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Conditions:
(a) Channelised flow starts on a rock surface.
(b) River incision slows progressively.
(c) Initially river incision, weathering and sliding produce a valley side slope at the angle of rest of the weathered material.
(d) The angle of rest is 45°.
(e) 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.
(f) As the valley side increases in length and the rate of incision slows creep is able to remove additional material from the top of the slope.
(g) Creep extends its influence through time down slope.
(h) The resulting valley side profile is composite reflecting the relative importance of sliding from the river up slope and creep from the top of the valley down slope.

Explanation of the simulation:
(a) The river is shown in blue, the rock in green and weathered fragments in yellow.
(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) A crucial difference between this and other simulations is that each click on the manual simulation represents a longer period of time. The animated simulation is therefore misleading.
(d) The incision is shown in stages - but the lower the stage the longer period of time it represents.
(e) At each stage incision produces basal steepening and triggers slides that work their way up slope. The slides are only triggered when the basal steepening exceeds a critical distance.
(f) As incision slows creep begins to affect the top of the slope. Initially it is able to remove one unit of weathered material in exess of what slides remove. This has the effect of making the upper slope gentler.
(g) The next stage has to occupy a longer time to allow another unit to be removed from the top of the slope at the gentler gradient. The longer time also allows a unit to be removed further down the slope. It must be remembered that as creep removes additional units as you go down the valley side it also has to remove the units above. In other words the work that has to be done is cumulative.
(h) To simulate this: creep removes one unit of weathered material from the top of the slope at each stage. For every three units removed from above only two can be removed immediately below. When three units have been removed from below another two units can be removed from below that. In this way the influence of creep extends slowly down slope.
(g) The left and right hand edges of the simulation represent the mid points of the interfluves. Eventually the interfluves become crested and are lowered.

Conclusion:
This simulation produces a composite valley profile. The lower slopes are at a constant angle - at the angle of rest of the weathered material. The upper slope become gently convex. The upper slope flattens until the interfluve begins to lower. However for the assumptions underlying this simulation the valley sides do not flatten over all. The lower slope maintains its gradient at 45°. Creep is a slow process and it takes a long while to extend its influence down slope. The only way for flattening of the valley sides to occur is if creep is significantly faster.