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The following points highlight the four main stages of arid cycle of erosion. The stages are: 1. Initial Stage 2. Youth Stage 3. Maturity Stage 4. Old Stage.
1. Initial Stage:
The initial stage of arid cycle of erosion of mountain-grit deserts is characterized by extremely dry climate (mean annual rainfall less than 25 cm), upliftment of deserts by folding or faulting, development of intermontane basins, initiation of inland and centripetal drainage etc.
Initially, the intermontane basins (of tectonic origin) have their independent drainage system and there is no drainage communication among the basins. The centres of isolated intermontane basins become base level of erosion. Evaporation is very active. Playa is formed due to accumulation of some water in the centre of the basins.
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Nearly all of the streams are consequent and drain into the enclosed basins. The cycle of erosion begins with the degradation of mountains and filling (through sedimentation) of the basins.
2. Youth Stage:
Erosion and deposition both go hand in hand and thus the initial reliefs are gradually reduced because of erosion of the mountains and filling of the basins (fig. 21.13(2)). The water divides are gradually narrowed and lowered because of backwasting and downwasting respectively. The lower segments of hillslopes are more vigorously dissected by rill and gully erosion caused by occasional rainstorms.
It is evident that there are two zones of geomorphic significance viz.:
(i) Zone of degradation on higher lands, and
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(ii) Zone of aggradation (of the basins).
Alluvial fans are formed at outer margins of the enclosed basins. Bajadas are formed due to coalescence of several alluvial fans. Deflation by wind becomes active whenever loose sands are available. Deflation causes smaller depressions known as blowouts. Playa lake is formed in the centre of the basin. Some sand dunes are formed here and there along the margins of playa. Relief is remarkably reduced by the end of youthful stage.
3. Maturity Stage:
Relief decreases rapidly because of progressive erosion of mountain divides and filling of enclosed basins. The progressive recession of water divides increases the size of enclosed basins. The level of these basins also rises because of gradual sedimentation. Bajadas are further enriched, widened, and extended upslope. Extensive rock-cut pediments are formed between the receding mountain fronts and bajada zone. Mountain divides are breached due to their progressive recession.
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This process leads to the integration of basins and drainage system. The higher basins become tributary to the lower basins as the water of the former is drained into the latter by the streams which have breached the divides between them (fig. 21.13(3)). Deflation of sands by winds becomes more active. This process causes the formation of bare surfaces-desert pavements.
4. Old Stage:
All the highlands are reduced to almost plain surface of very low height. Numerous residual hills project above the general flat desert surface. These low hills known as inselbergs or bornhardts look like islands in the vast deserts. Deflation of sands by wind is most active. Several pediments are coalesced and form extensive pediplains.