Where valley glaciers are born, at the head of the cirque, a bergschrund ("bearg-shroond") separates moving glacier material from the ice apron, the immobile ice and snow on the headwall of the cirque. 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 bergschrund is in Allalin Glacier in the Swiss Alps.
If there is no ice apron above the crack, just bare rock above, the crevasse is called a randkluft. Especially in summer, a randkluft may become wide because the dark rock next to it grows warm in the sunlight and melts the ice nearby.
