PCC - Erosion and Deposition Lesson

 

Erosion and Deposition

Wind, water, and gravity can transport and deposit sediments far from the place from which they originated. Characteristics of these deposited materials can give us clues as to how they arrived at their final location.

"Erratics" are the names given by geologists to rocks that have been transported on the underside of glaciers. Since glaciers are formed through repeated snowfalls that accumulate and compact into layers of ice, rocks that were on the surface before the glacier formed would be embedded in the bottom of the glacier. Recall that glaciers generally form at higher altitudes and "flow" to lower altitudes (usually towards a coastline); as the glacier moves downslope, the rocks embedded underneath are carried along with it. When the glacier melts, the erratics are deposited at their new locations, often hundreds of miles from their origin. Erratics can provide geologists with information about the pathways taken by ancient glaciers.

Sediments can be sorted according to size during the erosion process. In the case of wind or water (such as a river), higher velocity currents can mover larger fragments. Conversely, as wind or water currents slow down, the largest fragments are the first ones to settle out of the flow and deposit on the surface. The general trend in a river is that most of the larger rocks are found upstream, and only the finest of sediments are actually carried all the way to the ocean. In a river bend, the water flow tends to be fastest on the outside of the curve, so larger particles tend to settle on the inside of the bend, forming shoals.

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The source of erosion can leave behind telltale signs long after the process has ended. Valleys, for example, are shaped differently according to the erosion source that produced them. Rivers cut down into stone, and erosion on both sides causes the formation of talus slopes, giving the valley a distinct "V" shape. Valleys produced by glaciers, on the other hand, are essentially "scooped out", as the abrasive action of the ice and embedded erratics grind the bottom and sides of the valley away. When the glacier has receded, the valley often has a distinct "U" shape as evidence of the erosive force that created it.

 

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