Bergschrund
U.S. Geological Survey photo by Bruce Molnia (fair use policy)
Where valley glaciers are born, at the head of the cirque, is a large, deep crack in the ice or crevasse that separates moving glacier material from the ice apron, the immobile ice and snow on the headwall of the cirque. That crevasse is called a bergschrund (German, "mountain crack"). The bergschrund may be invisible in winter if snow covers it, but summer melting usually brings it out. It marks the top of the glacier. This horn in Alaska's Juneau Icefield has two bergschrunds visible.
If there is no ice apron, just bare rock above, the crevasse is called a randkluft. Especially in summer, a randkluft may widen because the dark rock next to it grows warm in the sunlight and melts the ice nearby.
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