Clearly, it also states "calving", which is quite the opposite of melting. All in all, it is a ****ed-up sentence.
True, sentence is poorly worded but calving is absolutely not the opposite of melting. Calving occurs at the edge/margins of the glacier, large masses of ice breaking free of the ice sheet, a process that
can be exasperated by melting of the ice sheet. "Can" being the operative word here but don't get hung up on that. The original question focused on the movement of the glaciers (northward vs southward). As I and others have posted, it depends on at what period/point in time is being discussed. Since the sentence stated that the glaciers were melting that indicated during a warming period when the ice sheets were receding (i.e., shrinking towards north). Even in periods of recession, the glaciers still affected the underlying topography.