The area now known as Camp Mitigwa is located
on the Des Moines lobe of Wisconsin glacial advance. This means that
only 12,000 to 14,000 years ago a sheet of ice nearly a half mile thick
sat on top of the area. This glacier extended down from Canada to where
the state capitol building stands today. The process at the glaciers
terminus formed the undulating hills of south Des Moines and the present
course of the Racoon River. A detailed chronology of the occupation
of north central Iowa by the Des Moines Lobe ice is possible because
of radiocarbon dating of peat from bogs as wells as logs and stumps
of the spruce, hemlock, and larch that were uprooted by the advancing
ice.
The melting of the ice deposited the various
materials that were encased in the ice. Debris that was transported
by wind on top of the ice and chunks of rock scraped up and broken
by the movement of the ice were mixed in the advancement of the glacier.
The glacier picked up material from different geologic deposits as
it moved across southern Canada, the Dakotas, and Minnesota. It passed
over large areas of fine grained Cretaceous bedrock, especially clay
rich shales and mudstones, limestones, and igneous bedrocks. Evidence
for the path of the glacier can be found in the kinds of rocks found
in the glacier's deposits. Igneous rocks that could not have formed
in Iowa are found mixed in the till of the Des Moines Lobe. Granite
and gabbros high in plagioclase (pinkish color) are indicative to
lava flows from northwestern Minnesota and continuing northwest into
Canada. These tough, quartz based (the clear-white color) igneous
rocks, have greater resistance to the crushing power of the mass of
ice and can be found in sizes from a few millimeters to several yards
in diameter, scattered across the landscape. Because they are rock
fragments transported by a glacier and deposited on bedrock of different
composition they are referred to as glacial erratics. This process
left behind thick deposits of compact, unsorted, pebbly loam called
glacial till.
The manner in which the glacier dropped its
cargo produced the landscape that we now see dominating north-central
Iowa. It generally produced low-relief landscapes punctuated by distinct
ridges marking the limits of major ice advances. These kinds of ridge
fronts are easily seen where the Altamont end moraine crosses Greene
and western Boone counties. The flat undulating surface of the glacial
till has a "swell and swale" shape with unconnected drainage.
The unconnected depressions are often called potholes and they collect
water from the local landscape forming wetlands. The small capture
basin of these wetlands make them dependant on snow melt and the long-term
variable rainfall. The low-relief landscape also produces a water
table that is close to the surface. The shallow aquifer often contributes
to the water of pothole wetlands. Though good for wetland habitat,
incomplete surface drainage is a serious impediment to agricultural
productivity. Many of the region's native wetlands were drained as
agriculture became more important. Today, numerous low spots contain
poorly drained, dark-colored soils indicating sites of previously
ponded water and abundant organic accumulations. Laying tile lines
beneath poorly drained areas enables water once contained by pothole
wetlands to be transported to surface drainage networks, such as streams.
There are only a few major valleys on the Des
Moines Lobe, and these are deep and narrow owing to the rapid excavation
by swift melt waters. The Des Moines River, in the most prominent
of these valleys, flows roughly down the axis of the Des Moines Lobe
landform region. The steep-sided valley, cut down through the 45 to
60 feet of glacial till, then into Pennsylvanian bedrock. Some uneroded
remnants of the valley's outwash deposits occur as terraces perched
along the valley sides. These features are often sites of commercial
sand-and-gravel production, such as the one on the opposite side of
the river from camp. Terraces also contain important geologic records
of successive episodes of post glacial erosion and deposition which
occurred in response to melting of ice from north-central Iowa. The
young, deep valleys that divide Mitigwa have changed in response to
changes in the Des Moines River (see Graded
Rivers & Des Moines River terraces).
