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The following points highlight the top four theories for the development of pediments. The theories are: 1. Lawson’s Recession Theory 2. Sheet-Flood Theory of McGee 3. Lateral Erosion Theory 4. Composite Theory.
1. Lawson’s Recession Theory:
According to A.C. Lawson (1915) pediments are developed because of gradual recession of mountain front caused by weathering and erosion. Due to backwasting of mountain front through mechanical disintegration of rocks and consequent aeolian and fluvial erosion mountain front is subjected to gradual recession. This process results in the formation of more and more smooth rock-cut plains.
As the mountain front recedes debris is deposited in the lower segment of the pediment and thus the alluvial deposits in the form of alluvial fans extend upslope on the pediments. Thus, the thickness of the veneer of deposits also increases. Pediments are exhumed due to removal of deposits under the impact of diastrophic movements. The thin cover resting on the pediment may also be removed through climate changes or from long continued aeolian deflation (fig. 21.11).
2. Sheet-Flood Theory of McGee:
According to W.J. McGee (1897) pediments are formed due to erosion of weathered bedrocks by sheet-floods originating from occasional torrential rainfall in deserts. The eroded materials are transported downslope and are deposited as bajadas. The advocates of this concept do not attach any importance to lateral erosion in the development of pediments because, according to them, concave surface of pediments cannot be formed by lateral erosion. The critics maintain that sheet-floods may have ancillary influence on pediments once they are formed but they cannot initiate the formation of pediments.
3. Lateral Erosion Theory:
G.K. Gilbert attributed the formation of pediments to lateral corrasion by streams. Later on S. Paige (1912), Eliot Black-welder (1931), D.W. Johnson (1932), Berkey, Morris etc. also advocated for lateral erosion by streams as powerful mechanism of pediment formation.
According to D.W. Johnson there are 3 zones along the mountains encircling an itermontane basin viz.:
(i) Inner zone,
(ii) Outer zone, and
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(iii) Intermediate zone (fig. 21.12).
Inner zone represents the mountain front where down-cutting (vertical erosion) by streams is most active. This zone is, thus, zone of degradation. Outer zone represents the lower segment which extends upto the margin of the intermontane basin (playa). In fact, this zone is characterized by deposition to form bajada. This zone, thus, is the zone of aggradation.
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Intermediate zone represents the zone between degradational zone (inner zone) and aggradational zone (outer zone) where lateral erosion is most active. This zone is also called as zone of pedimentation. The formation of pediment is initiated by active down-cutting of inner zone resulting into rapid rate of erosion of mountain front. The eroded materials are removed and deposited by ephemeral streams resulting from occasional strong rainstroms in the basins.
Thus, several alluvial fans are formed on the lower segments of hillslopes. These alluvial fans gradually coalesce to form bajadas. Gradually, the streams are graded and braided in the intermediate zone. Rock fans are formed in the intermediate zone due to lateral erosion by the steams. These rock fans gradually extend to become pediments.
4. Composite Theory:
It may be mentioned that the above mentioned theories of pediment formation are based on the mechanism of mono-geomorphic process. On the other hand, some geoinorphologists have opined that pediments are formed by more than one geomorphic process. For example, Kirk Bryan, W.M. Davis, R.P. Sharp, James Giluly, J.L. Rich, W.H. Bradley, L.C. King, T.J.D. Fair etc., have pleaded for composite origin of pediments.
(i) Kirk Bryan (1923) has postulated that pediments are developed in three ways viz.:
(a) due to lateral erosion by streams coming out from the mountain front,
(b) due to erosion of foothill zones by rills, and
(c) due to disintegration of mountain fronts under the impact of mechanical weathering.
(ii) According to W.M. Davis (1938) first the mountain front is disintegrated due to mechanical weathering and then the weathered materials are transported downslope by sheet-floods. These twin processes cause gradual recession of mountain front and consequent development of rockcut pediment surface. It is evident that Davis’ concept involves the views of A.C. Lawson (recession of mountain front) and McGee (sheet-flood erosion) for the development of pediments.
(iii) According to R.P. Sharp (1940) lateral erosion, weathering and rill wash play their relative roles in the development of pediments. According to him the pediments of Nevada (USA) owes their existence 40 per cent to rainwash.