Interpreting the Field Evidence of Past Ice Sheets: Structural Stability and Genericity

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Abstract
Ice sheets are always advancing or retreating, and these processes are not uniform along the ice edge. The result is that lines of maximum and minimum extent are envelopes of successive instantaneous ice edges. Generically, the envelopes can possess only two kinds of special points: crossings and cusps. These will often be arranged in characteristic patterns (unfoldings of higher mathematical singularities). The recognition of these features makes possible a novel approach to the interpretation of field evidence. It is shown, for example, how separate families of parallel erosional and depositional features seen at an interior site are related by coming together in lines of cusps at the envelopes, which may be marked by terminal moraines.