A mechanism is presented for subaerial calving of ice slabs and subaqueous calving of ice ledges from ice walls and ice shelves in lacustrine and marine glacial environments. The calving rate is equated with the maximum forward velocity of slab-bending at the time of calving. The rate of slab calving above water is shown to control the rate of block calving from ice ledges below water. Slabs theoretically calve along surface crevasses that are at a distance behind the calving front of about one-tenth the subaerial height of the ice wall or ice shelf. Blocks theoretically calve along bottom crevasses when subaqueous ice ledges produced by subaerial slab calving approach one-half of the ice thickness below water. These calculations are supported by studies of a calving ice wall on Deception Island, Antarctica, and by observations of calving from ice walls along the Antarctic Peninsula, from the Ross and Getz Ice Shelves in Antarctica, from large tabular icebergs in the Ross and Amundsen Seas of Antarctica, from Jakobshavns Isbrae in Greenland, and from tidewater glaciers in Alaska, Greenland, and Svalbard. Calving bays accelerate the retreat of Quaternary ice sheets along lacustrine and marine ice margins at the termination of Quaternary glaciation cycles. Rapid calving retreat causes sudden outbursts of icebergs from marine ice streams and meltwater from ice-dammed lakes, events that may trigger abrupt climatic changes. In the case of the Laurentide Ice Sheet, retreat of a calving bay along Hudson Strait would not only be accompanied by an iceberg outburst into the North Atlantic, the calving bay could also continue into Hudson Bay and carve out the heart of the Laurentide Ice Sheet itself, thereby terminating the last cycle of Quaternary glaciations.