Download River Bluffs

River Bluffs - 1
RIVER BLUFFS
Although the river gorge running through the Minneapolis campus is geologically recent,
the rocks exposed in the gorge are much older. They date back to a time long before
dinosaurs romped across Minnesota’s landscape; a time when the seas were high enough
to cover much of Minnesota; and a time when Minnesota straddled the equator.
2nd
The outcrops along the river consist of three distinct rock units.
Atop the bluffs, mixed sand, clay, cobbles and soil represent a fourth geologic division.
These are glacial deposits, a legacy of the state’s recent glaciation.
LABELS
A white sandy rock, the St. Peter Sandstone, is the oldest of the three units. It is exposed
in scattered outcrops from the river’s edge almost up to the vertical bluffs. Between the
massive sandstone and the layered rocks of the bluffs lies a thin shale unit called the
Glenwood Formation. The bluffs are built of distinctly layered carbonate rocks known as
the Platteville Formation (labeled limestone here).
PLATTEVILLE LIMESTONE change link to Platteville Formation
The Platteville Formation is the youngest (most recent) bedrock unit exposed in the river
gorge. Although it is often informally called the Platteville Limestone, most of the
Platteville layers along the river bluffs are dolostone.
A significant time gap is represented by the surface between the top of the Platteville
rock and the glacial deposits above. The dolostone below is roughly 455 million years
old, while the glacial deposits above formed within the past 18,000 years. Almost half a
billion years are missing, their only record being the surface between rock and sediment.
Surfaces, like this one, that represent significant amounts of geologic time are called
unconformities.
2ND
Stark contrasts exist between the Platteville and underlying rock units. While the
Platteville is composed of thin beds or layers, the St. Peter and Glenwood are more
massive rocks with less obvious internal layering.
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Differences in physical strength occur as well. The dolostone layers of the Platteville are
hard cohesive rocks, but the underlying units are physically weaker stone. Erosion can
easily remove the St. Peter and Glenwood to undercut the Platteville blocks.
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3RD
As the Platteville bluffs are undercut, another element of the rock comes into play.
Vertical breaks, called joints, cut through the rock section. Instead of eroding a grain at a
time, large blocks of the Platteville break off as erosion of underlying rock layers reach a
joint plane. This is how the bluffs’ vertical faces formed and continue to retreat.
It is also one of the reasons why a visitor to the bluffs has to be careful!
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Close up, it is easy to see how a combination of the rocks’ joints and bedding planes can
cause blocks to fall as erosion removes the underlying rock units.
LABELS
The highlighted surfaces outline a block that is nearing collapse.
From this perspective, a vertical joint appears as a zigzag line that intersects one of the
Platteville’s many horizontal bedding planes. Caught between the two breaks and the
underlying erosion surface a block of Platteville is almost at the point of collapsing.
When it does break loose, the blocks above are almost certain to follow.
5th
These Platteville blocks have already fallen, but nicely demonstrate the different types of
surfaces present in the bluffs. The flat upper surface of the fallen blocks is a bedding
plane, a break in the rocks’ original deposition. A number of parallel, vertical joints lie
perpendicular to this bedding plane. Other examples of both surfaces can be seen in the
outcrop behind the blocks.
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Formed of rock that originated as deposits on an ancient seafloor, the Platteville is
composed almost completely of dolomite and calcite. It formed in a shallow warm water
environment, well removed from any source of terrestrial sediment.
Consequently, the Platteville most likely formed in a tectonically quiet, offshore
continental shelf setting. Scattered fossils and burrows attest to its marine origin in a
shallow tropical sea.
GLENWOOD SHALE
A thin shale unit lies between the Platteville and St. Peter. Known as the Glenwood
Formation, this shale may be difficult to see as it is often covered by vegetation.
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Even where exposed the Glenwood may be overlooked as it erodes easily and retreats
back beneath the overlying Platteville layers.
2ND
Bluish-green in color and only a few centimeters thick, the Glenwood does not really
seem worthy of being designated as a formation.
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However, the Glenwood does represent a significantly different change in sedimentation.
The St. Peter Sandstone is composed of essentially pure quartz and the Platteville of
dolomite and calcite.
3RD – CHANGE ZOOM IN LABEL TO WHITE AND REMOVE LINK
Composed of clay minerals, the Glenwood represents a transition between the beach and
near shore sand deposits of the St. Peter and the offshore carbonate deposits of the
Platteville.
Although these units formed far from any active plate boundary, their existence still has a
plate tectonic origin. Seafloor ridges are hotter and higher during times of increased
seafloor spreading. As the volume of the seafloor ridge increases, sea level rises to flood
much of the land surface. These rock units formed at a time when the world’s plates were
moving much faster than they do now, and the seas rose to cover the campus area.
4TH – DELETE
5TH – DELETE
SAINT PETER SANDSTONE
From beneath the Mississippi to just below the gorge’s vertical bluffs, white, nearly pure
quartz sandstone forms the valley’s bedrock. Known as the St. Peter Sandstone, this is the
oldest rock unit exposed in this part of the Mississippi River valley.
2ND
The St. Peter Sandstone does not form vertical cliffs like the Platteville, nor does it erode
as easily as the Glenwood. Instead the sandstone forms gentle slopes.
LABELS
Weakly cemented, the sandstone is easily eroded. Even running a hand over outcrop can
dislodge sand grains.
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3RD
Because the St. Peter is easily eroded, vegetation covers much of the rock unit. Scattered
outcrops occur along the path beneath the bluffs. Here mud from the trail has discolored
the sandstone, giving it a brown hue. The sandstones natural color is a startling white.
Although the Platteville contains abundant burrows, the marks seen here are carved
graffiti, not natural features of the rock.
4TH
Weathered outcrops of the sandstone often have a grayish cast. Graffiti again mars the
rock, but the rounded surface is a product of the rock’s natural weathering.
Although most of the St. Peter is massive, a few bedding surfaces can be seen in the
lower half of the outcrop.
5TH
A fresh rock face reveals the St. Peter Sandstone’s characteristic white color. Composed
almost entirely of pure quartz sand, the sandstone is one of the world’s best known
sandstone formations. Its pure quartz composition makes it an excellent source for glass
manufacturing, water or wine purification systems and even children’s sandboxes.
6TH
Pure quartz sands typically are the product of extensive weathering, either from multiple
cycles of weathering or weathering in a tropical setting. The latter is considered the most
likely origin for the St. Peter Sandstone. At the time the sand was being deposited,
paleomagnetic evidence indicates the Twin Cities area was close to the equator.
A complete lack of other minerals also suggests that the sandstone formed in a nontectonic setting, probably as beach and near shore deposits on a continental shelf.