvariety of chalcedony 

(fibrous quartz) with a distinct banding 

in which successive layers differ in 

color and in degree of translucency. It 

most commonly originates as a cavity 

filling in volcanic rocks either as amygdules in volcanic flows or as irregularshaped masses in volcanic tuffs; both 

types occur in the Lake Superior region, including Isle Royale.

One of the most striking exam pies of 

an agate-bearing lava flow in the park is 

the Amygdaloid Island How. This flow, 

exposed only on Amygdaloid Island, 

has rather abundant rounded or 

almond-shaped agates with a characteristic flesh-pink color and commonly 

massive quartz or vuggy centers (fig. 

75). Agates from an individual flow 

tend to be somewhat similar in appearance; for example, those from the 

Amygdaloid Island Flow are pinkish, 

whereas those from the Long Island 

Flow tend to be bluish. In general, 

agates from the rocks on Isle Royale are

CENTIMETRES

AGATE AMYGDULES in fine-grained trap 

from Amygdaloid Island Flow. (Fig. 75)

paler and have less color contrast between color bands than those found in 

some other parts of the Lake Superior 

region.

The volcanic tuff that overlies the 

Greenstone Flow also contains numerous agates with a pink or red cast (fig. 

76). Unlike the ovoid agates typically 

occurring in the lava flows, these agates 

tend to have very irregular shapes similar to the so-called thunder-egg agates 

occurring in welded tuffs of the Columbia River Plateau in the northwestern United States, and they probably 

had a similar origin. The welded tuffs 

formed from volcanic ash that fell in a 

hot plastic condition that permitted it 

to be fused or welded into a generally 

cohesive mass. The agates themselves 

are interpreted as chalcedony deposited in cavities formed by pockets of 

gases accumulating during cooling, 

shrinkage, and partial crystallization of 

the tuff.

Because of their hardness, agates are 

more resistant to erosion than the enclosing rock and tend to become concentrated in gravel and other surficial

60
AGATE typical of that in the volcanic tuff above 

the Greenstone Flow. (Fig. 76)

deposits both ancient and modern. 

Thus agates are present in the conglomerates interbedded with the lava 

flows on the island and in the Copper 

Harbor Conglomerate, as well as in the 

modern surficial deposits. Most of the 

agates on beaches near conglomerate 

outcrops have, therefore, gone 

through two cycles of weathering and 

release from their host rock first from 

the volcanic rock in which they originated and then from the conglomerate 

in which they were subsequently incorporated. In addition to beach agates 

derived from nearby bedrock, some 

have been winnowed from glacial till by 

wave action and may have been transported a long distance from their original source; they commonly are quite 

different in some way from the majority of the agates on a given beach.

WHAT THE 

FUTURE HOLDS?

we pick up an agate from the 

beach and admire its color and pattern, 

we can reflect upon the enormous 

amount of change that has taken place 

since it originated as an amygdule in a

lava How a billion or so years ago. The 

time from the volcanic eruptions to the 

coming of the glacier that may have 

brought that agate to its resting place, 

however, was itself enormous, even in 

the geologic sense nearly one-fourth 

the age of the earth itself. And although things are relatively quiet on 

Isle Royale at the present time, geologically speaking, changes will continue to 

take place slowly but certainly. Storm 

waves will continue to batter the 

shoreline cliffs, wearing them back. 

Winter frost will loosen rock fragments 

on the hillsides for the summer rains to 

wash downslope. The streams will deposit silt in the landlocked lakes until 

they are filled and cease to exist as 

bodies of water.

These continuing changes take place 

so slowly that they are nearly imperceptible to us. They are quite different 

from the cataclysmic eruption of a volcano or the steady advance and envelopment of an ice sheet. But perhaps 

this is just a quiet moment in the continuing geologic history of Isle Royale. 

While renewed volcanic activity is unlikely, many scientists believe that we 

are now in an interglacial epoch and 

that massive ice sheets will again form 

on the Canadian Shield to move inevitably southward, again modifying the 

earth's surface as they grind their way 

over it. Perhaps the future glacier 

might even bring some new agates to 

the beach for another visitor to admire.

A WORD OF

THANKS

A,Ifred C. Lane opened his 

geologic report on Isle Royale with the 

following words: "The progress of 

knowledge is like the growth of a coral 

reef; each generation builds upon that 

which has been left behind by those

61
who have gone before." So it is with this 

report. I have drawn upon so many 

sources of information in presenting 

this geologic story of Isle Royale that it 

is impossible to acknowledge the individual contributions of each. This is 

especially true of the historical material 

and of numerous works that have 

delineated the regional geologic 

framework within which I have<placed 

Isle Royale. For those who wish to delve 

further into various aspects of the history and geology of this part of the 

Lake Superior region, several pertinent references, briefly annotated, are 

listed in the bibliography; these in turn 

will lead to additional source materials.

As part of the current work, a new 

multicolor geologic map of Isle Royale 

National Park has also been prepared 

(Huber, 1973c). It has been published 

separately and is at a scale of 1:62,500 

(approximately 1 in. = l mi). At that 

scale it naturally shows much more detail than any of the maps in this report 

and illustrates more graphically many 

of the geologic features described 

herein. It is thus highly recommended 

as a supplement to this report. Several 

reports more technical than this one 

also have been prepared, one of them 

with Roger G. Wolff (Huber, 1971, 

1973a, b; Wolff and Huber, 1973).

The study leading to the preparation 

of this report and the companion 

geologic map and other reports was 

conceived and carried out as a cooperative project between the Geological 

Survey and the National Park Service 

with two goals in mind: to take advantage of the geologic research potential 

of the park and to help interested visitors understand its geologic story. 

During the four summers that I spent 

mapping the geology of Isle Royale, the 

hospitality and enthusiastic support of 

every member of the Park Service staff 

contributed immeasurably to the success of the project. In the geologic 

mapping I was ably assisted by Robert 

J. Larson in 1966, Harrison T. Southworth in 1967, Charles E. Bartberger in 

1968, and David R. Chivington in 1969. 

Henry R. Cornwall and Walter S. 

White aided considerably through 

numerous discussions that drew upon 

their broad knowledge of Keweenawan 

geology. Most of the text and many illustrations for the discussion of the 

postglacial lake stages are based largely 

on the work of William R. Farrand 

(figs. 56, 57, 58, 60) and the unpublished Ph.D. dissertation (Univ. of 

Michigan, 1932) of George M. Stanley 

(figs. 55, 61, 64).

DEFINITION OF 

TERMS

A,-n attempt has been made to 

minimize the use of technical terms in 

this volume, but some jargon is inevitable in any discussion of technical matters. Most of the strictly geologic terms 

that are apt to be stumbling blocks are 

defined where they first appear in the 

text; for ease of reference, some of 

them have been summarized in this 

short glossary. Geologic time terms 

have not been included. (See the section on "Time as a Geologic Concept.") 

Alluvial fan. A sloping fan-shaped 

mass of loose rock material deposited 

by a stream where it emerges from an 

upland into a broad valley or plain. 

Alluvium. A general term for clay, silt, 

sand, and gravel deposited by running water, such as a stream. 

Amygdule. A gas cavity or vesicle in 

volcanic rock that has been filled with 

secondary minerals such as calcite or 

quartz.

Basalt. The most common type of volcanic rock, generally fine grained,

62
dark, heavy, and with a silica (SiC»2) 

content of about 50 percent or less.

Basement. The assemblage of 

metamorphic and igneous rocks underlying the stratified rocks in any 

particular region.

Breccia. A rock consisting of consolidated rock fragments like a conglomerate, except that most of the 

fragments are angular rather than 

rounded.

Chlorastrolite. A variety of the mineral 

pumpellyite, a complex hydrous calcium silicate. (See section on 

"Chlorastrolite Michigan's State 

Gem.")

Chlorite. A green complex silicate 

mineral, similar in structure to the 

micas and usually of secondary 

origin.

Cleavage. The tendency of a mineral to 

break along definite planes, producing smooth surfaces.

Columnar jointing. Parallel prismatic 

columns, polygonal in cross section, 

formed by contraction during cooling in some lava flows.

Conglomerate. A rock, the consolidated equivalent of gravel.

Drumlin. A streamlined hill of glacial 

till elongate in the direction of flow of 

the glacier that formed it. The cragand-tail variety is streamed out behind a bedrock knob.

Erratic (glacial). A rock fragment, 

usually of a large size, that has been 

transported from a distant source, 

especially by the action of glacial ice.

Fault. A break in rock strata along 

which displacement has occurred.

Felsite. A fine-grained light-colored

. volcanic rock, generally reddish. (See 

section on "Volcanic Rocks" for fuller description.)

Flood basalt (plateau basalt). A basaltic 

lava flow or successive lava flows that 

spread over very large areas from 

fissure eruptions.

Formation. In geology, an assemblage 

of rocks that have some character in 

common that distinguishes them 

from adjacent rocks.

Greenstone. (1) Informal name for the 

mineral Chlorastrolite (pumpellyite); 

(2) a volcanic rock with a greenish 

hue, such as makes up Greenstone 

Ridge.

Igneous rock. A rock formed by 

solidification of hot molten material, 

either at depth in the earth's crust 

(plutonic) or erupted at the earth's 

surface (volcanic).

Keweenawan Supergroup. A sequence 

of geologic formations of which the 

Portage Lake Volcanics and the 

Copper Harbor Conglomerate are a 

part; Keweenawan is also used informally to denote that part of Precambrian time during which the rock 

sequence was deposited.

Metamorphic rock. Rock changed 

materially in composition or appearance by heat, pressure, or 

infiltrations at depth in the earth's 

crust.

Moraine. An accumulation of glacial 

till with an initial topographic expression of its own, usually a ridge, 

unrelated to the surface upon which 

it lies. Several varieties have been 

recognized; for example, a recessional moraine is one formed at the 

margin of a receding ice sheet during 

a pause in its recession.

Ophite. A volcanic rock with a mottled 

texture, often exhibiting a knobby 

surface when weathered. (See section on "Volcanic Rocks" for fuller

. description.)

Pegmatite. A volcanic rock in which 

elongate plagioclase laths produce a 

matted appearance. (See section on 

"Volcanic Rocks" for fuller description.)

Playa. A level or nearly level area that 

occupies the lowest part of a com63
pletely closed basin and that is covered with water at irregular intervals, 

forming a temporary lake.

Porphyrite. A volcanic rock with a texture produced by well-defined 

plagioclase crystals scattered 

through a finer grained matrix. (See 

section on "Volcanic Rocks" for fuller description.)

Prehnite. A calcium aluminum silicate 

mineral. (See ' section on 

"Prehnite The Little Pink Pebbles.")

Pyroclastic rock. Rock formed by consolidation of ash or other fragmental 

material explosively ejected from a 

volcano.

Rhyolite. A light-colored commonly 

reddish volcanic rock with a high 

silica (SiO2) content of 70 percent or 

more.

Sedimentary rock. Rock formed by 

consolidation of sediment deposited 

at the surface of the earth, chiefly in, 

or through the action of, water.

Silica (SiOg). Occurs as the natural 

mineral quartz, including various 

fine-grained varieties such as chert, 

chalcedony, agate, and others. It also 

occurs in most rock-forming minerals (silicates) and is a major factor in 

the chemical classification of volcanic 

rocks.

Stratigraphy. The arrangement of 

rock strata, especially as to geographic position and chronologic 

order of sequence.

Syncline. In simple form, a concaveupward, or troughlike, fold in 

stratified rock.

Talus. An accumulation of coarse angular rock fragments derived from and 

resting at the base of a cliff or very 

steep slope.

Till. Glacially transported material deposited directly by ice, without 

transportation or sorting by water.

Trap. A fine-grained dark-colored volcanic rock. (See section on "Volcanic 

Rocks" for fuller description.),

Tuff. Rock formed from consolidation 

of volcanic ash.

Vesicle. Small cavity formed by the expansion of bubbles of gas during 

solidification of a volcanic rock.

REFERENCES 

AND

ADDITIONAL 

READING

Agassiz, Louis, 1850, Lake Superior 

its physical character, vegetation, 

and animals, compared with those 

of other and similar regions; with a 

narrative of the tour, by J. Elliot 

Cabot: Boston, Gould, Kendall 

and Lincoln, 428 p. Reprinted 

1970 (American Environmental 

Studies), New York, Arno Press. 

For the history buffs, a marvelous narrative of Agassiz' expedition along the 

north shore of Lake Superior nearly as 

far west as the Pigeon River, with frequent geologic observations.

Bird, J. B., 1972, The natural landscapes of Canada; a study in regional earth science: Toronto, 

Wiley Pub. Canada Limited, 191 p. 

A description of the physical landscape 

and its origin, including a very readable summary of the glaciation of the 

Canadian Shield.

Courier, E. W., 1972, The chlorastrolite; now Michigan's official gem: 

Conglomerate, v. 33, no. 2, p. 3, 8, 

10, 12.

The .nature and history of chlorastrolite,from its discovery to its new honor.

Dorr, J. A., Jr., and Eschman, D. F., 

1970, Geology of Michigan: Ann 

Arbor, Univ. Michigan Press, 476 p. 

Excellent overview of the geology of 

Michigan, for the general reader.

64
Farrand, W. R., 1969, The Quaternary 

history of Lake Superior: Internal. 

Assoc. Great Lakes Research, 12th 

Conf'., Great Lakes Research, 

1969, Proc., p. 181-197. 

Summary of the glacial and postglacial 

lakes in the Lake Superior basin and 

their changing outlets.

Flint, R. F., 1971, Glacial and Quaternary geology: New York, John 

Wiley & Sons, 892 p. 

Standard text with broad coverage of 

continental glaciation in North 

America.

Foster, J. W., and Whitney, J. D., 1850, 

Report on the geology and topography of a portion of the Lake 

Superior Land District in the State 

of Michigan; Pt. 1, Copper lands: 

U.S. 31st Cong., 1st sess. House 

Executive Doc. 69, 224 p. 

Primarily of historical interest with 

sketches of early explorations in the 

Lake Superior region and the initiation 

of copper mining, including descriptions of early activities on Isle Royale.

Griffin, J. B., ed., 1961, Lake Superior 

copper and the Indians miscellaneous studies of Great Lakes 

prehistory: Michigan Univ. Mus. 

Anthropology, Anthropol. Paper 

17, 189 p., 33 pi.

Includes extensive data on prehistoric 

activities of Indian copper miners on 

Isle Royale.

Halls, H. C., 1966, A review of the 

Keweenawan geology of the Lake 

Superior region, in The earth beneath the continents a volume of 

geophysical studies in honor of 

Merle A. Tuve: Am. Geophys. 

Union Geophys. Mon. 10 (Natl. 

Acad. Sci.-Natl. Research Council 

Pub. 1467), p. 3-27. 

Summary of the correlation of individual groups of Keweenawan rocks 

around the Lake Superior basin.

Huber, N. K., 1971, Pink copperbearing prehnite from Isle Royale 

National Park, Michigan: Earth 

Science, v. 24, no. 1, p. 9-14. 

Data on pink prehnite and discussion 

of its common confusion with thomsonite.

 1973a, Glacial and postglacial 

geologic history of Isle Royale National Park, Michigan: U.S. Geol. 

Survey Prof. Paper 754-A, 15p. 

Comprehensive discussion of material 

simplified for this report.

1973b, The Portage Lake Volcanics (middle Keweenawan) on 

Isle Royale, Michigan: U.S. Geol. 

Survey Prof. Paper 754-C, 32p. 

Detailed description of the volcanic 

rock sequence on Isle Royale. 

-1973c, Geologic map of Isle

Royale National Park, Keweenaw 

County, Michigan: U.S. Geol. Survey Misc. Geol. Inv. Map 1-796, 

scale 1:62,500.

Details of geology not possible to show 

on smaller scale maps in this report.

Kelley, R. W., and Farrand, W. R., 

1967, The glacial lakes around 

Michigan: Michigan Geol. Survey 

Bull. 4, 23 p.

Excellent summary for the general 

reader.

Lane, A. C., 1898, Geological report on 

Isle Royale, Michigan: Michigan 

Geol. Survey, v. 6, pt. 1, 281 p. 

First comprehensive report on the geology of Isle Royale includes descriptions 

of exploration activities on the island 

through 1896.

Mech, L. D., 1966, The wolves of Isle 

Royale: National Park Service 

Fauna Series 7, 210 p. 

A fascinating account with emphasis 

on the predator-prey relationship oj the 

wolf and moose.

Poindexter, O. F., Martin, H. M., and 

Bergquist, S. G., 1965, Rocks and 

minerals of Michigan [5th ed.]: 

Michigan Geol. Survey Bull. 2,

65
101 p.

Introduction to the origin, loation, and 

basic uses of rocks and minerals found 

in Michigan.

Rakestraw, Lawrence, 1965, Historic 

mining on Isle Royale: Isle Royale 

Nat. Hist. Assoc., 20 p. 

Summary of post-Indian mining and 

exploration activities on Isle Royale, 

1843-92.

U.S. Geological Survey, 1957, Topographic map of Isle Royale National 

Park (culture revised 1970): scale 

1:62,500.

Available in contour or sliaded-relief 

edition.

White, W. S., 1960, The Keweenawan 

lavas of Lake Superior, an example of Hood basalts: Am. Jour. Sci. 

(Bradley volume), v. 258-A, p. 

367-374. 

The rationale behind the interpretation

of the Portage Lake Volcanics as flood 

basalts.

 1968, The native-copper deposits of northern Michigan, in 

Ridge, J. D., ed., Ore deposits of 

the" United States, 1933-1967: 

New York, Am. Inst. Mining, 

Metall., and Petroleum Engineers, 

v. 1, p. 303-326.

One of the more recent summaries of the 

nature and origin of Michigan's 

native-copper deposits.

Wolff, R. G., and Huber N. K., 1973, 

The Gopper Harbor Gonglomerate (middle Keweenawan) on Isle 

Royale, Michigan, and its regional 

implications: U.S. Geol. Survey 

Prof. Paper 754-B, 15 p. 

Detailed description of the Copper 

Harbor Conglomerate on Isle Royale 

including discussion of its depositional 

environment.

The real significance of wildgjncss is a cultural matter. It 

is far more than hunting, fisfung, hiking, camping, or 

canoeing; it has to do with the human spirit.

Sigurd F. Ql£&£.~'"rhe Spiritual Aspects of Wilderness," 1956

66 U.S. GOVERNMENT PRINTING OFFICE: 1975-0-689-909/55