GEOLOGIC
AND GLACIAL HISTORY
Geologic History
The
physical features that we see on the Door Peninsula today are an expression of
its bedrock geology. Most of the
rest of the
state of Wisconsin is blanketed with sediments deposited by the
advance and retreat of glaciers during the Pleistocene epoch of geologic
history. The Door Peninsula, however, is characterized by exposure at
the surface of formations of resistant Silurian limestones and dolomitic
limestones, which formed at the bottom of a shallow, warm salt-water sea that
covered this part of North America during the Silurian period (Kluessendorf and
Mikulic, nd).
The Silurian period occurred between ~425 and ~405 Ma (Bates &
Jackson, 1984). The geologic time scale
is linked here.
Almost
2 million years prior to the Silurian period, Precambrian (> 570 Ma) igneous
and metamorphic rocks were flooded by Cambrian (~570 – 500 Ma)
oceans, in which thick deposits of sandstone were laid down.
Ancient life forms, such as trilobites, brachiopods, and snails thrived
in these oceans. Cambrian rocks are
not exposed in Door County, as they lie more than 1,000 feet below the surface
(Sherill,
1978).
During
the next period in geologic history, the Ordovician (~500 – 425 Ma),
deposits of limestone, sandstone, dolomite and shale were laid down as sea level
fluctuated and the region was flooded and exposed several times over this 75
million year time span. Directly
underlying the Silurian dolomite formations are rocks of Ordovician age.
The Maquoketa Shale, which is the youngest of Ordovician rock units, is
the only formation of Ordovician age, which is exposed on the Peninsula.
Thus, the Maquoketa Shale is the oldest stratum found on the Peninsula.
Exposure of the Maquoketa Shale occurs only along the south shore of Green Bay
in the southwestern part of the Peninsula. Otherwise, it lies as much as 300
feet below the surface. At the end
of the
Ordovician, extensive glaciation in Earth’s south polar region caused a
dramatic lowering of sea level, which resulted in emergence of all of Wisconsin
and most of North
America (Berry and Boucot, 1973; Sheehan, 1973).
The
figure above illustrates the stratigraphic units of the pre-Cambrian thru the Ordovician
periods, which compose the rocks of the Door Peninsula.
General lithologies and thicknesses of each unit are shown (Kluessendorf
and Mikulic, nd).
The
dolomite rocks, which compose the Door Peninsula, were formed during the next
period in geologic history, the Silurian. At the beginning of the Silurian,
glaciers underwent melting and subsequent retreat, which in turn caused a rise
in global sea level. As sea level
rose, the mid-continent of North America was flooded with seawater.
The exposed surface of Ordovician rocks, specifically, the Maquoketa
Shale, was then covered with thick, muddy deposits of sediment derived from
erosion of these deposits and underlying rocks.
These sediments were deposited at the bottom of this inland sea, which
occupied a depression in the earth’s crust, the center of which is now the
state of Michigan. Thick deposits of limestone formed at the bottom of this
inland sea as the calcareous remains of ancient organisms settled and underwent
compression. These limestone
deposits then underwent chemical alteration to dolomite. These thick, massive dolomite deposits now constitute the
rock formations that are exposed on the Green Bay side of the Door Peninsula.
Specifically, the Byron and Hendricks Formations, which together form the
Burnt Bluff Group, form these extensive cliff exposures, which
range from 100 to 200 feet high, along the western shore of the Door Peninsula
north of Sturgeon Bay. One of the
most prominent of these cliffs occurs on the western shore of Eagle Harbor in
Peninsula State Park. Abundant
fossils of brachiopods and corals occur in the Schoolcraft and Cordell Dolomite
formations, following deposition of the Hendricks Formation (Kluessendorf and
Mikulic, nd).
The
Engadine Group represents the youngest Silurian strata found on the Door
Peninsula. This unit is exposed
only sporadically on the Door Peninsula and only on the Lake Michigan shore.
This is due to the general eastward dip of strata on the peninsula (Kluessendorf
and Mikulic, nd). Kluessendorf
and Mikulic (nd) describe this unit as an extremely dense,
crystalline, even-textured dolomite, containing fewer numbers of fossils than
the underlying Cordell and Schoolcraft Formations.
Any rocks deposited on the Door Peninsula area younger than Silurian in age have
been subsequently removed by erosion. Thus, no rocks younger than the Engadine
Group are present on the Door Peninsula. This
area was uplifted above sea level sometime toward the end of the Paleozoic Era
(~230 Ma), and has remained a land surface ever since, with the exception of the
formation of lakes associated with glaciation in later time periods (Kluessendorf
and Mikulic, nd).
A
downwarping of the earth’s crust sometime following the Silurian period
resulted in a tilting eastward toward the Michigan Basin of all the Paleozoic
strata that had been formed in Wisconsin up to that time (Kluessendorf and
Mikulic, nd). The layers 
of Silurian dolomite were then forced to “bow” upward, along with the older
Cambrian and Ordovician rocks and the younger Devonian rocks, forcing the
Michigan Basin into a “bowl” shape. The
exposed edges of this “bowl” then formed a cuesta.
As the softer, underlying rocks were eroded away, large blocks of the
resistant dolomite cap rock broke off creating a vertical face on the western
edge of the cuesta.
This vertical face now forms the prominent line of dolomite bluffs
exposed on the Green Bay side of the Door Peninsula (Niagara Escarpment
Commission, 1999). These bluffs owe
their prominence to both the resistance of the
Silurian
dolomite of which they are composed,
and
the relatively
non-resistant Ordovician- and Devonian-age rocks on either side of it (below and
above it) (UWGB website). Because
the strata were tilted toward the east, the youngest rocks on the Peninsula are
exposed along its eastern edge, and progressively older strata are exposed to
the west (Kluessendorf and Mikulic, nd). The cuesta formed by the downwarping of Cambrian- through Silurian-aged rock units in the Michigan Basin is
illustrated above. Lake
Michigan currently resides between the state of Michigan and the western side of
the cuesta, and Lake Huron resides between the state of Michigan and the eastern
side of the cuesta.
The
resistant Silurian dolomite formations form the Niagara
Escarpment (see figure above) a ridge formed by the edge of this 650-mile sickle-shaped cuesta
(Niagara Escarpment Commission, 1999) which begins in Iowa, runs through
northern Illinois, into Wisconsin where it forms the Door Peninsula, over the
northern edges of Lake Michigan and Huron where it forms the Garden Peninsula
and part of Mantoulin Island and the Bruce Peninsula, and then southeast across
Ontario and into New York, where it is currently being eroded by Niagara Falls.
From there, it continues north into Canada (UWGB website).
In Wisconsin, the cuesta dips down at about a 10°
angle, generally toward the east. In
Door County, the cuesta di
ps an average of 3°
(see figure below, Paull & Paull, 1988).
This is the dip slope of the ridge, which is essentially the surface of
the rock layer. The eastern edge of
the escarpment reflects this regional eastward dip.
On the western edge of the Door Peninsula, the dips of the rocks are much
steeper, and form the prominent bluffs and the face of the cuesta, the
escarpment. (Niagara Escarpment Commission, 1999).
The
Niagara Escarpment is purely an erosional feature, marking the edge of a thick,
resistant series of rock layers. It
is not connected to any fault or other tectonic disturbance (UWGB website).
Because
of its sedimentary nature, the Silurian dolomite rocks fracture horizontally and
vertically. These fractures greatly influence the rates and direction of
groundwater flow in eastern Wisconsin, especially in Door County, where there is
only a thin cover of glacial deposits (Niagara Escarpment Commission, 1999).
The nature of the rocks that create the beauty of the Door Peninsula
paradoxically is also responsible for some problems related to groundwater flow
and contamination here (Paull & Paull, 1988).
During
the Pleistocene epoch of geologic history (the last 2 million years), the Lake
Michigan lobe of the Laurentide Ice Sheet advanced and retreated many times over
the state of Wisconsin, dramatically altering the cuesta and eroding its
surface. The advancing and
retreating glaciers carried massive loads of debris and deposited them on the
cuesta. These deposits, as much as
several hundred feet thick in places, dramatically altered the appearance of the
cuesta. As a result, it is
difficult to detect the exact location of the escarpment (the steep face of the
cuesta) at certain places on the Peninsula (Niagara Escarpment Commission,
1999).
At
the same time as the glaciers were influencing the cuesta, the cuesta was
influencing the ice movement. The hard, resistant rocks of the Niagara
Escarpment served as a divide between the Lake Michigan lobe and the Green Bay
lobe of the Laurentide Ice Sheet. The
Green Bay lobe was forced to carve out a path in the soft Maquoketa Shale to the
west of the escarpment, forming the basin that would become Green Bay. The Lake
Michigan Lobe was forced to the east side of the escarpment and carved out the
basin that would become Lake Michigan (Kluessendorf and Mikulic, nd). Specific glacial features of the Door Peninsula will be
discussed on Liberty Grove Drumlin Field page.
Glacial History
During
the Pleistocene epoch of geologic history, glacial lobes of the Laurentide Ice
Sheet advanced and retreated several times over the mid-North American
continent, as mentioned above. Specifically, the Green Bay Lobe was responsible for the glacial
features we currently see on the Peninsula.
The glacial till deposits of Liberty Grove and others of the same age are
the oldest glacial sediments in Door County.
A good example of this till is visible at the Drumlin Field stop.
It was probably deposited about 16,000 to 17,000 years ago (Schneider,
1993) during the late Woodfordian (Cary) stage of the Wisconsinan Glaciation.
The
south edge of the Laurentide Ice Sheet was extremely lobate.
These drumlins were created by the Green Bay Lobe during the Wisconsinan
glaciation stage. The Green Bay
lobe was cut off from the main body of ice by the sharp edge of the Niagara
Escarpment, and eroded the less resistant Ordovician rock to form Green Bay.
The lobe then flowed south southeastward across the escarpment, a narrow
(7-20 miles wide) high (almost 200 ft above present lake level) projection,
cutting
valleys in a northwest to southeast orientation.
Four
bedrock valleys cut across the peninsula.
These
valleys are a) between the end of the
peninsula and Washington Island, b) between
Ellison Bay and Rowleys Bay, c) between Ephraim and Baileys Harbor, and d) near
the city of Sturgeon Bay (Sherril, 1978). Glacial
striations and drumlin orientation support this.
In the northern part of Door County, the flow was S. 5-15 deg. E.,
whereas in the southern half the direction was S. 15-25 deg. E.
Evidence
indicates that the Lake Michigan lobe never covered any part of the Door
Peninsula
(Schneider, 1993). The presence of
the Green Bay Lobe may have prevented it. Kettle
Moraine State Park, in southeastern Wisconsin, is situated at the margins of the
Green Bay and Lake Michigan lobes.
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