east of Bemis-Altamont moraine system of the Des Moines Lobe- lake
free, well-integrated drainage, landscape dominated by effects of stream
erosion which increase as one approaches the Mississippi River
deep, flat-floored valleys and intervening ridges
meandering channel patterns on broad floodplaines
terraces along valley sides are remnants of older floodplains dissected
by rivers as a result of downcutting due to increase in discharge associated
with melting of glaciers
numerous bedrock exposures along valley sides attest to thin glacial
cover and depth of erosion by rivers
west of moraines - landscape dominated by glacial processes, many lakes,
few river valleys, youthful surface, glacial retreat a mere 13,000 years
ago
Glacial Geology
Area east of Bemis-Altamont moraine was ice-free during Wisconsinan
Covered during earlier glaciations as indicated by scattered erratics
and patches of till
effects largely removed by erosion
"driftless area"
vertical sections of sediment show multiple layers of till with intervening
paleosols indicating cycles of advance and retreat of the ice
problem working out history lies in a record that has been covered
by Wisconsinan wind-blown silt (loess) and deeply weathered so that the
original materials are converted to clay
Area west of Bemis-Altamont moraine affected by two different Wisconsinan
advances
Superior Lobe - earlier advance from northeast and building of St.
Croix moraine. In the Twin Cities area, this moraine is northeast-southwest
trending ridge with lots of hills and lakes in intervening depressions.
Des Moines Lobe - later advance from west and northwest and building
of Bemis and Altamont moraines from till rich in shale and limestone fragments.
The Bemis and Altamont moraines are a north-south trending ridge with lots
of hills with many lakes in intervening depressions
Two lobes superimposed one atop another in the Twin Cities area, so
that the topography becomes a complex pattern of hills and depressions
in the two superimposed moraines. Also in the Twin Cities, vertical sections
of Wisconsinan deposits reveal two tills with intervening paleosol
Within and beyond ice margins, meltwater streams deposited sand and
gravel
as valley trains along major river valleys, and
as outwash plains, where many outwash streams coalesced across broad
floodplains
Mississippi River valley filled with outwash
this outwash has since been largely removed by later erosion by the
Mississippi, so that scattered terrace remnants of the old outwash deposits
are all that remain
this outwash dammed tributary streams to form temporary lakes from
which sediment settled out to form lake plains upon draining of the lakes
lake plains later incised by streams in the tributary valleys to leave
terraces standing above the modern level of the floodplains
Paleozoic shallow marine sandstones, shales and carbonates
Cambrian, Ordovician and Devonian age
many of the units are fossiliferous, particularly the limestones and
shales, and contain brachiopods, trilobites and crinoids. Worm burrows
are also common.
Cambrian rocks are dominated by sandstones, siltstones and shales and
are not as fossiliferous as the Ordovician rocks. Some of the sandstones
contain abundant glauconite and are called "greensands".
Ordovician rocks are dominated by dolomites and limestones and limy
siltstones and shales. Only one sandstone, the St. Peter sandstone, is
important in the Ordovician. The vertical sucession of St. Peter Sandstone-
Glenwood Shale - Platteville Limestone is a classic sequence that indiates
transgression of the sea over the craton.
Devonian rocks are less important and include the carbonates of the
Cedar Valley Formation
Pattern of transgression of the sea and deposition of Paleozoic sediments
were influenced by structural patterns in the basin.
The Hollandale Embayment of the Forest City Basin controlled most of
the deposition, but a smaller basin, the Twin Cities Basin, sank more rapidly
and accumulated a thicker interval of sediment.
Uplifted areas in the Hollandale Embayment such as the Red Wing-Rochester Anticline and
the Hudson-Afton Anticline near to the Twin Cities also influenced patterns
of deposition
Mesozoic non-marine conglomerate and sandstone interfinger west in
the Mankato area with marine black shale. Mesozoic sediments in southeastern
Minnesota exist as discontinuous patches lying on an erosion surface in
the underlying Paleozoic rocks.
Places of Interest
Twin Cities
Underlain by Paleozoic sediments deposited in a structural basin that traps
a huge reservoirs of groundwater
Note from the cross section that as one travels from the center of
the basin toward the basin edges, the rocks exposed at the surface become
older.
Most exposures
reveal the light-colored, poorly cememted St. Peter sandstone at the base,
a thin interval of soft gray-green shale called the Glenwood, and a caprock
of resistant fossiliferous limestone called the Platteville
These three bedrock formations exposed in valley walls are not continuous
beneath the glacial deposits
This old topography can be mapped by making maps of the thickness of
the drift.
In areas where the drift is thicker, there must be valleys in the old
pre-glacial bedrock surface
In areas where drift is thinner, there must be high places in the old
pre-glacial bedrock surface
Location of buried valleys sometimes marked by lake chains, where thicker
glacial ice in the valleys later collapsed along with the sediment to form
depressions at the surface
Lake chain from Golden Valley to Minneapolis including Sweeny Lake
at the west and Lakes Calhoun and Harriet at the east
Chain crossing St. Paul and northern suburbs including Snail Lake to
Lake Phalen
Glacial sediments lie atop the Paleozoic rocks.
lower glacial sediments date from the 20,000 year advance of the Superior
Lobe and contain fragments of igneous and metamorphic rocks and iron formations
upper glacial sediments date from the 14,000 year advance of the Des
Moines Lobe and contain fragments of shale and limestone
topography developed on the glacial deposits is irregular and hilly
morainal topography with lakes in depressions between the hills
Hills in St. Paul developed on St. Croix Moraine.
This belt of hilly lake country stretches east to the St. Croix River
and south to Pine Bend, the big bend in the Mississippi north of Hastings.
Dead ice moraine topography is due to stagnation of Superior Lobe.
Made of red sandy till with igneous and metamorphic rock fragments.
Aprons of outwash grade into Mississippi River south of the moraine.
Mpls-St. Paul international airport built on such a gently sloping outwash
apron.
Post-glacial erosion by the Mississippi and Minnesota Rivers
left the finishing touches on the landscape in the form of steep-sided
valleys and waterfalls in gorges
Minnesota River course determined by Glacial River Warren which drained
Glacial Lake Agassiz
Flood of River Warren entered Mississippi at Fort Snelling, cutting
deeply into the glacial sediments and the bedrock. Segment of the Mississippi
above the junction with the River Warren was left hanging, and a waterfall
developed. As this waterfall retreated, it produced the deep gorge of the
Mississippi River in the Twin Cities. The falls have now retreated to a
position marked by St. Anthony
Falls in downtown Minneapolis.
Caves and ravines throughout the Twin Cities created by removal of soft St. Peter sandstone
by groundwater seeping through the formation in the form of springs at
the surface
sand eroded at surface along sides of valleys
opening expands beneath caprock of resistant Platteville Limestone
cave is created
when roof of cave collapses, a ravine in the landscape is created
Hastings and Red Wing
Bluffs near Hastings
Jordan sandstone at river level and Oneota dolomite above
faults in the section have vertical displacements as great as 80 feet
Terraces
evidence of several episodes of filling of the river valley by outwash,
and excavation of outwash during interglacial episodes
Langdon Terrace on north side of river is 3 miles wide and is a remnant
of the outwash surface from the Des Moines Lobe
Waterfalls
The Vermillion River enters the Mississippi Valley in south Hastings
through a 1 mile long gorge with a waterfall at its head just east of US
61.
Gorge cut because River Warren flood excavated the Mississippi Valley
alluvium, lowered the level of the Mississippi, meaning that tributary
streams now had to drop to a lower elevation to reach their new mouth.
A waterfall developed, and as the waterfall retreated upstream, a gorge
was cut along the tributary river.
Sand plains
south of Hastings are great outwash sand plains grading eastward to
the Mississippi Valley terraces from sources in moraines of the Superior
and Des Moines Lobes to the west
remnants of bedrock stand as castle-like features in the landscape,
indicating that the glacial ice did not cover parts of this area during
Wisconsinan time
Barn Bluff in Red Wing
outlier of bedrock cut off from the main upland by an ancestral Mississippi
River which flowed through the area now occupied by US 61
Franconia greensand up through Oneota Dolomite
Red Wing Fault exposed
at the west end of Barn Bluff where yellow-tan Jordan sandstone lies side-by-side
with the Franconian Greensand.
130 feet of vertical movement indicated
crushed rock along fault zone which is about 3 feet wide
Fault is part of the upfolded Red Wing-Rochester anticline which was
formed sometime before the Cretaceous (Cretaceous sediments are not offset,
so principle of cross-cutting relationships can be used to indicate a Pre-Cretacous
date for the anticline and associated fault).
Clay deposits
Cretaceous clays south of Red Wing
instrumental in the growth of the pottery industry in Red Wing
Cave Country
Exposures of limestone and dolomite (carbonate rocks) in southeastern
Minnesota have resulted in formation of caves, because carbonate bedrock
is soluble
More than 150 caves have been reported, most in Fillmore and Olmstead
Counties in Ordovician Dubuque and Galena Formations.
Mystery Cave is now a state park, 6 miles southeast of Spring Valley
in Fillmore COunty. Niagara Cave near the MN-IA border 5 miles soutwest
of Harmony is a commercial cave which can also be visited.
ground water solution dissolves the soluble bedrock because groundwater
is a weak acid (carbonic acid) as the result of its picking up of carbon
dioxide from decaying plant material in the soils
solution is most effective at the water table, where mixing of soil
water with ground water makes it especially chemically active
pathways of water movement in the rock aid in the solution process.
Such pathways include fractures in the rocks or bedding planes in the rock
or interconnected pore spaces between mineral particles in the rocks. Cave
geometry is often governed by the network of interconnecting fractures
and bedding planes.
outlet for the groundwater is needed, so deeply entrenched valleys
in the bedrock provide low places in the water table where the caves can
drain, thereby providing for continuous flow of new acidic groundwater
through the cave system
when water table drops below the level of the cave, the cave drains and formation of cave deposits
begins. Stalactites and stalagmites
and flow stones decorate the
cave interior as chemical sediment in the form of calcium carbonate is
precipitated from carbonated and mineralized ground water seeping into
the roof of the cave.
Downcutting by the Mississippi River was instrumental in the beginning
of cave formation, because this downcutting lowered the water table on
a regional basis. Radiometric dating of cave deposits suggests that they
are older than 30,000 years, meaning that they were formed before Glacial
River Warren resulted in Mississippi River downcutting. Perhaps there was
an earlier pre-Wisconsinan version of Glacial River Warren???
Sinkholes are formed
at the surface by solution often enhanced by collapse of small underground
caves. A topography dominated by sinkholes and caves and other features
such as sinking streams, blind river valleys and springs is called Karst
Topography
Rochester and Vicinity
This region and almost all the region of southeastern Minnesota east
of Interstate 35 is beyond the limits of the last glaciation
few lakes
deeply dissected upland, especially as one approaches the Mississippi
River
cover of loess (wind-blown silt derived from glacial deposits
to the west) atop the older drift and bedrock makes up parent material
for soil
soils are generally thin because of removal by mass wasting
and stream erosion
In Rochester, St. Peter and younger formations, arched up along the
axis of the Red Wing-Rochester Anticline, are exposed along stream valleys
Hillsides in the city underlain by white sand and blocks of white sandstone
from the St. Peter
Along US 14 west of Rochester, the Ordovician transgressive sequence
of the white St. Peter sandstone, greenish Glenwood shale
and overlying gray-tan dolomitic limestone of the Platteville is
exposed
Along US 52 south of Rochester at Golden Hill the Decorah shale
lies atop the Glenwood-Platteville. The Decorah is overlain in turn by
the Galena Dolomite. Lots of fossils can be collected in
these roadcuts
Collapse of sinkholes because of underground solution creates
land use problems in Rochester
Presence of karst topography creates problems for groundwater
quality (see lecture set 14)
