ABSTRACT
A threshold in geomorphology or physical geography is an upper limit to some cumulative process, beyond which that particular sequence of events is terminated, and a totally new sequence introduced. Examples of threshold or crescendo effects may be recognized in different categories according to energy source and site. The threshold is a temporal constraint that permits a Davisian stage designation into youth-maturity-old age, which is susceptible to quantitative analysis. Two threshold boundary conditions are seen in connection with subduction. First, following the taphrogenic continental rupture there will be a long-term “trailing-edge” sedimentary cycle during which a miogeosynclinal wedge will build up on the continental margin. The second threshold is reached when the oceanic part of the plate is consumed and the one drifting continent collides with another. According to the glaciologists there are two basically distinct types of glacier, cold, dry-based ones, and wet-based ones associated either with heat-flow warmth or with climatically milder situations.
