The process known as weathering breaks up rocks so that they can
be carried away by the process known as erosion. Water, wind, ice,
and waves are the agents of erosion that wear away at the surface
of the earth.
Water Erosion
Water is the most important erosional agent and erodes most
commonly as running water in streams. However, water in all its
forms is erosional. Raindrops (especially in dry environments)
create splash erosion that moves tiny particles of soil. Water
collecting on the surface of the soil collects as it moves towards
tiny rivulets and streams and creates sheet erosion. In streams,
water is a very powerful erosional agent. The faster water moves in
streams the larger objects it can pick up and transport. This is
known as critical erosion velocity. Fine sand can be moved by
streams flowing as slowly as three-quarters of a mile per hour.
Streams erode their banks in three different ways: 1) the hydraulic
action of the water itself moves the sediments, 2) water acts to
corrode sediments by removing ions and dissolving them, and 3)
particles in the water strike bedrock and erode it. The water of
streams can erode in three different places: 1) lateral erosion
erodes the sediment on the sides of the stream channel, 2) down
cutting erodes the stream bed deeper, and 3) headward erosion
erodes the channel upslope.
Wind Erosion
Erosion by wind is known as aeolian (or eolian) erosion (named
after Aeolus, the Greek god of winds) and occurs almost always in
deserts. Aeolian erosion of sand in the desert is partially
responsible for the formation of sand dunes. The power of the wind
erodes rock and sand.
Ice Erosion
The erosive power of moving ice is actually a bit greater than
the power of water but since water is much more common, it is
responsible for a greater amount of erosion on the earth's surface.
Glaciers can perform to erosive functions - they pluck and abrade.
Plucking takes place by water entering cracks under the glacier,
freezing, and breaking off pieces of rock that are then transported
by the glacier. Abrasion cuts into the rock under the glacier,
scooping rock up like a bulldozer and smoothing and polishing the
rock surface.
Wave Erosion
Waves in oceans and other large bodies of water produce coastal
erosion. The power of oceanic waves is awesome, large storm waves
can produce 2000 pounds of pressure per square foot. The pure
energy of waves along with the chemical content of the water is
what erodes the rock of the coastline. Erosion of sand is much
easier for the waves and sometimes, there's an annual cycle where
sand is removed from a beach during one season, only to be returned
by waves in another.
The Silver Cliff highway tunnel, located on the environmentally
and aesthetically sensitive North Shore of Lake Superior. The
tunnel is part of an 18-year, $104-million program to upgrade U.S.
Route 61 in northeastern Minnesota. The existing highway was
threatened by steep, unstable cliffs and contained tight curves not
meeting current design standards. The Minnesota Department of
Transportation project consisted of open cut rock slopes for the
approaches, portal structures, and mined rock tunnel. The tunnel
was constructed in a closely-jointed diabase rock mass, with
numerous thick infilled seams intersecting the tunnel. The
arch-shaped tunnel was excavated 53' wide by 36' high and finished
46' wide by 31' high. Tunnel excavation was by controlled blasting,
with multiple top headings and benches. Rock support and lining
consisted of external portal beams, rockbolts and reinforced
shotcrete. A final concrete lining provided structural support for
the rock mass and the membrane waterproofing system.
Questions to Answer:
1) What day was the Silver Creek Cliff Tunnel opened?
2) Mark waypoints on your GPS at one end of the tunnel. Walk to the
other. What is the distance from your start point? (for safety
reasons please walk on the Gitchi-Game State Trail)
You can if you choose to get a more accurate reading for
question two by climbing up above the tunnel and placing waypoints
that way. While you are up there be sure to grab
GCPPEJ
3) All four types of erosion can be seen here. Describe some of the
ways you see erosion at work.
4) Post a picture of your caching party at your favorite spot along
the cliff. (optional)
Please do not post answers in your log, even
encrypted. This will result in your log being deleted. Please email
the answers to the cache owner.
Be sure to grab the nearby traditional
cache that is located at this overlook.