The
word Minneopa means, "water falling twice" in the Dakota Indian
language. This double waterfall is the result of Minneopa Creek
cutting into and eroding layers of Jordan sandstone (a sedimentary
rock composed mainly of sand-size mineral or rock grains) at
different rates; the top and bottom layers of softer sandstone are
eroded by water faster than the middle and hardest layer of
sandstone. The 50 foot deep gorge below the falls was formed by
Minneopa Creek downcutting and removing soft sandstone. A shelf is
formed when the middle layer is exposed from on top and undercut
from beneath. The repetition of this erosion cycle causes the slow
progressive upstream movement of the falls.
The
forces that created Minneopa Falls began during the last ice age
15,000 years ago. As the final glacier retreated it left behind
Lake Agassiz, which covered most of the northern part of Minnesota.
Lake Agassiz was drained by the Glacial River Warren whose waters
carved what is now the Minnesota River Valley. As Lake Agassiz
receded to create the lakes of Northern Minnesota, Glacial River
Warren was consequently reduced in size and the water now flows as
the Minnesota River. It was at this time that Minneopa Falls first
appeared near the confluence of Minneopa Creek and the Minnesota
River and gradually its position was moved farther up the creek
through the processes described above.
Downcutting: is a geological
process that deepens the channel of a stream or valley by removing
material from the stream's bed or the valley's floor. How fast
downcutting occurs depends on the stream's base level, which is the
lowest point to which the stream can erode. Sea level is the
ultimate base level, but many streams have a higher "temporary"
base level because they empty into another body of water that is
above sea level or encounter bedrock that resists erosion. A
concurrent process called lateral erosion refers to the widening of
a stream channel or valley. When a stream is high above its base
level, downcutting will take place faster than lateral erosion; but
as the level of the stream approaches its base level, the rate of
lateral erosion increases. This is why streams in mountainous areas
tend to be narrow and swift, forming V-shaped valleys, while
streams in lowland areas tend to be wide and slow-moving, with
valleys that are correspondingly wide and
flat-bottomed.
TO LOG THIS
CACHE:
1) Since the end of the last Ice
age how far have the falls been estimated to travel upstream from
its start?
2) What evidence is there
that downcutting is taking place and the falls are in constant
motion upstream?
To log this cache e-mail me
the
answer
HERE
.