Valley of the Au Sable EarthCache
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Before you is the valley of the Au Sable River. It stretches from where you are standing to where the trees are at the same level straight ahead of you, basically as far as you can see. All the material between here and there has been washed out by erosion over the millennia.
Fluvial erosion is caused by water flowing over land surfaces. This water may concentrate in channels as streams and rivers or flow in thin sheets down slopes. Essentially all land surfaces are subjected to modification by running water, and it is among the most important surface processes. Erosion is the combination of processes in which the materials of the earth's surface are loosened, dissolved, or worn away, and transported from one place to another by natural agents. Water is one of the most powerful agents in erosion. Acting on gravity, water tears away at the earth and transports the sediment until it no longer has the energy to do so.
Erosion by moving water happens in two ways. First, the movement of water across the bed has an effect called hydraulic action. Second, the sediment being transported in the river wears away the bed. This is called abrasion. The fragments themselves are ground down becoming smaller and more rounded. The sediment is transported as either bedload (the coarser fragments which move close to the bed) or the suspended load (finer fragments carried in the water). There is also a component carried as dissolved material.
Valleys are cut and areas become dissected as sediment is moved from land areas to the lake basins. With increasing dissection and the lowering of the landscape, the land area passes through a series of stages known as the fluvial erosion cycle.
The most distinctive fluvial landform is the stream valley. Valleys range greatly in size and shape, as do the streams that flow in them. They enlarge both through down and lateral cutting by the stream and mass wasting processes acting on the valley sides.
Many streams flow in a meandering channel, and stream velocity is greatest around the outside of meander bends. Erosion is concentrated in this area, and a steep, cut bank forms. If the river meander impinges against a valley wall, the valley will be widened actively.
A stream terrace represents a former floodplain which has been abandoned as a result of downcutting by the stream. It is a relatively flat surface with a steep slope that separates it from the current floodplain or from a lower terrace. Terraces are common features in valleys and are the result of significant changes to the stream system through time.
Fluvial erosion also has regional effects. Streams and their valleys form a drainage network which reflects the original topography and geologic conditions in the drainage basin. A branched drainage pattern is the most common.
The amount of matter carried by a large river is enormous. The names of many rivers are derived from the color that the transported matter gives the water. For example, the Huang He in China is literally translated "Yellow River", and the Mississippi River in the United Stated is also called "the Big Muddy." It has been estimated that the Mississippi River carries 406 million tons of sediment to the sea each year.
After the glaciers retreated at the end of the last ice age, Northeastern Michigan began to rise because of the removal of the enormous weight of the glaciers. As it rose the rivers cut their valleys deeper and deeper. Because most of this area is neither rock nor sandstone the Au Sable had little trouble eroding out the valley before you.
Here the valley is over 2 miles across and 220ft deep. From here you can see hillsides of this material.
To Log: E-mail us with the name of this material, the angle of the hillside it forms, and where you think all the missing material went. Although not required a picture is appreciated.
Down the stairs is both another Earthcache and a Regular cache, both are worth a visit. It is 350 steps to the bottom.
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