Please start at Evolution.html

Conditions:
(a) Channelised flow starts on a rock surface.
(b) The rock is resistant and strong enough to support a vertical slope.
(c) River incision is rapid to begin with but slows.
(d) The valley side is subject to physical weathering and the free fall of the weathered debris into the river.
(e) The angle of rest of this weathered debris is 45°.
(f) As the river continues to incise it triggers slides in the weathered debris. The slides will only be triggered when the basal steepening is of a certain order of magnitude.
(g) When incision is very slow there is time for the weathered debris to be comminuted by further weathering - physical and/or chemical. This finer material will have a lower angle of rest.(22.5°)
(h) A lower, gentler, constant slope evolves at the angle of rest of the finer material.

Explanation of the simulation:
(a) The river is shown in blue, the rock in green, large weathered fragments in yellow and fine weathered fragments in blue
(b) The rock is divided up into rectangles to represent the fragments it weathers to. The angle of rest of the weathered material is given by the diagonal of the rectangles. The dimensions of the rectangles can be varied to give the angle of rest required.
(c) Each click on the manual simulation represents a constant unit of time.
(d) The incision is shown in stages.
(e) Initially incision is rapid. Weathering and free fall of the weathered debris form a steep slope.
(f) As incision slows a gentler lower slope is formed at the angle of rest of the larger fragments
(g) River incision produces basal steepening of the lower slope. When this steepening exceeds a certain critical amount shallow debris slides are triggered. These work their way up slope lowering the slope but maintaining its angle.
(h) At a slower rate of incision there is time for the larger weathered fragments to weather down into finer material. This leads to the formation of a lower constant slope
(i) This slope too will be lowered by slides triggered by river incision and basal steepening. These slides are very shallow and do not involve a lot of material.
(j) It is important to realise that once a gentler basal slope has evolved the steeper slope above is lowered by the larger fragments weathering to finer debris. These are very unstable on the steep, 45° slope and slide quickly towards the river.
(k) When incision has slowed again weathering and free fall of debris at the top of the slope may exceed the rate of lowering of the slopes in the weathered material. In this way the upper steep section may disappear out of the valley profile.

It should be clear that where the processes assumed for the simulation occur, namely, river incision, weathering, free fall of weathered fragments and shallow slides, the number of slope angles in the landscape will be very few. One or two angles should be dominant. These will be the angles of rest of weathered debris.