Time and temporal gradients

When the size of the area analysed increases, the importance of intrasystem age differences increases dramatically. In local to regional-scale (<50km) glacial geology there is usually little reason to consider this time-transgressiveness, and glacial history is typically described in terms of a relative age-sequence of flow directions (event sequence). However, for continental-scale, apparently continuous, glacial lineation systems it is reasonable to assume that the glacial lineations were formed relatively close inside the retreating margin. They witness, consequently, massive reorganizations of the ice-sheet margin and the location of its dispersal centre. The logical conclusions are that a 'continuity line' that traces glacial lineations in the up-glacier direction does not necessarily reflect a true flowline at a specific time and that realistic inversion models need to include a deconvolution procedure for time-transgressive systems. The latter conclusion follows from the argument that time-transgressive map representations, for example a 1000km-long deglaciation landform system, cannot be used for validation of time-slice output from numerical ice-sheet models.

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