Open questions and future research needs
It should be clear that there is considerable scope for further work in understanding the role of landslides in mountain range evolution. We identify a number of open questions, which may serve as stepping stones for future research directions:
4.1. How widespread are threshold hillslopes and how can they be objectively identified from topographic data?
We identify several important points in this regard. First, we see a need for a standard methodology for objectively detecting and quantifying threshold hillslopes in a given mountain belt. So far, data have been compiled only for selected regions, such as the northwestern Himalaya (Burbank, 2002) or the northwestern USA (Montgomery, 2001). If mean slope or slope-angle histograms are indeed used as measures for threshold hillslopes, they will mask effects of variations in hillslope strength, materials, and processes (Fig. 12), and the role of failure-prone or failure-resistant slope segments such as steep inner gorges or slot canyons. In many mountain belts, the preservation of fluvial strath terraces with ages of up to 104 yr as indicators of rapid fluvial bedrock incision (Burbank et al., 1996 and Schaller et al., 2005) is somehow at odds with the requirement of rapid slope adjustment by bedrock landsliding, which would likely eliminate such evidence (Fig. 10A).