Lineated valley fill (LVF) in fretted valleys at the dichotomy boundary has been interpreted as glacial in origin. Unknown are (1) the original thickness of the glacier ice, (2) the amount of ice-surface lowering, through sublimation and retreat, to its presently observed level, and (3) whether there were multiple periods of glaciation. We address these questions through analysis of an LVF glacial system. The elevation difference between the upper limit of a previous highstand and the current surface of the LVF at the study site is ~920 m. We interpret this difference to be the minimum amount of ice-surface lowering of the glacier system. Consistent with a general lowering of the ice surface are multiple moraines and/or trimlines, and changes in LVF flow patterns, as the ice retreated and decreased in thickness. The superposition of several lobes onto the current surface of the LVF indicates that a phase of alpine glaciation followed the lowering of the valley glacial system. These data suggest that the Late Amazonian glaciation that produced LVF in this region involved significantly larger amounts of ice than previously thought, and that subsequent alpine glaciation followed.

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