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Geology
The Upper
Peninsula is distinctive in that it is composed of bedrock that
can be broken into two broad categories: the Precambrian and Phanerozoic
Eons, roughly splitting the peninsula in half. The western half
is a complex mix of Precambrian and Cambrian rocks while the eastern
half of the peninsula is younger, composed mainly of rocks from
the Cambrian through Silurian Period.
The exceptional
geology of the U.P. will provide
the group with a chance to explore the two very diverse geologic
times. As stated above, the western part of Upper Michigan (which
we will explore in Days 1 - 4) dates back to the Precambrian and
Cambrian times. (This also extends to some of the underlying eastern
portion of the area.) Home to some of the Earth's oldest rocks,
the Canadian Shield dates
back to the Archean and Proterozoic Eons between 2.7 and 2.5 billion
years ago! The western U.P. is found at the southern-most extent
of the Canadian
Shield. The Precambrian timeline comprises most of our geologic
history and, as such, gave rise to a conglomeration of differing
rock types over the western part of the peninsula. We will have
the opportunity to see such groups as: Igneous Rocks: intrusive
& extrusive; Sedimentary Rocks: clastic, chemical & biochemical
and Metamorphic: meta-intrusive igneous rocks, meta-volcanics,
meta-sediments. Essentially all of the state's metallic mineral
resources (iron, copper, copper sulfides, silver, etc.) occur
in rocks of Precambrian age and , in fact , were formed during
this time.
The eastern
part of Upper Michigan (which we will explore in Days 5 - 7) dates
back to the Phanerozoic Eon. Beginning about 540 million years
ago and ending about 360 million years later, this area is represented
by a wide variety of strictly sedimentary rocks that were deposited
in the Cambrian, Ordovician and Silurian Periods. This older bedrock
makes up the northern extent of what is generally considered the
younger Michigan Basin. Most of the Paleozoic rocks were deposited
as marine sediments.The concepts of transgression
and regression played a very important role in the depositional
environments of eastern Upper Michigan during the Paleozoic. A
noted exception in our area of interest is the Jacobsville
Sandstone deposited during the Cambrian as a stream and lake
deposit. The different types of rocks we may witness from the
Paleozoic of Upper Michigan include: Cambrian: shale, sandstone,
hematitic sandstone, conglomerate; Ordovician: sandstone, glauconitic
sandstone, dolomitic sandstone, shale, sandy dolomite, dolomitic
limestone, shaley limestone, cherty limestone and Silurian: limestone,
reef limestone, dolomite, shale, evaporites (rock salt, rock gypsum,
rock anhydrite, potash-salt rocks).
What is unusual
about much of the eastern U.P. bedrock is that it is overlain
by a depositional layer of glacially derived material put down
during the last great Ice
Age of the Pleistocene Epoch. The complete epoch involved
four major glacial periods, but because of the large transformations
of landscape caused by glacier movement, the only deposits that
we see are from the Wisconsinan, the final phase of this glaciation.
As the ice moved, it transformed the existing surfaces through
the processes of grinding, eroding and depositing material. It
changed the shape of existng landscapes, created new pathways
for rivers to flow while blocking others; it filled in valleys
while gouging out still others to create the major Great Lakes
basins. The area is mostly covered in glacial till comprised of
a mixture of sediment with many rocks and stones, laid down as
the ice rapidly melted. The eastern region will provide us with
many examples of this time as we move from area to area and see
the many moraines, plains and
some drumlins left behind.
Between the glaciers' retreat and the weathering of the Canadian
Shield, Upper Michigan will provide this field study with many
opportunities to witness a myriad of landforms as we transit the
outline of this great peninsula.
One final
area we will have the ability to observe during our study is related
to the size of the Great Lakes. Upper Michigan is known for its
large dunes that line the shores
of Lakes Superior and Michigan. Composed mostly of quartz particles,
the sands of the peninsula are left behind by the complex wave
actions of the lakes. As smaller sediment is washed back into
the lake, the larger particles get thrown onto the shores by storms
and wave action. Over time, these particles dry out and then become
available for wind to blow them further inland until they settle
and form dunes. Michigan is home to the largest complex of dunes
associated with a freshwater body source. The large source of
sand makes these dunes another source for commercial enterprise
and we will see how steps have been taken to ensure their continued
existance.
Hydrology
Though not
as complex as the geology of Upper Michigan, the hydrologic
cycle of the region is influenced by three main factors: 1.
Precipitation 2. The Great Lakes 3. Groundwater flow.
1. This topic
will be discussed in detail in the Upper Michigan Weather section,
so I will say here only that the peninsula gets its share of weather
and the associated precipitation that goes with it.
2. Besides
weather influences, the Great Lakes present a profound influence
on the local hydrology. As well as being a major producer and
recipient of precipitation, the lakes are one of the largest inland
areas of freshwater in the world! Water supply needs vary across
the area and in some locations the demand is larger than the supply.
Everything from industry, to agriculture, to homes utilize the
importation of water from the Great Lakes.
3. For the
third source, I will focus on the two major formations that compose
the Upper Peninsula groundwater regions: The Crystalline-Rock
Aquifer and the Jacobsville Aquifer. The
Crystalline-Rock Aquifer (Day 4 of our trip) is found in the
western part of the peninsula associated with the Precambrian
rock formations. Normally considered a barrier to ground-water
movement because of their low permeability, these rocks become
an important source of water when no other source of water exists,
especially for domestic and farm wells. Water generally moves
through secondary openings, such as joints, fractures, or faults,
in the crystalline rocks. The pressure induced by the overlying
rock's weight causes these openings to become largely insignificant
below depths of a few hundred feet and the availability of water
is limited to this same general depth. Withdrawls from the aquifer
are usually about 30 percent for public supply, about 65 percent
for domestic and commercial purposes, and about 5 percent for
agricultural purposes. The Jacobsville
Aquifer (Day 5 of our trip) is found running under large portions
of the peninsula along the Lake Superior shoreline and gradually
tapering off from north to south over the U.P. It is composed
of feldspathic to quartzitic sandstone and shale. Although the
Jacobsville aquifer is a sandstone, it is well cemented and has
low permeability. Water moves primarily through joints and fractures
in the sandstone, and these openings decrease to insignificance
at depths of 100 to 150 feet. The aquifer is generally considered
a confined aquifer. Freshwater withdrawals are nearly equally
divided between domestic and commercial uses.
Michigan is
known as a "hard water" state. "Hard" water contains a lot of
calcium carbonate. When compared to soft water, it requires more
soap to get the same amount of bubbles or lather when trying to
get clean. Most water, whether taken from ground water supplies
or from rivers, are "softened" for domestic use and usually require
treatment for industrial processing. If these steps aren't taken,
corrosion and deposits can occur, which can cause potential damage
to pipes and equipment or at least shortening their lifespan.
Rivers,
Water Falls, and Wetlands
Upper Michigan
can be divided into a series of watersheds as shown by this graphic.
There are a few major rivers in the U.P. and a multitude of smaller
rivers that decorate the landscape. The complex intertwinings
provide a large drainage network into the smaller lakes, as well
as the Great Lakes that nearly surround the peninsula. One river
in particular, the St. Marys River, is a major factor in the state's
commerce and is well known for the famous Soo Locks, which were
built to connect Lakes Superior and Huron for shipping. The locks
provide an in-depth look into how humans have manipulated their
environment to their advantage.
With bedrock
being so close to the surface and the large amount of rivers in
Upper Michigan, the area is a haven for numerous waterfalls.
In one of the peninsula's more spectacular examples, the Upper
and Lower Tahquamenon Falls (Day 6 on our journey), has the soft
sandstone which has been eroded from under the harder dolomite
layer above it, creating the environment needed for the water
to cascade over the rocks in some dazzling displays of color.
In still
another display of the diversity of this landmass, the peninsula
is also home to some picturesque wetlands.
As described in our Environmental Science course, these wetlands
are the "kidneys of the landscape". These ecotones are
home to a wide variety of plants and animals. They provide the
transition from an area of the deep water of the lakes to the
higher, drier landscapes inland on the peninsula. During Day 6
of our study, we will see that it is just this kind of diversity
that led me to choose the Upper Peninsula of Michigan as my major
field study area.
Now let's
continue our journey by reading a primer on the peninsula's......
Weather
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