Chapter Four
Land Weathering, Erosion, and Deposition
Weathering
Rock exposures undergo constant change when subjected to wind, rain, temperature, and
chemical agents. These agents produce sedimentary materials (particles) that are transported
and subsequently deposited to form new rocks.
The mechanism of breakdown is accomplished by mechanical and chemical means (Fig.80).
Erosion (sedimentary transport)
Rock materials are moved during weathering. Removal of rock materials is called erosion.
Rock materials are removed during erosion and transported to where they are deposited.
Along the way, the rock materials are further broken down, rounded, and sorted.
1) Down slope Movement:
One) Mass Wasting: down slope movement of rock and soil materials with or without the
aid of running water (fig.2.11).
Down slope movement are responsible for the relocation of large amounts of weathered rock
materials.
b) Hydrologic Cycle: rainfall upon the landscape moves rock material and concentrates it in
streams, where it is transported and deposited (Fig.89).
Continuity of cycle creates an endless source of erosive and transportive power. Rock
materials on the land portions of the earth’s surface find their way to the sea for re-deposition,
forming new rocks.
2) Stream Erosion:
A rushing stream hurries down the mountain carrying abrasive particles degrading the
bedrock, and cutting down its valley (Fig. 2.15).
Water velocity has a direct relation upon how much sediment a stream can carry.
Bed Load: material moved by sliding and rolling along the channel bottom (Fig.95).
Suspended Load: materials carried by a stream in suspension above bed load.
Dissolve Load: is added to the stream by chemical weathering and dissolving of bedrock.
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3) Other transport agents:
Wind and glaciers, compared to flowing water, they move only minor amounts of sediment.
High winds can carry clay, silt, and sand. A glacier moves slowly but with great weight of
unsorted rock material (Fig. 2.17).
Deposition
After rock materials are eroded and transported, they are deposited where they will
eventually be re-transformed into rocks.
It can be transported by rivers and deposited as floodplains or carried further to the sea.
Floodplain Deposits
The gradual shift from erosion to deposition results in the formation of a floodplain (Figs.99
and 100). As the river rises over its banks and spills over the floodplain, its velocity
immediately diminishes and reduces its carrying capacity. Coarse materials settle adjacent to
the channel. The floodwaters move more and more slowly with distance from the channel,
allowing finer and finer particles to settle.
Delta Deposits
When a sediment-carrying stream enters a lake or the sea, its velocity decreases and some of
the coarse sediment settles to form a delta (Fig.111). The remaining finer sediment is
transported father from shore into the sea (Fig.112).
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