our state parks... keyholes to the past and to geologic

OUR STATE PARKS...
KEYHOLES TO THE PAST AND
TO GEOLOGIC HISTORY
RETA E. BRADLEY
{Minnesota Geologic Survey
+
*
*
Staff)
"No aspect of Minnesota scenery is a finished creation existent
from
the beginning. All geological features represent one stage or another in
the process by which the architecture of the Earth is being
constructed
or destroyed. The story is still being written."—George
M.
Schwartz,
Director 'Emeritus, MINNESOTA
GEOLOGICAL
SURVEY.
M
M
*
Citizens of the State of Minnesota are fortunate in having an extensive and
expanding State Park system, established to preserve for the enjoyment of all
a part of our natural heritage. Not only are these sanctuaries a holiday delight, but like the spacecraft that hurtles toward the keyhole in the sky, our
State Parks unlock the secrets of our Earth's long and varied geologic history.
The state can be divided into four general areas, each of which is characterized by distinct physiographic features that are represented in the State Parks.
1) SUPERIOR
UPLAND:
Interior uplands from which small rivers
cascade over a series of ridges before finally plunging over blocky
ledges into Lake Superior.
2) SOUTHERN
PLAINS:
In the east, an upland plateau, dissected into
a series of broad valleys and narrow ridges by many rivers and
smaller tributaries hastening to join the Mississippi River on its
journey to the sea; and in the west, the prairie lands, gently rolling
and gradually gaining in elevation toward the great plains.
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Figure 1 . . .
This Map
Minnesota's
Reveals
Most
Prominent
Physical Characteristics
Relative
Our
Location
And
Of
Various
State Parks,
Indicated
By Black Dots
. . .
3) LAKE COUNTRY:
Hummocky and rugged hills surround the depressions that play host to Minnesota's more than 10,000 lakes.
4) GREAT
R1VERWAYS:
Broad fertile valleys, entrenched to their
present magnitude by floods that lasted not for days or weeks but for
hundreds of years.
This, then, is the face of Minnesota, reflecting not only its original countenance but the rearrangement of its physiognomy by the never ceasing physical
and chemical forces of nature.
Current scientific conjecture places the age of the Earth at about 4V2 billion
years. To simplify reference to such a great span of time, geologists have divided the total earth record into four major eras. (See Figure 2.)
Each area has its own characteristic rock types, flora and fauna. Each, in
turn, can be subdivided into smaller units having equally significant features.
A little arithmatic reveals that approximately 3Vz billion years of the total
Earth history is recorded within the 84,059 square miles of Minnesota. To
complete the broken geologic record, excursions must be made outside the
state: to the Appalachian Mountains for the "missing" Paleozoic age deposits
and to the western United States for the missing Mesozoic and Cenozoic age
deposits.
M A Y • J U N E 1969
51
Geologic Time Scale
COMPOSITION
i
mainly
flood
plains,
silt
and
sand
mainly
silt,
sand,
„„„ „„„
(travel,
cobbles
1,000,000
lculde'rs
MISSING IN
70,000,000
mainly
135, 000, 00V
MISSING
350,000,000
400,000,000
440,000,000
500,000,000
MINNESOTA
olay,silt.sand
IN
MINNESOTA
mainly
limestone
and
dolomite
MISSING IN MINNESOTA
mainly
dolomite
and
limestone,
some
sandstone
and
shale
mainly
sandstone
600, 000, 000
mainly
igneous
rocks,
sedimentary
rocks , and
metamorphosed
igneous
and sedimentary
rocks;
local
deposits
of
ironformation
4,500,000,000
Figure 2 . . . Earth's deposits over eons past are shown in this "Geologic Time
Scale" and its relationship to Minnesota. Most recent periods, or ages, are shown
in this "layer-cake" of time. Layers which have not been identified in Minnesota are indicated as "missing" . . .
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Keyhole A . . . Jay Cooke State
Park area shows ancient sea floor
folds and
fractures.
Keyhole C . . .
Tower-Soudan
Park site reveals layers of iron
ore deposited over centuries.
Precambrian age rocks are the foundation of all the continents.
They
have been exposed some place on each continent as a result of the processes of weathering and erosion that have stripped away overlying material. The southern extension of the North American Precambrian
core,
exposed throughout Canada, is found in Minnesota in the area designated as the SUPERIOR
UPLAND.
(Figure 1.) Here, studies have revealed, exists one of the most complete rock records of the Precambrian
Era.
The oldest observable Precambrian rocks in the Superior Upland are igneous rocks, which were derived f r o m molten material f r o m deep within the
Earth. The igneous rocks have two forms: extrusive, or, lavas that poured
forth on the surface f r o m a rupture in the skin, and intrusive, or molten material that crystallized before it reached the Earth's surface and was subsequently exposed by erosion of overlying rocks. The lava flows are represented
by the well known Ely Greenstone of the Ely and Tower areas, and the intrusive rocks by the Giant's Range and Vermilion granites.
The Precambrian Era was also a time of intense diastrophism, a process by
which the Earth's crust was deformed to produce the continents and the ocean
basins. Mountains analagous to the Rocky Mountains were formed in a belt
that extended across the whole of northern Minnesota. In the process of
mountain building, heat and pressure changed some of the rocks into schists
M A Y • J U N E 1969
55
or gneisses. Such rocks are well represented in scenic Jay Cooke State Park.
(Keyhole A.)
Later in the Precambrian Era, after the mountains of northern
Minnesota were worn down to a probable low-lying surface, lavas again
were extruded in the area now known as the North Shore of Lake Superior. The lava flows are responsible for the many waterfalls along the
North Shore, as seen at Gooseberry Falls State Park (illustrations) and
provide some of the most spectacular lake shore vistas.
More quiescent periods in the Precambrian Era are reflected by accumulations of sediments that later were compacted into rocks. The iron-formations
(or taconite) of the Mesabi range, and the older Vermilion range are spectacular examples of these rocks. Tower-Soudan State Park (Keyhole C) stands as
a monument to the economic significance of these iron deposits to the state of
Minnesota.
With the close of the Precambrian Era, a new era, the Paleozoic (Figure 2)
began. Paleozoic means "ancient life" and the fossil remains are clues to the
periods when animals and plants first developed. In contrast to the chaotic
and diastrophic events of the Precambrian Era, the temperament of the Paleozoic Era was generally mild. Repeated slow and gentle uplift and lowering of
the landmass relative to sea level caused the warm shallow seas to advance
from the southeast and retreat several times over the older basement of crystalline rocks. Runoff from the land masses carried erosion products into the
seas. Gradually these sediments were built into the flat-lying "layer-cake" sequence so characteristic of southeastern Minnesota land areas. If these deposits ever covered northern and western Minnesota, they were removed and no
evidence of their past existence remains.
The sands of Cambrian age were buried by sediments deposited during the maximum
advance of the Paleozoic seas in Ordovician
time.
Chemical precipitates — limestone (calcium carbonate) and
dolomite
(magnesium
carbonate),
were the dominant
type of
sedimentation.
These sandstone and limestone-dolomite
deposits form many of the
bluffs along the Mississippi River South of the Twin Cities and provide
the setting for some of the most beautiful parks in southeastern
Minnesota.
The combined action of surface streams and percolating underground waters has produced a DISSECTED
PLAIN, (Figure 1) on the flat-lying Paleozoic rocks. This dissected plain is marked by many deep valleys (Keyhole D )
and small gullies that form a dendritic (likened to a branching tree) drainage
pattern.
The Devonian seas left a meager record, and their withdrawal marked the
end of sedimentation in Minnesota during the Paleozoic Era. For about 200
million years the interior part of the North American continent generally
stood above sea level.
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Keyhole D . . .
Nerstrand
Woods Park has hidden waterfall undercutting limestone ledge.
Keyhole E . . . Glacial
Lake
Park terminal moraine is revealed by rolling, rugged terrain.
During the Mesozoic Era, (Figure 2) the seas once again invaded Minnesota in the PRAIRIE
LAND (Figure 1), region — this time from the west.
The record of this interval is sketchy. Fossil plant remains indicate that the
climate just prior to the advance of the late Cretaceous sea tended to be warm
and humid. Intense tropic-like weathering developed a thick mantle of white
clay over all the underlying rocks.
The vast sea encroached over the mantle deposits and as the water became
deeper, beds of clay, later changed to shale, were laid down on the older reworked clay residuum. Subsequently overridden by the glaciers of the Ice Age
and covered by sand and gravel, the Cretaceous clays and shales are exposed
only along the Minnesota River and its tributaries or in artificial cuts in the
prairie lands.
The Cenozoic Era (Figure 2) ushered in geologic events that resulted
in the familiar geomorphic features seen today. The climate
became
colder. Minnesota's romance with the Cenozoic became a "cold shoulder" as the Pleistocene epoch began. Snow began to accumulate in the
highlands of Canada, and as it thickened to several thousands of feet the
bottom layers gradually recrystallized to ice. Responding to increasing
weight, the borders of the ice masses began to flow southward and coalesce. The great continental glaciers of the Ice Age (Pleistocene)
were
born.
Like a great bulldozer, the glaciers moved across the land, scraping, grindM A Y • J U N E 1969
55
Keyhole
F .
At Interstate
Park,
Gorge of the
St. Croix River Cuts
Ancient
Lava Flows
Zones of
Into
.
Weakness
in River Bed
Aids Erosion
.
ing and incorporating into the flowing bottom layer a heterogeneous collection of soil and rock material of every size and composition. The stolen material litters the landscape, piled into huge mounds and hummocks interspersed
with abundant depressions that provide housing for the LAKE
COUNTRY.
(Figure 1).
Not one, but three major ice sheets advanced across Minnesota, then retreated back to the north. The last major chill began about 50,000 years ago
and turned Minnesota into a wintry battle field as four ice lobes advanced,
crunched and ground, pushed and shoved, and over-rode one another during
five phases of advance and retreat.
Where each lobe met the disastrous warm front, deposits of soil, sand,
gravel, and cobbles were dumped into rolling, hummocky, rugged hills called
terminal moraines (Keyhole E). Meltwaters rushing from the ice deposited
finer-materials in alluvial fans. Wasting ice behind the terminous left ground
moraines, less conspicuous than the terminal moraines but characterized by
irregular sags and swells that also have become the sites for lakes.
As the last onslought of continental ice succumbed to warming climatic conditions and shrank toward its source, Holocene (recent) time
began. The ice which at places reached thicknesses of about 10,000 feet,
released a deluge of meltwaters laden with silt, sand, and
boulders.
Armed with these abrasive tools, the streams began excavation.
The
angry torrents poured into valleys and running brim-full carved out the
Great
Waterways.
Glacial Lake Agassiz, empounded by the retreating ice face to the north and
the moraines to the south, spread over northwestern Minnesota and the Dakotas, and before its demise covered 200,000 square miles of the United States
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FRONTENAC
\
'
•
!
\
STATE
PARK
This lovely Park on Lake Pepin, a widening of the Mississippi
River
between Red Wing and Lake City, is the home of Old Frontenac with all
its historic lore, from the time of Father Louis Hennepin
in 1680, to
lumberjacks
and pioneer settlers. It is a favorite of
warbler-watchers
and mycologists.
Park highlands command
magnificent
view of Lake
Pepin.
and C a n a d a . T h e Red River valley and its lowlands (Figure 1) m a r k e d its
extent. Successive sandy beach ridges record its subsequent levels. Erosion of the
boulder d a m at Lake Traverse permitted the lake waters to escape by way of
Glacial River W a r r e n down the Minnesota valley.
On the eastern side of the state, Glacial Lake Superior, 600 feet above its
present level, was ponding as the Superior lobe gouged and cleared the lake
basin and melted eastward. T w o outlets, one by way of the Brule River in
Wisconsin and the other by way of the St. Louis River to the Kettle River,
channeled the torrents into the St. Croix valley (Keyhole F ) .
From 11,700 to 9,200 B.P. (before present) the glacial lakes drained
to the south, cutting the Minnesota
and St. Croix valleys deeper and
broader. When the two rivers joined forces, the Mississippi valley (Figure
1) below the junction succumbed
to their determined
energies.
Northern drainage routes opened as the ice retreated and Lake Agassiz
vanished from Minnesota soil leaving the northward-draining
Red River to
service the land it once
occupied.
Drainage f r o m Glacial Lake Superior established its outlet to the east, but
the G r e a t Lakes didn't settle into their present basins until about 2000 B.P.
E a c h river has its own story, each valley was carved by the scour-power of
floods that lasted f o r hundreds of years, and each today supports a broad fertile flood plain with a stream meandering to a final destination.
A keyhole is f o r peeking. Better still, open the door, step over the threshold,
and look not only into the past, but through the keyhole to the future.
Picture Credits
Page 6, National Park Service; Pages 1-2-8-14-18-43, art by Ken Haag; Page 7,
U. S. Fish and Wildlife Service; Pages 10-30-37-39-64, Walter Wettschreck, chief
photog., MCD; Page 12, Wayland Porter, Bureau Engineering Services, MCD; Pages
21-23-25, Herb Harper; Page 28, Joan O'Malley; Pages 32-33, David W. Lime,
North Central Forest Experiment Station; Page 41, School of Forestry, U of M;
Pages 46-48-49; Dick Manly; Pages 51-52-53-55-56, Minnesota Geologic Survey.
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