Hey Silbrulf,
do I understand you correctly if I say you want to show both the reach of the glacier in it's minimized state (in summer) and in it's maximized state (winter)?

If so I'd say draw the summer state as a regular glacier as that will permanently be glacier area. How to draw that depends on your style, so I can't help with that, yet.
The simplest solution to then show the maximum reach of your glacier would be with dotted lines.
Does that help?

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