During most of the 20th century three models of hillslope evolution were widely diffused: slope decline, slope replacement and parallel slope retreat. Until the 1950s models of hillslope form evolution were "central" in geomorphology. The modern understanding is that the evolution of slopes is much more complex than the classical models of decline, replacement and retreat imply.
Slope declineEdit
Slope decline was proposed by William Morris Davis in his cycle of erosion theory. It consists of a gradual decrease in slope angle as stream incision slows down. This is accompaigned as slopes becomes more gentle they accumulate with fine-grained regolith stemming from weathering.
Slope replacementEdit
Slope replacement first proposed by Walther Penck challenging Davis' ideas on slope development. Slope replacement describes an evolution of slopes that is associated with decreasing rates of over-all erosion (denudation). It begins with a flattening of the lowermost slope that propagates upward and backward making the uppermost slope recede and decrease its angle while it remains steeper than the lower portions.
Parallel slope retreatEdit
Slopes evolve by parallel retreat when rock strength remains constant and basal debris, like talus, is continuously removed. These conditions are often met in areas where hard horizontal rock layers of basalt or hard sedimentary rock overlie softer rocks. Slopes tend to cease to evolve by parallel retreat once overlying hard layers covering softer rock have been fully eroded. As rock strength is related to weathering and weathering to climate in the case a slope retreat over large distances or over long time-spans retreat is unlikely to remain fully parallel. This is because climate changes over time and over long distances.
Parallel slope and scarp retreat, albeit proposed by early geomorphologists, was notably championed by Lester Charles King.King considered scarp retreat and the coalescence of pediments into pediplains a dominant processes across the globe. Further he claimed that slope decline was a special case of slope development seen only in very weak rocks that could not maintain a scarp. Slopes that are convex upslope and concave downslope and have no free face were held by King to be a form that became common in the late Tertiary. King argued that this was the result of more slowly acting surface wash caused by carpets of grass which in turn would have resulted in relatively more soil creep.
Unequal activityEdit
The notion that slopes in an area do not develop all at the same time is known as unequal activity. Colin Hayter Crick, who coined the term, proposed that unequal activity may be regulated by removal of debris at the base of slopes. Following this thought erosion by the sea and lateral stream migration are of prime importance as these processes are effective in removing debris.] Unequal activity does also imply there are great disparities between stream erosion near stream channels and apparently unchanged uplands, and between headwaters with limited erosion and the more active middle and lower courses of streams. From this it is derived that landscapes and slopes with limited river erosion may in many cases be considered as stagnant in their evolution
