Talus is also referred to as scree, which is,
a collection of broken rock fragments at the base of crags, mountain cliffs, volcanoes or valley shoulders that has accumulated through periodic rockfall from adjacent cliff faces. Landforms associated with these materials are often called talus deposits.
Such deposits are formed as a result of,
physical and chemical weathering acting on a rock face, and erosive processes transporting the material downslope.
The predominant processes that degrade a rock slope depend largely on the regional climate (see below), but also on the thermal and topographic stresses governing the parent rock material. Example process domains include:
- Physical weathering
- Chemical weathering
- Biotic processes
- Thermal stresses
- Topographic stresses
Another key aspect of talus slopes is the channelization of the deposits. The wasting away of the rock face is not uniform. In the weaker sections of the rock face channels form which act like a funnels when rocks from above fall. Initially the fallen rocks accumulate on any flat regions in the rock face, called berms or catch benches. When the angle of repose for the material on the bench is reached additional fall material falls over the edges and through the channels of weaker rock face regions. This can be seen in the picture below of a talus fan region on north shore of Isfjord, Svalbard, Norway.
The most likely formation method of the talus slopes on Mars would be physical weathering or thermal stresses and possibly topographical stresses.
With physical weathering, water enters cracks within the rock face. When the water freezes it induces stresses in the rock which eventually cause it to spall. Thermal stresses would cause the rock to spall because of stresses induced by changes in temperature of the rock face.
As to your second question, the role of talus slopes in the formation of the valleys on Mars, they have no role in the overall formation of valleys. Instead they are the result of the rock walls of valleys.
The accumulation of scree at the base of the valley walls will act as a buttress supporting the lower reaches of the walls preventing the lower reaches of the walls from spalling. They can however, also be sources of localized landslides should the talus fans be destabilized by ground tremors or possible inflows of water.